diff --git a/conf/flight_plans/EMAV2008.xml b/conf/flight_plans/EMAV2008.xml
index 1d81947daf..57fa34c38a 100644
--- a/conf/flight_plans/EMAV2008.xml
+++ b/conf/flight_plans/EMAV2008.xml
@@ -47,7 +47,7 @@
       <attitude roll="0" throttle="0" vmode="throttle"/>
     </block>
     <block name="Takeoff" strip_button="Takeoff (wp CLIMB)" strip_icon="takeoff.png">
-      <exception cond="GetPosAlt() > ground_alt+40" deroute="Standby"/>
+      <exception cond="GetPosAlt() > GetAltRef()+40" deroute="Standby"/>
       <set value="0" var="kill_throttle"/>
       <go wp="CLIMB"/>
     </block>
@@ -86,11 +86,11 @@
       <circle radius="nav_radius" until="NavQdrCloseTo(DegOfRad(baseleg_out_qdr)-(nav_radius/fabs(nav_radius))*10) && 15 > fabs(GetPosAlt() - WaypointAlt(WP_BASELEG))" wp="BASELEG"/>
     </block>
     <block name="final">
-<!--      <exception cond="ground_alt + 6 > GetPosAlt()" deroute="flare"/> -->
+<!--      <exception cond="GetAltRef() + 6 > GetPosAlt()" deroute="flare"/> -->
       <go approaching_time="0" from="AF" hmode="route" wp="SF"/>
     </block>
     <block name="shortfinal">
-<!--      <exception cond="ground_alt + 6 > GetPosAlt()" deroute="flare"/> -->
+<!--      <exception cond="GetAltRef() + 6 > GetPosAlt()" deroute="flare"/> -->
       <go approaching_time="0" from="SF" hmode="route" wp="TD"/>
     </block>
     <block name="flare">
diff --git a/conf/flight_plans/EMAV2009.xml b/conf/flight_plans/EMAV2009.xml
index 28c86bd6f1..e9f4de4d36 100644
--- a/conf/flight_plans/EMAV2009.xml
+++ b/conf/flight_plans/EMAV2009.xml
@@ -26,7 +26,7 @@
       <attitude roll="0" throttle="0" vmode="throttle"/>
     </block>
     <block name="Takeoff" strip_button="Takeoff (wp CLIMB)" strip_icon="takeoff.png">
-      <exception cond="GetPosAlt() > ground_alt+25" deroute="Standby"/>
+      <exception cond="GetPosAlt() > GetAltRef()+25" deroute="Standby"/>
       <set value="0" var="kill_throttle"/>
       <set value="0" var="autopilot_flight_time"/>
       <go from="HOME" pitch="15" throttle="1.0" vmode="throttle" wp="CLIMB"/>
diff --git a/conf/flight_plans/EMAV2009_safety.xml b/conf/flight_plans/EMAV2009_safety.xml
index 9982b628ca..5e2dca0b32 100644
--- a/conf/flight_plans/EMAV2009_safety.xml
+++ b/conf/flight_plans/EMAV2009_safety.xml
@@ -2,7 +2,7 @@
 
 <procedure>
   <exceptions>
-    <exception cond="Or(! InsideGreen(GetPosX(), GetPosY()), GetPosAlt() > ground_alt + 150)" deroute="Center"/>
+    <exception cond="Or(! InsideGreen(GetPosX(), GetPosY()), GetPosAlt() > GetAltRef() + 150)" deroute="Center"/>
   </exceptions>
 
   <blocks>
diff --git a/conf/flight_plans/IS.xml b/conf/flight_plans/IS.xml
index 6d636cb7b1..598371aed5 100644
--- a/conf/flight_plans/IS.xml
+++ b/conf/flight_plans/IS.xml
@@ -101,15 +101,15 @@
     </block>
 
     <block name="climb 75">
-      <circle radius="50+(GetPosAlt()-ground_alt)/2" wp="1" throttle="0.75" pitch="15" vmode="throttle" until="10 > PowerVoltage()"/>
+      <circle radius="50+(GetPosAlt()-GetAltRef())/2" wp="1" throttle="0.75" pitch="15" vmode="throttle" until="10 > PowerVoltage()"/>
     </block>
 
     <block name="climb 1">
-      <circle radius="50+(GetPosAlt()-ground_alt)/2" wp="1" climb="1" pitch="5" vmode="climb" until="10 > PowerVoltage()"/>
+      <circle radius="50+(GetPosAlt()-GetAltRef())/2" wp="1" climb="1" pitch="5" vmode="climb" until="10 > PowerVoltage()"/>
     </block>
 
     <block name="descent 0">
-      <circle radius="50+(GetPosAlt()-ground_alt)/2" wp="1" throttle="0.0" pitch="-15" vmode="throttle" until="ground_alt+50 > GetPosAlt()"/>
+      <circle radius="50+(GetPosAlt()-GetAltRef())/2" wp="1" throttle="0.0" pitch="-15" vmode="throttle" until="GetAltRef()+50 > GetPosAlt()"/>
       <deroute block="wait"/>
     </block>
 
@@ -147,7 +147,7 @@
     </block>
 
     <block name="final">
-      <exception cond="ground_alt + 10 > GetPosAlt()" deroute="flare"/>
+      <exception cond="GetAltRef() + 10 > GetPosAlt()" deroute="flare"/>
       <go from="AF" hmode="route" vmode="glide" wp="TD"/>
     </block>
 
diff --git a/conf/flight_plans/basic.xml b/conf/flight_plans/basic.xml
index e404260c69..e690348416 100644
--- a/conf/flight_plans/basic.xml
+++ b/conf/flight_plans/basic.xml
@@ -35,7 +35,7 @@
       <attitude roll="0" throttle="0" vmode="throttle"/>
     </block>
     <block key="t" name="Takeoff" strip_button="Takeoff (wp CLIMB)" strip_icon="takeoff.png" group="home">
-      <exception cond="GetPosAlt() > ground_alt+25" deroute="Standby"/>
+      <exception cond="GetPosAlt() > GetAltRef()+25" deroute="Standby"/>
       <set value="0" var="kill_throttle"/>
       <set value="0" var="autopilot_flight_time"/>
       <go from="HOME" throttle="1.0" vmode="throttle" wp="CLIMB" pitch="15"/>
@@ -76,7 +76,7 @@
       <circle radius="nav_radius" until="And(NavQdrCloseTo(DegOfRad(baseleg_out_qdr)-(nav_radius/fabs(nav_radius))*10), 10 > fabs(GetPosAlt() - WaypointAlt(WP__BASELEG)))" wp="_BASELEG"/>
     </block>
     <block name="final">
-      <exception cond="ground_alt + 10 > GetPosAlt()" deroute="flare"/>
+      <exception cond="GetAltRef() + 10 > GetPosAlt()" deroute="flare"/>
       <go from="AF" hmode="route" vmode="glide" wp="TD"/>
     </block>
     <block name="flare">
diff --git a/conf/flight_plans/corsica.xml b/conf/flight_plans/corsica.xml
index 02db9b1e70..7b96b5ad8f 100644
--- a/conf/flight_plans/corsica.xml
+++ b/conf/flight_plans/corsica.xml
@@ -21,7 +21,7 @@
   </includes>
 
   <exceptions>
-    <exception cond="Or(! InsideZRT(GetPosX(), GetPosY()), GetPosAlt() > ground_alt + 300)" deroute="Kill"/>
+    <exception cond="Or(! InsideZRT(GetPosX(), GetPosY()), GetPosAlt() > GetAltRef() + 300)" deroute="Kill"/>
   </exceptions>
   <blocks>
     <block name="Wait GPS">
@@ -33,7 +33,7 @@
       <attitude roll="0" throttle="0" vmode="throttle"/>
     </block>
     <block name="Takeoff" strip_button="Takeoff (wp CLIMB)" strip_icon="takeoff.png">
-      <exception cond="GetPosAlt() > ground_alt+25" deroute="Standby Menton"/>
+      <exception cond="GetPosAlt() > GetAltRef()+25" deroute="Standby Menton"/>
       <set value="0" var="kill_throttle"/>
       <set value="0" var="autopilot_flight_time"/>
       <go throttle="1.0" vmode="throttle" wp="CLIMB" pitch="15"/>
@@ -66,7 +66,7 @@
       <circle radius="nav_radius" until="And(NavQdrCloseTo(DegOfRad(baseleg_out_qdr)-10), 10 > fabs(GetPosAlt() - WaypointAlt(WP__BASELEG)))" wp="_BASELEG"/>
     </block>
     <block name="final">
-      <exception cond="ground_alt + 10 > GetPosAlt()" deroute="flare"/>
+      <exception cond="GetAltRef() + 10 > GetPosAlt()" deroute="flare"/>
       <go from="AF" hmode="route" vmode="glide" wp="TD"/>
     </block>
     <block name="flare">
@@ -74,7 +74,7 @@
       <attitude roll="0.0" throttle="0.0" until="FALSE" vmode="throttle"/>
     </block>
     <block name="Kill">
-      <exception cond="And(InsideZRT(GetPosX(), GetPosY()), ground_alt + 300 > GetPosAlt())" deroute="Resume Mission"/>
+      <exception cond="And(InsideZRT(GetPosX(), GetPosY()), GetAltRef() + 300 > GetPosAlt())" deroute="Resume Mission"/>
       <attitude roll="0" vmode="throttle" throttle="0.2"/>
     </block>
     <block name="Resume Mission">
diff --git a/conf/flight_plans/creidlitz.xml b/conf/flight_plans/creidlitz.xml
index d4b34b0250..58bfa754c3 100644
--- a/conf/flight_plans/creidlitz.xml
+++ b/conf/flight_plans/creidlitz.xml
@@ -33,7 +33,7 @@
       <attitude roll="0" throttle="0" vmode="throttle"/>
     </block>
     <block name="Takeoff" strip_button="Takeoff (wp CLIMB)" strip_icon="takeoff.png">
-      <exception cond="GetPosAlt() > ground_alt+25" deroute="Standby"/>
+      <exception cond="GetPosAlt() > GetAltRef()+25" deroute="Standby"/>
       <set value="0" var="kill_throttle"/>
       <set value="0" var="autopilot_flight_time"/>
       <go from="HOME" throttle="1.0" vmode="throttle" wp="CLIMB"/>
@@ -45,7 +45,7 @@
       <circle radius="nav_radius" wp="PROFILE"/>
     </block>
     <block name="profile_down" strip_button="Profile Down" strip_icon="down_profile.png">
-      <circle pitch="RadOfDeg(-10)" radius="nav_radius" throttle="0" until="(ground_alt+150 > GetPosAlt())" vmode="throttle" wp="PROFILE"/>
+      <circle pitch="RadOfDeg(-10)" radius="nav_radius" throttle="0" until="(GetAltRef()+150 > GetPosAlt())" vmode="throttle" wp="PROFILE"/>
       <deroute block="Standby"/>
     </block>
     <block name="MOB" strip_button="Circle around here" strip_icon="mob.png">
diff --git a/conf/flight_plans/fp_tp_auto.xml b/conf/flight_plans/fp_tp_auto.xml
index 444a7fc7c3..1c91224cc7 100644
--- a/conf/flight_plans/fp_tp_auto.xml
+++ b/conf/flight_plans/fp_tp_auto.xml
@@ -44,7 +44,7 @@
       <attitude roll="0" throttle="0" vmode="throttle"/>
     </block>
     <block name="Takeoff" strip_icon="takeoff.png" strip_button="Takeoff (wp CLIMB)">
-      <exception cond="GetPosAlt() > ground_alt+25" deroute="Standby"/>
+      <exception cond="GetPosAlt() > GetAltRef()+25" deroute="Standby"/>
       <set value="0" var="kill_throttle"/>
       <go wp="CLIMB"/>
     </block>
@@ -65,19 +65,19 @@
       <circle pitch="auto" radius="75" throttle="0.7" wp="1"/>
     </block>
     <block name="Climb 75% throttle">
-      <circle pitch="10" radius="50+(GetPosAlt()-ground_alt)/2" throttle="0.75" until="(10 > PowerVoltage()) || (GetPosAlt() > ground_alt+ 1350)" vmode="throttle" wp="1"/>
+      <circle pitch="10" radius="50+(GetPosAlt()-GetAltRef())/2" throttle="0.75" until="(10 > PowerVoltage()) || (GetPosAlt() > GetAltRef()+ 1350)" vmode="throttle" wp="1"/>
     </block>
     <block name="Climb 0m/s">
       <circle climb="0" radius="nav_radius" until="10 > PowerVoltage()" vmode="climb" wp="1"/>
     </block>
     <block name="Climb 1m/s">
-      <circle climb="1" radius="50+(GetPosAlt()-ground_alt)/2" until="10 > PowerVoltage()" vmode="climb" wp="1"/>
+      <circle climb="1" radius="50+(GetPosAlt()-GetAltRef())/2" until="10 > PowerVoltage()" vmode="climb" wp="1"/>
     </block>
     <block name="Climb nav_climb m/s">
-      <circle climb="nav_climb" radius="nav_radius" until="(10 > PowerVoltage()) || (GetPosAlt() > ground_alt+ 1350)" vmode="climb" wp="1"/>
+      <circle climb="nav_climb" radius="nav_radius" until="(10 > PowerVoltage()) || (GetPosAlt() > GetAltRef()+ 1350)" vmode="climb" wp="1"/>
     </block>
     <block name="Descent 0% throttle">
-      <circle pitch="-5" radius="50+(GetPosAlt()-ground_alt)/2" throttle="0.0" until="ground_alt+50 > GetPosAlt()" vmode="throttle" wp="1"/>
+      <circle pitch="-5" radius="50+(GetPosAlt()-GetAltRef())/2" throttle="0.0" until="GetAltRef()+50 > GetPosAlt()" vmode="throttle" wp="1"/>
       <deroute block="Standby"/>
     </block>
     <block name="Route 1-2">
@@ -114,7 +114,7 @@
       <circle radius="nav_radius" until="NavQdrCloseTo(DegOfRad(baseleg_out_qdr)-(nav_radius/fabs(nav_radius))*10) && 10 > fabs(GetPosAlt() - WaypointAlt(WP_BASELEG))" wp="BASELEG"/>
     </block>
     <block name="final">
-      <exception cond="ground_alt + 10 > GetPosAlt()" deroute="flare"/>
+      <exception cond="GetAltRef() + 10 > GetPosAlt()" deroute="flare"/>
       <go from="AF" hmode="route" vmode="glide" wp="TD"/>
     </block>
     <block name="flare">
@@ -140,7 +140,7 @@
       </while>
     </block>
     <block name="Heading 30">
-      <heading alt="ground_alt+50" course="30" until="FALSE"/>
+      <heading alt="GetAltRef()+50" course="30" until="FALSE"/>
     </block>
   </blocks>
 </flight_plan>
diff --git a/conf/flight_plans/generic.xml b/conf/flight_plans/generic.xml
index b1fc8ab8ea..5cdf61eefc 100644
--- a/conf/flight_plans/generic.xml
+++ b/conf/flight_plans/generic.xml
@@ -24,7 +24,7 @@
       <circle radius="-75" wp="HOME"/>
     </block>
     <block name="climb 75">
-      <circle radius="50+(GetPosAlt()-ground_alt)/2" wp="1" throttle="0.75" pitch="15" vmode="throttle" until="10>PowerVoltage()"/>
+      <circle radius="50+(GetPosAlt()-GetAltRef())/2" wp="1" throttle="0.75" pitch="15" vmode="throttle" until="10>PowerVoltage()"/>
     </block>
     <block name="stack 1">
       <circle radius="GetPosAlt()/2" wp="1"/>
diff --git a/conf/flight_plans/grosslobke_demo.xml b/conf/flight_plans/grosslobke_demo.xml
index 6cdcaa5866..80343d88f0 100644
--- a/conf/flight_plans/grosslobke_demo.xml
+++ b/conf/flight_plans/grosslobke_demo.xml
@@ -53,7 +53,7 @@
       <set value="TRUE" var="h_ctl_disabled"/>
       <set value="(-0.6*MAX_PPRZ)" var="h_ctl_aileron_setpoint"/>
       <set value="(0.9*MAX_PPRZ)" var="h_ctl_elevator_setpoint"/>
-      <while cond="GetPosAlt() > ground_alt + 220"/>
+      <while cond="GetPosAlt() > GetAltRef() + 220"/>
       <set value="0" var="h_ctl_aileron_setpoint"/>
       <set value="0" var="h_ctl_elevator_setpoint"/>
     </block>
@@ -78,7 +78,7 @@
       <circle radius="nav_radius" until="NavQdrCloseTo(DegOfRad(baseleg_out_qdr)-(nav_radius/fabs(nav_radius))*10) && 20 > fabs(GetPosAlt() - WaypointAlt(WP__BASELEG))" wp="_BASELEG"/>
     </block>
     <block name="final">
-      <exception cond="ground_alt + 10 > GetPosAlt()" deroute="flare"/>
+      <exception cond="GetAltRef() + 10 > GetPosAlt()" deroute="flare"/>
       <go from="AF" hmode="route" vmode="glide" wp="TD"/>
     </block>
     <block name="flare">
diff --git a/conf/flight_plans/hsif.xml b/conf/flight_plans/hsif.xml
index 19eb10dfb3..39808c6349 100644
--- a/conf/flight_plans/hsif.xml
+++ b/conf/flight_plans/hsif.xml
@@ -18,7 +18,7 @@
       <call fun="NavSetGroundReferenceHere()"/>
     </block>
     <block name="Takeoff">
-      <exception cond="GetPosAlt() > ground_alt+25" deroute="HSIF"/>
+      <exception cond="GetPosAlt() > GetAltRef()+25" deroute="HSIF"/>
       <set value="0" var="kill_throttle"/>
       <set value="0" var="autopilot_flight_time"/>
       <go from="HOME" pitch="15" throttle="1.0" vmode="throttle" wp="CLIMB"/>
diff --git a/conf/flight_plans/ingolfsskali.xml b/conf/flight_plans/ingolfsskali.xml
index f980b7a2cb..8cb378e8f1 100644
--- a/conf/flight_plans/ingolfsskali.xml
+++ b/conf/flight_plans/ingolfsskali.xml
@@ -58,22 +58,22 @@
     </block>
     <block name="profile">
       <for from="1" to="5" var="i">
-        <circle alt="ground_alt+50* $i" radius="nav_radius" until="stage_time>120" wp="1"/>
+        <circle alt="GetAltRef()+50* $i" radius="nav_radius" until="stage_time>120" wp="1"/>
       </for>
     </block>
     <block name="climb">
       <circle radius="nav_radius" wp="CLIMB"/>
     </block>
     <block name="descent">
-      <circle radius="nav_radius" wp="CLIMB" vmode="throttle" throttle="0" pitch="RadOfDeg(my_nav_pitch)"  until="(ground_alt+150 > GetPosAlt())"/>
+      <circle radius="nav_radius" wp="CLIMB" vmode="throttle" throttle="0" pitch="RadOfDeg(my_nav_pitch)"  until="(GetAltRef()+150 > GetPosAlt())"/>
     <deroute block="wait"/>
     </block>
     <block name="wind">
-      <attitude pitch="RadOfDeg(my_nav_pitch)" roll="my_nav_roll" throttle="0.0" until="(ground_alt+150 > GetPosAlt())" vmode="throttle"/>
+      <attitude pitch="RadOfDeg(my_nav_pitch)" roll="my_nav_roll" throttle="0.0" until="(GetAltRef()+150 > GetPosAlt())" vmode="throttle"/>
       <deroute block="wait"/>
     </block>
     <block name="wait" strip_button="Wait">
-      <circle radius="nav_radius" wp="CLIMB" alt="ground_alt+200"/>
+      <circle radius="nav_radius" wp="CLIMB" alt="GetAltRef()+200"/>
     </block>
     <block name="roll">
       <set value="TRUE" var="h_ctl_disabled"/>
diff --git a/conf/flight_plans/joystick.xml b/conf/flight_plans/joystick.xml
index 6969aeb403..5e979f785e 100644
--- a/conf/flight_plans/joystick.xml
+++ b/conf/flight_plans/joystick.xml
@@ -33,7 +33,7 @@
       <attitude roll="0" throttle="0" vmode="throttle"/>
     </block>
     <block name="Takeoff" strip_button="Takeoff (wp CLIMB)" strip_icon="takeoff.png">
-      <exception cond="GetPosAlt() > ground_alt+25" deroute="Standby"/>
+      <exception cond="GetPosAlt() > GetAltRef()+25" deroute="Standby"/>
       <set value="0" var="kill_throttle"/>
       <set value="0" var="autopilot_flight_time"/>
       <go from="HOME" throttle="1.0" vmode="throttle" wp="CLIMB" pitch="15"/>
@@ -82,7 +82,7 @@
       <circle radius="nav_radius" until="NavQdrCloseTo(DegOfRad(baseleg_out_qdr)-(nav_radius/fabs(nav_radius))*10) && 10 > fabs(GetPosAlt() - WaypointAlt(WP__BASELEG))" wp="_BASELEG"/>
     </block>
     <block name="final">
-      <exception cond="ground_alt + 10 > GetPosAlt()" deroute="flare"/>
+      <exception cond="GetAltRef() + 10 > GetPosAlt()" deroute="flare"/>
       <go from="AF" hmode="route" vmode="glide" wp="TD"/>
     </block>
     <block name="flare">
diff --git a/conf/flight_plans/kalscott.xml b/conf/flight_plans/kalscott.xml
index 49d59220b2..c9540c1192 100644
--- a/conf/flight_plans/kalscott.xml
+++ b/conf/flight_plans/kalscott.xml
@@ -26,48 +26,48 @@
       <attitude roll="0" throttle="0" vmode="throttle"/>
     </block>
     <block name="Takeoff" strip_button="Takeoff (wp CLIMB)" strip_icon="takeoff.png">
-      <exception cond="GetPosAlt() > ground_alt+25" deroute="Circle"/>
+      <exception cond="GetPosAlt() > GetAltRef()+25" deroute="Circle"/>
       <set value="0" var="kill_throttle"/>
       <set value="0" var="autopilot_flight_time"/>
       <go from="HOME" throttle="1.0" vmode="throttle" wp="CLIMB" pitch="15"/>
     </block>
     <block name="Circle" strip_button="Circle" strip_icon="home.png">
-      <circle radius="nav_radius" wp="CIRCLE" alt="ground_alt+200" until="GetPosAlt() > ground_alt+195"/>
-      <circle radius="nav_radius" wp="CIRCLE" alt="ground_alt+200" until="NavCircleCount() > 2"/>
-      <circle radius="nav_radius" wp="CIRCLE" alt="ground_alt+300" until="GetPosAlt() > ground_alt+295"/>
-      <circle radius="nav_radius" wp="CIRCLE" alt="ground_alt+300" until="NavCircleCount() > 2"/>
-      <circle radius="nav_radius" wp="CIRCLE" alt="ground_alt+400" until="GetPosAlt() > ground_alt+395"/>
-      <circle radius="nav_radius" wp="CIRCLE" alt="ground_alt+400" until="NavCircleCount() > 2"/>
-      <circle radius="nav_radius" wp="CIRCLE" alt="ground_alt+300" until="ground_alt+305 > GetPosAlt()"/>
-      <circle radius="nav_radius" wp="CIRCLE" alt="ground_alt+300" until="NavCircleCount() > 2"/>
-      <circle radius="nav_radius" wp="CIRCLE" alt="ground_alt+200" until="ground_alt+205 > GetPosAlt()"/>
-      <circle radius="nav_radius" wp="CIRCLE" alt="ground_alt+200" until="NavCircleCount() > 2"/>
+      <circle radius="nav_radius" wp="CIRCLE" alt="GetAltRef()+200" until="GetPosAlt() > GetAltRef()+195"/>
+      <circle radius="nav_radius" wp="CIRCLE" alt="GetAltRef()+200" until="NavCircleCount() > 2"/>
+      <circle radius="nav_radius" wp="CIRCLE" alt="GetAltRef()+300" until="GetPosAlt() > GetAltRef()+295"/>
+      <circle radius="nav_radius" wp="CIRCLE" alt="GetAltRef()+300" until="NavCircleCount() > 2"/>
+      <circle radius="nav_radius" wp="CIRCLE" alt="GetAltRef()+400" until="GetPosAlt() > GetAltRef()+395"/>
+      <circle radius="nav_radius" wp="CIRCLE" alt="GetAltRef()+400" until="NavCircleCount() > 2"/>
+      <circle radius="nav_radius" wp="CIRCLE" alt="GetAltRef()+300" until="GetAltRef()+305 > GetPosAlt()"/>
+      <circle radius="nav_radius" wp="CIRCLE" alt="GetAltRef()+300" until="NavCircleCount() > 2"/>
+      <circle radius="nav_radius" wp="CIRCLE" alt="GetAltRef()+200" until="GetAltRef()+205 > GetPosAlt()"/>
+      <circle radius="nav_radius" wp="CIRCLE" alt="GetAltRef()+200" until="NavCircleCount() > 2"/>
       <deroute block="Circle"/>
     </block>
    <block name="Racetrack E-W" strip_button="Racetrack E-W" strip_icon="oval.png">
-      <oval p1="1" p2="2" radius="nav_radius" alt="ground_alt+200" until="GetPosAlt() > ground_alt+195"/>
-      <oval p1="1" p2="2" radius="nav_radius" alt="ground_alt+200" until="nav_oval_count > 2"/>
-      <oval p1="1" p2="2" radius="nav_radius" alt="ground_alt+300" until="GetPosAlt() > ground_alt+295"/>
-      <oval p1="1" p2="2" radius="nav_radius" alt="ground_alt+300" until="nav_oval_count > 2"/>
-      <oval p1="1" p2="2" radius="nav_radius" alt="ground_alt+400" until="GetPosAlt() > ground_alt+395"/>
-      <oval p1="1" p2="2" radius="nav_radius" alt="ground_alt+400" until="nav_oval_count > 2"/>
-      <oval p1="1" p2="2" radius="nav_radius" alt="ground_alt+300" until="ground_alt+305 > GetPosAlt()"/>
-      <oval p1="1" p2="2" radius="nav_radius" alt="ground_alt+300" until="nav_oval_count > 2"/>
-      <oval p1="1" p2="2" radius="nav_radius" alt="ground_alt+200" until="ground_alt+205 > GetPosAlt()"/>
-      <oval p1="1" p2="2" radius="nav_radius" alt="ground_alt+200" until="nav_oval_count > 2"/>
+      <oval p1="1" p2="2" radius="nav_radius" alt="GetAltRef()+200" until="GetPosAlt() > GetAltRef()+195"/>
+      <oval p1="1" p2="2" radius="nav_radius" alt="GetAltRef()+200" until="nav_oval_count > 2"/>
+      <oval p1="1" p2="2" radius="nav_radius" alt="GetAltRef()+300" until="GetPosAlt() > GetAltRef()+295"/>
+      <oval p1="1" p2="2" radius="nav_radius" alt="GetAltRef()+300" until="nav_oval_count > 2"/>
+      <oval p1="1" p2="2" radius="nav_radius" alt="GetAltRef()+400" until="GetPosAlt() > GetAltRef()+395"/>
+      <oval p1="1" p2="2" radius="nav_radius" alt="GetAltRef()+400" until="nav_oval_count > 2"/>
+      <oval p1="1" p2="2" radius="nav_radius" alt="GetAltRef()+300" until="GetAltRef()+305 > GetPosAlt()"/>
+      <oval p1="1" p2="2" radius="nav_radius" alt="GetAltRef()+300" until="nav_oval_count > 2"/>
+      <oval p1="1" p2="2" radius="nav_radius" alt="GetAltRef()+200" until="GetAltRef()+205 > GetPosAlt()"/>
+      <oval p1="1" p2="2" radius="nav_radius" alt="GetAltRef()+200" until="nav_oval_count > 2"/>
       <deroute block="Racetrack E-W"/>
     </block>
     <block name="Racetrack N-S" strip_button="Racetrack N-S" strip_icon="oval.png">
-      <oval p1="2" p2="3" radius="nav_radius" alt="ground_alt+200" until="GetPosAlt() > ground_alt+195"/>
-      <oval p1="2" p2="3" radius="nav_radius" alt="ground_alt+200" until="nav_oval_count > 2"/>
-      <oval p1="2" p2="3" radius="nav_radius" alt="ground_alt+300" until="GetPosAlt() > ground_alt+295"/>
-      <oval p1="2" p2="3" radius="nav_radius" alt="ground_alt+300" until="nav_oval_count > 2"/>
-      <oval p1="2" p2="3" radius="nav_radius" alt="ground_alt+400" until="GetPosAlt() > ground_alt+395"/>
-      <oval p1="2" p2="3" radius="nav_radius" alt="ground_alt+400" until="nav_oval_count > 2"/>
-      <oval p1="2" p2="3" radius="nav_radius" alt="ground_alt+300" until="ground_alt+305 > GetPosAlt()"/>
-      <oval p1="2" p2="3" radius="nav_radius" alt="ground_alt+300" until="nav_oval_count > 2"/>
-      <oval p1="2" p2="3" radius="nav_radius" alt="ground_alt+200" until="ground_alt+305 > GetPosAlt()"/>
-      <oval p1="2" p2="3" radius="nav_radius" alt="ground_alt+200" until="nav_oval_count > 2"/>
+      <oval p1="2" p2="3" radius="nav_radius" alt="GetAltRef()+200" until="GetPosAlt() > GetAltRef()+195"/>
+      <oval p1="2" p2="3" radius="nav_radius" alt="GetAltRef()+200" until="nav_oval_count > 2"/>
+      <oval p1="2" p2="3" radius="nav_radius" alt="GetAltRef()+300" until="GetPosAlt() > GetAltRef()+295"/>
+      <oval p1="2" p2="3" radius="nav_radius" alt="GetAltRef()+300" until="nav_oval_count > 2"/>
+      <oval p1="2" p2="3" radius="nav_radius" alt="GetAltRef()+400" until="GetPosAlt() > GetAltRef()+395"/>
+      <oval p1="2" p2="3" radius="nav_radius" alt="GetAltRef()+400" until="nav_oval_count > 2"/>
+      <oval p1="2" p2="3" radius="nav_radius" alt="GetAltRef()+300" until="GetAltRef()+305 > GetPosAlt()"/>
+      <oval p1="2" p2="3" radius="nav_radius" alt="GetAltRef()+300" until="nav_oval_count > 2"/>
+      <oval p1="2" p2="3" radius="nav_radius" alt="GetAltRef()+200" until="GetAltRef()+305 > GetPosAlt()"/>
+      <oval p1="2" p2="3" radius="nav_radius" alt="GetAltRef()+200" until="nav_oval_count > 2"/>
       <deroute block="Racetrack N-S"/>
     </block>
   </blocks>
diff --git a/conf/flight_plans/kv_svalbard.xml b/conf/flight_plans/kv_svalbard.xml
index 2831ac9162..7dcfa2ba9b 100644
--- a/conf/flight_plans/kv_svalbard.xml
+++ b/conf/flight_plans/kv_svalbard.xml
@@ -35,7 +35,7 @@
       <attitude roll="0" throttle="0" vmode="throttle"/>
     </block>
     <block name="Takeoff" strip_button="Takeoff (wp CLIMB)" strip_icon="takeoff.png">
-      <exception cond="GetPosAlt() > ground_alt+25" deroute="Standby"/>
+      <exception cond="GetPosAlt() > GetAltRef()+25" deroute="Standby"/>
       <set value="0" var="kill_throttle"/>
       <set value="0" var="autopilot_flight_time"/>
       <go from="HOME" throttle="1.0" vmode="throttle" wp="CLIMB"/>
@@ -47,7 +47,7 @@
       <circle radius="nav_radius" wp="PROFILE"/>
     </block>
     <block name="profile_down" strip_button="Profile Down" strip_icon="down_profile.png">
-      <circle radius="nav_radius" wp="PROFILE" vmode="throttle" throttle="0" pitch="RadOfDeg(-10)"  until="(ground_alt+150 > GetPosAlt())"/>
+      <circle radius="nav_radius" wp="PROFILE" vmode="throttle" throttle="0" pitch="RadOfDeg(-10)"  until="(GetAltRef()+150 > GetPosAlt())"/>
     <deroute block="Standby"/>
     </block>
     <block name="MOB" strip_button="Circle around here" strip_icon="mob.png">
diff --git a/conf/flight_plans/landing.xml b/conf/flight_plans/landing.xml
index f1f2aed593..b38a4de861 100644
--- a/conf/flight_plans/landing.xml
+++ b/conf/flight_plans/landing.xml
@@ -21,8 +21,8 @@
       <circle radius="nav_radius" until="And(NavQdrCloseTo(DegOfRad(baseleg_out_qdr)-(nav_radius/fabs(nav_radius))*10), 10 > fabs(GetPosAlt() - WaypointAlt(WP__BASELEG)))" wp="_BASELEG"/>
     </block>
     <block name="final">
-      <exception cond="ground_alt + 10 > GetPosAlt()" deroute="flare"/>
-      <go from="AF" hmode="route" vmode="glide" wp="TD" alt="ground_alt"/>
+      <exception cond="GetAltRef() + 10 > GetPosAlt()" deroute="flare"/>
+      <go from="AF" hmode="route" vmode="glide" wp="TD" alt="GetAltRef()"/>
     </block>
     <block name="flare">
       <go approaching_time="0" from="AF" hmode="route" throttle="0.0" vmode="throttle" wp="TD"/>
diff --git a/conf/flight_plans/mav06.xml b/conf/flight_plans/mav06.xml
index 746dd37c1f..69bf4e8212 100644
--- a/conf/flight_plans/mav06.xml
+++ b/conf/flight_plans/mav06.xml
@@ -32,7 +32,7 @@
     </block>
 
     <block name="final">
-      <exception cond="ground_alt + 10 > GetPosAlt()" deroute="flare"/>
+      <exception cond="GetAltRef() + 10 > GetPosAlt()" deroute="flare"/>
       <go alt="baseleg_alt" approaching_time="16" from="AF" hmode="route" wp="TD"/>
       <go approaching_time="2" from="AF" hmode="route" throttle="0.1" vmode="throttle" wp="TD"/>
     </block>
@@ -48,7 +48,7 @@
     </block>
 
     <block name="climb">
-      <exception cond=" GetPosAlt() > ground_alt +30" deroute="wait"/>
+      <exception cond=" GetPosAlt() > GetAltRef() +30" deroute="wait"/>
       <attitude pitch="15" roll="0" throttle="0.8" until="10> stage_time" vmode="throttle"/>
     </block>-->
     <block name="wait" strip_button="Wait">
diff --git a/conf/flight_plans/mav07.xml b/conf/flight_plans/mav07.xml
index 83c5ce09bb..7433aac8f2 100644
--- a/conf/flight_plans/mav07.xml
+++ b/conf/flight_plans/mav07.xml
@@ -72,7 +72,7 @@
       </while>
     </block>
     <block name="takeoff">
-      <exception cond="GetPosAlt() > ground_alt + 25" deroute="standby"/>
+      <exception cond="GetPosAlt() > GetAltRef() + 25" deroute="standby"/>
       <set value="0" var="kill_throttle"/>
       <go wp="TAKEOFF"/>
     </block>
@@ -132,7 +132,7 @@
       <circle radius="nav_radius" until="NavQdrCloseTo(DegOfRad(baseleg_out_qdr)-(nav_radius/fabs(nav_radius))*10) && 10 > fabs(GetPosAlt() - WaypointAlt(WP_BASELEG))" wp="BASELEG"/>
     </block>
     <block name="final">
-      <exception cond="ground_alt + 10 > GetPosAlt()" deroute="flare"/>
+      <exception cond="GetAltRef() + 10 > GetPosAlt()" deroute="flare"/>
       <call fun="compute_TOD(WP_AF, WP_TD, WP_TOD, GLIDE_AIRSPEED, GLIDE_VSPEED)"/>
       <go approaching_time="0" from="AF" hmode="route" wp="TOD"/>
       <go from="TOD" hmode="route" pitch="stage_time*(GLIDE_PITCH/3)" throttle="0" vmode="throttle" wp="TD"/>
diff --git a/conf/flight_plans/mav08.xml b/conf/flight_plans/mav08.xml
index 700b08d8a7..5ce3f7ae2c 100644
--- a/conf/flight_plans/mav08.xml
+++ b/conf/flight_plans/mav08.xml
@@ -41,7 +41,7 @@
       <attitude roll="0" throttle="0" vmode="throttle"/>
     </block>
     <block name="Takeoff" strip_button="Takeoff (wp CLIMB)" strip_icon="takeoff.png">
-      <exception cond="GetPosAlt() > ground_alt+25" deroute="Standby"/>
+      <exception cond="GetPosAlt() > GetAltRef()+25" deroute="Standby"/>
       <set value="0" var="kill_throttle"/>
       <set value="0" var="autopilot_flight_time"/>
       <go from="HOME" pitch="15" throttle="1.0" vmode="throttle" wp="CLIMB"/>
@@ -100,7 +100,7 @@
       <circle radius="nav_radius" until="NavQdrCloseTo(DegOfRad(baseleg_out_qdr)-(nav_radius/fabs(nav_radius))*10) && 10 > fabs(GetPosAlt() - WaypointAlt(WP__BASELEG))" wp="_BASELEG"/>
     </block>
     <block name="final">
-      <exception cond="ground_alt + 10 > GetPosAlt()" deroute="flare"/>
+      <exception cond="GetAltRef() + 10 > GetPosAlt()" deroute="flare"/>
       <go from="AF" hmode="route" vmode="glide" wp="TD"/>
     </block>
     <block name="flare">
diff --git a/conf/flight_plans/mission_fw.xml b/conf/flight_plans/mission_fw.xml
index 8359db5d26..5c53485f74 100644
--- a/conf/flight_plans/mission_fw.xml
+++ b/conf/flight_plans/mission_fw.xml
@@ -26,7 +26,7 @@
       <attitude roll="0" throttle="0" vmode="throttle"/>
     </block>
     <block key="t" name="Takeoff" strip_button="Takeoff (wp CLIMB)" strip_icon="takeoff.png" group="home">
-      <exception cond="GetPosAlt() > ground_alt+25" deroute="Standby"/>
+      <exception cond="GetPosAlt() > GetAltRef()+25" deroute="Standby"/>
       <set value="0" var="kill_throttle"/>
       <set value="0" var="autopilot_flight_time"/>
       <go from="HOME" throttle="1.0" vmode="throttle" wp="CLIMB" pitch="15"/>
@@ -52,7 +52,7 @@
       <circle radius="nav_radius" until="And(NavQdrCloseTo(DegOfRad(baseleg_out_qdr)-(nav_radius/fabs(nav_radius))*10), 10 > fabs(GetPosAlt() - WaypointAlt(WP__BASELEG)))" wp="_BASELEG"/>
     </block>
     <block name="final">
-      <exception cond="ground_alt + 10 > GetPosAlt()" deroute="flare"/>
+      <exception cond="GetAltRef() + 10 > GetPosAlt()" deroute="flare"/>
       <go from="AF" hmode="route" vmode="glide" wp="TD"/>
     </block>
     <block name="flare">
diff --git a/conf/flight_plans/muret_demo_1.xml b/conf/flight_plans/muret_demo_1.xml
index 439ed05688..d0271dafe1 100644
--- a/conf/flight_plans/muret_demo_1.xml
+++ b/conf/flight_plans/muret_demo_1.xml
@@ -19,7 +19,7 @@
       </while>
     </block>
     <block name="circle">
-      <circle wp="road_end" ALT="ground_alt+50" radius="75"/>
+      <circle wp="road_end" ALT="GetAltRef()+50" radius="75"/>
     </block>
     <block name="go too far">
       <go wp="too far"/>
diff --git a/conf/flight_plans/muret_for.xml b/conf/flight_plans/muret_for.xml
index 961a2816c4..1837fd6b7f 100644
--- a/conf/flight_plans/muret_for.xml
+++ b/conf/flight_plans/muret_for.xml
@@ -14,7 +14,7 @@
     </block>
     <block name="for">
       <for from="0" to="3" var="i">
-        <circle alt="ground_alt+50+50*$i" radius="75" until="stage_time>10" wp="HOME"/>
+        <circle alt="GetAltRef()+50+50*$i" radius="75" until="stage_time>10" wp="HOME"/>
       </for>
       <circle wp="HOME" radius="50"/>
     </block>
diff --git a/conf/flight_plans/nav_modules.xml b/conf/flight_plans/nav_modules.xml
index 3f142d3d55..b527228872 100644
--- a/conf/flight_plans/nav_modules.xml
+++ b/conf/flight_plans/nav_modules.xml
@@ -50,7 +50,7 @@
       <attitude roll="0" throttle="0" vmode="throttle"/>
     </block>
     <block group="home" key="t" name="Takeoff" strip_button="Takeoff (wp CLIMB)" strip_icon="takeoff.png">
-      <exception cond="GetPosAlt() > ground_alt+25" deroute="Standby"/>
+      <exception cond="GetPosAlt() > GetAltRef()+25" deroute="Standby"/>
       <set value="0" var="kill_throttle"/>
       <set value="0" var="autopilot_flight_time"/>
       <go from="HOME" pitch="15" throttle="1.0" vmode="throttle" wp="CLIMB"/>
@@ -82,7 +82,7 @@
       <call fun="nav_bungee_takeoff_run()"/>
     </block>
     <block group="nav_pattern" name="Poly Survey" strip_button="Poly Survey">
-      <call fun="nav_survey_polygon_setup(WP_S1,5,80,30,10,50,ground_alt+60)"/>
+      <call fun="nav_survey_polygon_setup(WP_S1,5,80,30,10,50,GetAltRef()+60)"/>
       <call fun="nav_survey_polygon_run()"/>
     </block>
     <block group="nav_pattern" name="Border line 1-2" strip_button="Border Line">
@@ -130,7 +130,7 @@
       <circle radius="nav_radius" until="And(NavQdrCloseTo(DegOfRad(baseleg_out_qdr)-(nav_radius/fabs(nav_radius))*10), 10 > fabs(GetPosAlt() - WaypointAlt(WP__BASELEG)))" wp="_BASELEG"/>
     </block>
     <block name="final">
-      <exception cond="ground_alt + 10 > GetPosAlt()" deroute="flare"/>
+      <exception cond="GetAltRef() + 10 > GetPosAlt()" deroute="flare"/>
       <go from="AF" hmode="route" vmode="glide" wp="TD"/>
     </block>
     <block name="flare">
diff --git a/conf/flight_plans/nordlys.xml b/conf/flight_plans/nordlys.xml
index f7778a7820..e526b31724 100644
--- a/conf/flight_plans/nordlys.xml
+++ b/conf/flight_plans/nordlys.xml
@@ -37,7 +37,7 @@
       <set value="TRUE" var="h_ctl_disabled"/>
       <set value="(-0.6*MAX_PPRZ)" var="h_ctl_aileron_setpoint"/>
       <set value="(0.9*MAX_PPRZ)" var="h_ctl_elevator_setpoint"/>
-      <while cond="GetPosAlt() > ground_alt + 220"/>
+      <while cond="GetPosAlt() > GetAltRef() + 220"/>
       <set value="0" var="h_ctl_aileron_setpoint"/>
       <set value="0" var="h_ctl_elevator_setpoint"/>
     </block>
@@ -70,7 +70,7 @@
       <circle radius="nav_radius" wp="PROF_LAND"/>
     </block>
     <block name="profile_down land" strip_button="Profile Down Land" strip_icon="down_profile.png">
-      <circle pitch="RadOfDeg(-10)" radius="nav_radius" throttle="0" until="(ground_alt+100 > GetPosAlt())" vmode="throttle" wp="PROF_LAND"/>
+      <circle pitch="RadOfDeg(-10)" radius="nav_radius" throttle="0" until="(GetAltRef()+100 > GetPosAlt())" vmode="throttle" wp="PROF_LAND"/>
       <deroute block="wait"/>
     </block>
     <block name="Land Right AF-TD" strip_button="Land right (wp AF-TD)" strip_icon="land-right.png">
@@ -87,7 +87,7 @@
       <circle radius="nav_radius" until="NavQdrCloseTo(DegOfRad(baseleg_out_qdr)-(nav_radius/fabs(nav_radius))*10) && 20 > fabs(GetPosAlt() - WaypointAlt(WP__BASELEG))" wp="_BASELEG"/>
     </block>
     <block name="final">
-      <exception cond="ground_alt + 10 > GetPosAlt()" deroute="flare"/>
+      <exception cond="GetAltRef() + 10 > GetPosAlt()" deroute="flare"/>
       <go from="AF" hmode="route" vmode="glide" wp="TD"/>
     </block>
     <block name="flare">
diff --git a/conf/flight_plans/poles.xml b/conf/flight_plans/poles.xml
index 7241254523..ccf1487b8c 100644
--- a/conf/flight_plans/poles.xml
+++ b/conf/flight_plans/poles.xml
@@ -32,7 +32,7 @@
       <attitude roll="0" throttle="0" vmode="throttle"/>
     </block>
     <block key="t" name="Takeoff" strip_button="Takeoff (wp CLIMB)" strip_icon="takeoff.png">
-      <exception cond="GetPosAlt() > ground_alt+25" deroute="Standby"/>
+      <exception cond="GetPosAlt() > GetAltRef()+25" deroute="Standby"/>
       <set value="0" var="kill_throttle"/>
       <set value="0" var="autopilot_flight_time"/>
       <go from="HOME" pitch="15" throttle="1.0" vmode="throttle" wp="CLIMB"/>
@@ -73,7 +73,7 @@
       <circle radius="nav_radius" until="And(NavQdrCloseTo(DegOfRad(baseleg_out_qdr)-(nav_radius/fabs(nav_radius))*10), 10 > fabs(GetPosAlt() - WaypointAlt(WP__BASELEG)))" wp="_BASELEG"/>
     </block>
     <block name="final">
-      <exception cond="ground_alt + 10 > GetPosAlt()" deroute="flare"/>
+      <exception cond="GetAltRef() + 10 > GetPosAlt()" deroute="flare"/>
       <go from="AF" hmode="route" vmode="glide" wp="TD"/>
     </block>
     <block name="flare">
diff --git a/conf/flight_plans/rotorcraft_basic_safety.xml b/conf/flight_plans/rotorcraft_basic_safety.xml
index 75e3641daf..c11631d0b5 100644
--- a/conf/flight_plans/rotorcraft_basic_safety.xml
+++ b/conf/flight_plans/rotorcraft_basic_safety.xml
@@ -39,7 +39,7 @@
   </sectors>
   <exceptions>
 	<!-- Check inside Flight Area, then goto Standby -->
-    <exception cond="Or(!InsideFlight_Area(GetPosX(), GetPosY()), GetPosAlt() > ground_alt + 50) && !(nav_block == IndexOfBlock('Wait GPS')) && !(nav_block == IndexOfBlock('Geo init')) && !(nav_block == IndexOfBlock('Holding point')) && !(nav_block == IndexOfBlock('landed')) && !(nav_block == IndexOfBlock('Standby'))" deroute="Standby"/>
+    <exception cond="Or(!InsideFlight_Area(GetPosX(), GetPosY()), GetPosAlt() > GetAltRef() + 50) && !(nav_block == IndexOfBlock('Wait GPS')) && !(nav_block == IndexOfBlock('Geo init')) && !(nav_block == IndexOfBlock('Holding point')) && !(nav_block == IndexOfBlock('landed')) && !(nav_block == IndexOfBlock('Standby'))" deroute="Standby"/>
   </exceptions>
   <blocks>
     <block name="Wait GPS">
diff --git a/conf/flight_plans/rotorcraft_basic_superbitrf.xml b/conf/flight_plans/rotorcraft_basic_superbitrf.xml
index 635610b6ae..aae3d4b9ce 100644
--- a/conf/flight_plans/rotorcraft_basic_superbitrf.xml
+++ b/conf/flight_plans/rotorcraft_basic_superbitrf.xml
@@ -39,7 +39,7 @@
   </sectors>
   <exceptions>
     <!-- Check inside small flight area, then goto Center(Standby) -->
-    <exception cond="Or(! InsideFlight_Area(GetPosX(), GetPosY()), GetPosAlt() > ground_alt + 50) && !(nav_block == BLOCK_Init) && !(nav_block == BLOCK_Geo_init) && !(nav_block == BLOCK_Landed)" deroute="Standby"/>
+    <exception cond="Or(! InsideFlight_Area(GetPosX(), GetPosY()), GetPosAlt() > GetAltRef() + 50) && !(nav_block == BLOCK_Init) && !(nav_block == BLOCK_Geo_init) && !(nav_block == BLOCK_Landed)" deroute="Standby"/>
     <!-- Check if battery is empty, then Land Here -->
     <exception cond="electrical.bat_low && !(nav_block == BLOCK_Land) && !(nav_block == BLOCK_Flare) && !(nav_block == BLOCK_Landed)" deroute="Land_Here"/>
     <!-- Check if time is up(10 seconds range), then Land Here -->
diff --git a/conf/flight_plans/slayer_training.xml b/conf/flight_plans/slayer_training.xml
index 794ec94e96..ab416079d4 100644
--- a/conf/flight_plans/slayer_training.xml
+++ b/conf/flight_plans/slayer_training.xml
@@ -35,7 +35,7 @@
       <deroute block="climb"/>
     </block>
     <block name="climb">
-      <attitude pitch="10" roll="0" throttle="0.9" until="GetPosAlt() > ground_alt+30" vmode="throttle"/>
+      <attitude pitch="10" roll="0" throttle="0.9" until="GetPosAlt() > GetAltRef()+30" vmode="throttle"/>
     </block>
     <block name="standby" strip_button="STDBY">
       <circle radius="nav_radius" wp="STDBY"/>
@@ -77,7 +77,7 @@
       <circle radius="nav_radius" until="NavQdrCloseTo(DegOfRad(baseleg_out_qdr)-(nav_radius/fabs(nav_radius))*10) && 10 > fabs(GetPosAlt() - WaypointAlt(WP_BASELEG))" wp="BASELEG"/>
     </block>
     <block name="final">
-      <exception cond="ground_alt + 10 > GetPosAlt()" deroute="flare"/>
+      <exception cond="GetAltRef() + 10 > GetPosAlt()" deroute="flare"/>
       <go from="AF" hmode="route" vmode="glide" wp="TD"/>
     </block>
     <block name="flare">
diff --git a/conf/flight_plans/tcas.xml b/conf/flight_plans/tcas.xml
index c5eeda466c..e3612c132d 100644
--- a/conf/flight_plans/tcas.xml
+++ b/conf/flight_plans/tcas.xml
@@ -33,7 +33,7 @@
       <attitude roll="0" throttle="0" vmode="throttle"/>
     </block>
     <block name="Takeoff" strip_button="Takeoff (wp CLIMB)" strip_icon="takeoff.png">
-      <exception cond="GetPosAlt() > ground_alt+25" deroute="Standby"/>
+      <exception cond="GetPosAlt() > GetAltRef()+25" deroute="Standby"/>
       <set value="0" var="kill_throttle"/>
       <set value="0" var="autopilot_flight_time"/>
       <go from="HOME" throttle="1.0" vmode="throttle" wp="CLIMB"/>
@@ -74,7 +74,7 @@
       <circle radius="nav_radius" until="NavQdrCloseTo(DegOfRad(baseleg_out_qdr)-(nav_radius/fabs(nav_radius))*10) && 10 > fabs(GetPosAlt() - WaypointAlt(WP__BASELEG))" wp="_BASELEG"/>
     </block>
     <block name="final">
-      <exception cond="ground_alt + 10 > GetPosAlt()" deroute="flare"/>
+      <exception cond="GetAltRef() + 10 > GetPosAlt()" deroute="flare"/>
       <go from="AF" hmode="route" vmode="glide" wp="TD"/>
     </block>
     <block name="flare">
diff --git a/conf/flight_plans/versatile.xml b/conf/flight_plans/versatile.xml
index 5e4023afba..0887a889cf 100644
--- a/conf/flight_plans/versatile.xml
+++ b/conf/flight_plans/versatile.xml
@@ -45,7 +45,7 @@
       <attitude roll="0" throttle="0" vmode="throttle"/>
     </block>
     <block name="Takeoff" strip_icon="takeoff.png" strip_button="Takeoff (wp CLIMB)">
-      <exception cond="GetPosAlt() > ground_alt+25" deroute="Standby"/>
+      <exception cond="GetPosAlt() > GetAltRef()+25" deroute="Standby"/>
       <set value="0" var="kill_throttle"/>
       <go wp="CLIMB"/>
     </block>
@@ -70,25 +70,25 @@
       <circle pitch="auto" radius="75" throttle="0.7" wp="1"/>
     </block>
     <block name="Climb 75% throttle">
-      <circle pitch="10" radius="50+(GetPosAlt()-ground_alt)/2" throttle="0.75" until="(10 > PowerVoltage()) || (GetPosAlt() > ground_alt+ 1350)" vmode="throttle" wp="1"/>
+      <circle pitch="10" radius="50+(GetPosAlt()-GetAltRef())/2" throttle="0.75" until="(10 > PowerVoltage()) || (GetPosAlt() > GetAltRef()+ 1350)" vmode="throttle" wp="1"/>
     </block>
     <block name="Climb 0m/s">
       <circle climb="0" radius="nav_radius" until="10 > PowerVoltage()" vmode="climb" wp="1"/>
     </block>
     <block name="Climb 1m/s">
-      <circle climb="1" pitch="5" radius="50+(GetPosAlt()-ground_alt)/2" until="10 > PowerVoltage()" vmode="climb" wp="1"/>
+      <circle climb="1" pitch="5" radius="50+(GetPosAlt()-GetAltRef())/2" until="10 > PowerVoltage()" vmode="climb" wp="1"/>
     </block>
     <block name="Climb nav_climb m/s">
-      <circle climb="nav_climb" radius="nav_radius" until="(10 > PowerVoltage()) || (GetPosAlt() > ground_alt+ 1350)" vmode="climb" wp="1"/>
+      <circle climb="nav_climb" radius="nav_radius" until="(10 > PowerVoltage()) || (GetPosAlt() > GetAltRef()+ 1350)" vmode="climb" wp="1"/>
     </block>
 
     <block name="Circle 0% throttle">
-      <circle pitch="fp_pitch" radius="nav_radius" throttle="0.0" until="ground_alt+50 > GetPosAlt()" vmode="throttle" wp="1"/>
+      <circle pitch="fp_pitch" radius="nav_radius" throttle="0.0" until="GetAltRef()+50 > GetPosAlt()" vmode="throttle" wp="1"/>
       <deroute block="Standby"/>
     </block>
 
     <block name="Oval 0% throttle">
-      <oval p1="1" p2="2" pitch="fp_pitch" radius="nav_radius" throttle="0.0" until="ground_alt+50 > GetPosAlt()" vmode="throttle"/>
+      <oval p1="1" p2="2" pitch="fp_pitch" radius="nav_radius" throttle="0.0" until="GetAltRef()+50 > GetPosAlt()" vmode="throttle"/>
       <deroute block="Standby"/>
     </block>
 
@@ -127,7 +127,7 @@
       <circle radius="nav_radius" until="NavQdrCloseTo(DegOfRad(baseleg_out_qdr)-(nav_radius/fabs(nav_radius))*10) && 10 > fabs(GetPosAlt() - WaypointAlt(WP_BASELEG))" wp="BASELEG"/>
     </block>
     <block name="final">
-      <exception cond="ground_alt + 10 > GetPosAlt()" deroute="flare"/>
+      <exception cond="GetAltRef() + 10 > GetPosAlt()" deroute="flare"/>
       <go from="AF" hmode="route" vmode="glide" wp="TD"/>
     </block>
     <block name="flare">
@@ -153,7 +153,7 @@
       </while>
     </block>
     <block name="Heading 30">
-      <heading alt="ground_alt+50" course="30" until="FALSE"/>
+      <heading alt="GetAltRef()+50" course="30" until="FALSE"/>
     </block>
     <block name="For loop (circles wp 1)">
       <for from="0" to="3" var="i">
@@ -168,7 +168,7 @@
     </block>
     <block name="Fly in Square">
       <exception cond="! InsideSquare(GetPosX(), GetPosY())" deroute="Come back wp 1"/>
-      <attitude alt="ground_alt+75" roll="0" vmode="alt"/>
+      <attitude alt="GetAltRef()+75" roll="0" vmode="alt"/>
     </block>
     <block name="Come back wp 1">
       <exception cond="InsideSquare(GetPosX(), GetPosY())" deroute="Fly in Square"/>
diff --git a/conf/flight_plans/versatile_airspeed.xml b/conf/flight_plans/versatile_airspeed.xml
index 3b9525bd00..ec1b0d3fdc 100644
--- a/conf/flight_plans/versatile_airspeed.xml
+++ b/conf/flight_plans/versatile_airspeed.xml
@@ -45,7 +45,7 @@
       <attitude roll="0" throttle="0" vmode="throttle"/>
     </block>
     <block name="Takeoff" strip_icon="takeoff.png" strip_button="Takeoff (wp CLIMB)">
-      <exception cond="GetPosAlt() > ground_alt+25" deroute="Standby"/>
+      <exception cond="GetPosAlt() > GetAltRef()+25" deroute="Standby"/>
       <set value="0" var="kill_throttle"/>
       <set value="CLIMB_AIRSPEED" var="v_ctl_auto_airspeed_setpoint" />
       <go wp="CLIMB"/>
@@ -69,25 +69,25 @@
       <circle pitch="auto" radius="75" throttle="0.7" wp="1"/>
     </block>
     <block name="Climb 75% throttle">
-      <circle pitch="10" radius="50+(GetPosAlt()-ground_alt)/2" throttle="0.75" until="(10 > PowerVoltage()) || (GetPosAlt() > ground_alt+ 1350)" vmode="throttle" wp="1"/>
+      <circle pitch="10" radius="50+(GetPosAlt()-GetAltRef())/2" throttle="0.75" until="(10 > PowerVoltage()) || (GetPosAlt() > GetAltRef()+ 1350)" vmode="throttle" wp="1"/>
     </block>
     <block name="Climb 0m/s">
       <circle climb="0" radius="nav_radius" until="10 > PowerVoltage()" vmode="climb" wp="1"/>
     </block>
     <block name="Climb 1m/s">
-      <circle climb="1" pitch="5" radius="50+(GetPosAlt()-ground_alt)/2" until="10 > PowerVoltage()" vmode="climb" wp="1"/>
+      <circle climb="1" pitch="5" radius="50+(GetPosAlt()-GetAltRef())/2" until="10 > PowerVoltage()" vmode="climb" wp="1"/>
     </block>
     <block name="Climb nav_climb m/s">
-      <circle climb="nav_climb" radius="nav_radius" until="(10 > PowerVoltage()) || (GetPosAlt() > ground_alt+ 1350)" vmode="climb" wp="1"/>
+      <circle climb="nav_climb" radius="nav_radius" until="(10 > PowerVoltage()) || (GetPosAlt() > GetAltRef()+ 1350)" vmode="climb" wp="1"/>
     </block>
 
     <block name="Circle 0% throttle">
-      <circle pitch="fp_pitch" radius="nav_radius" throttle="0.0" until="ground_alt+50 > GetPosAlt()" vmode="throttle" wp="1"/>
+      <circle pitch="fp_pitch" radius="nav_radius" throttle="0.0" until="GetAltRef()+50 > GetPosAlt()" vmode="throttle" wp="1"/>
       <deroute block="Standby"/>
     </block>
 
     <block name="Oval 0% throttle">
-      <oval p1="1" p2="2" pitch="fp_pitch" radius="nav_radius" throttle="0.0" until="ground_alt+50 > GetPosAlt()" vmode="throttle"/>
+      <oval p1="1" p2="2" pitch="fp_pitch" radius="nav_radius" throttle="0.0" until="GetAltRef()+50 > GetPosAlt()" vmode="throttle"/>
       <deroute block="Standby"/>
     </block>
 
@@ -163,7 +163,7 @@
       </while>
     </block>
     <block name="Heading 30">
-      <heading alt="ground_alt+50" course="30" until="FALSE"/>
+      <heading alt="GetAltRef()+50" course="30" until="FALSE"/>
     </block>
     <block name="For loop (circles wp 1)">
       <for from="0" to="3" var="i">
@@ -178,7 +178,7 @@
     </block>
     <block name="Fly in Square">
       <exception cond="! InsideSquare(GetPosX(), GetPosY())" deroute="Come back wp 1"/>
-      <attitude alt="ground_alt+75" roll="0" vmode="alt"/>
+      <attitude alt="GetAltRef()+75" roll="0" vmode="alt"/>
     </block>
     <block name="Come back wp 1">
       <exception cond="InsideSquare(GetPosX(), GetPosY())" deroute="Fly in Square"/>
diff --git a/conf/flight_plans/versatile_carto_fixe_muret.xml b/conf/flight_plans/versatile_carto_fixe_muret.xml
index dfd522ded7..fe9a0eb356 100644
--- a/conf/flight_plans/versatile_carto_fixe_muret.xml
+++ b/conf/flight_plans/versatile_carto_fixe_muret.xml
@@ -61,7 +61,7 @@ float varsweepsize=110;
     </block>
 
     <block name="Takeoff" strip_button="Takeoff (wp CLIMB)" strip_icon="takeoff.png">
-      <exception cond="GetPosAlt() > ground_alt+25" deroute="Standby"/>
+      <exception cond="GetPosAlt() > GetAltRef()+25" deroute="Standby"/>
       <set value="0" var="kill_throttle"/>
       <go wp="CLIMB"/>
     </block>
@@ -111,8 +111,8 @@ float varsweepsize=110;
     <block name="Monter">
       <call fun="NavSetWaypointHere(WP_MOB)"/>
       <set value="DEFAULT_CIRCLE_RADIUS" var="nav_radius"/>
-      <circle alt="ground_alt+105" radius="nav_radius" wp="MOB"/>
-      <exception cond="GetPosAlt() > ground_alt+100" deroute="Carto survey S1-S2-S3"/>
+      <circle alt="GetAltRef()+105" radius="nav_radius" wp="MOB"/>
+      <exception cond="GetPosAlt() > GetAltRef()+100" deroute="Carto survey S1-S2-S3"/>
     </block>
 
 
@@ -139,8 +139,8 @@ float varsweepsize=110;
     <block name="Descendre">
       <call fun="NavSetWaypointHere(WP_MOB)"/>
       <set value="DEFAULT_CIRCLE_RADIUS" var="nav_radius"/>
-      <circle alt="ground_alt+35" radius="nav_radius" wp="MOB"/>
-      <exception cond="ground_alt+40 > GetPosAlt()   " deroute="Carto survey SP1-SP2-SP3 fin"/>
+      <circle alt="GetAltRef()+35" radius="nav_radius" wp="MOB"/>
+      <exception cond="GetAltRef()+40 > GetPosAlt()   " deroute="Carto survey SP1-SP2-SP3 fin"/>
     </block>
 
     <block name="Carto survey SP1-SP2-SP3 fin" strip_button="Survey (wp S1-S2)" strip_icon="survey.png">
diff --git a/conf/flight_plans/xsens_cachejunction.xml b/conf/flight_plans/xsens_cachejunction.xml
index 3d2fd30938..fca2f69c32 100644
--- a/conf/flight_plans/xsens_cachejunction.xml
+++ b/conf/flight_plans/xsens_cachejunction.xml
@@ -23,7 +23,7 @@
   <exceptions/>
   <blocks>
     <block name="Takeoff" strip_button="Takeoff (wp CLIMB)" strip_icon="takeoff.png">
-      <exception cond="GetPosAlt() > ground_alt+25" deroute="Standby"/>
+      <exception cond="GetPosAlt() > GetAltRef()+25" deroute="Standby"/>
       <set value="0" var="kill_throttle"/>
       <set value="0" var="autopilot_flight_time"/>
       <go from="HOME" pitch="15" throttle="1.0" vmode="throttle" wp="CLIMB"/>
@@ -91,7 +91,7 @@
       <circle radius="nav_radius" until="NavQdrCloseTo(DegOfRad(baseleg_out_qdr)-(nav_radius/fabs(nav_radius))*10) && 10 > fabs(GetPosAlt() - WaypointAlt(WP__BASELEG))" wp="_BASELEG"/>
     </block>
     <block name="final">
-      <exception cond="ground_alt + 10 > GetPosAlt()" deroute="flare"/>
+      <exception cond="GetAltRef() + 10 > GetPosAlt()" deroute="flare"/>
       <go from="AF" hmode="route" vmode="glide" wp="TD"/>
     </block>
     <block name="flare">
diff --git a/conf/flight_plans/zamboni_survey_test.xml b/conf/flight_plans/zamboni_survey_test.xml
index 810c6a779d..9e43f0bb35 100644
--- a/conf/flight_plans/zamboni_survey_test.xml
+++ b/conf/flight_plans/zamboni_survey_test.xml
@@ -34,7 +34,7 @@
       <attitude roll="0" throttle="0" vmode="throttle"/>
     </block>
     <block key="t" name="Takeoff" strip_button="Takeoff (wp CLIMB)" strip_icon="takeoff.png" group="home">
-      <exception cond="GetPosAlt() > ground_alt+25" deroute="Standby"/>
+      <exception cond="GetPosAlt() > GetAltRef()+25" deroute="Standby"/>
       <set value="0" var="kill_throttle"/>
       <set value="0" var="autopilot_flight_time"/>
       <go from="HOME" throttle="1.0" vmode="throttle" wp="CLIMB" pitch="15"/>
@@ -66,7 +66,7 @@
       <circle radius="nav_radius" until="And(NavQdrCloseTo(DegOfRad(baseleg_out_qdr)-(nav_radius/fabs(nav_radius))*10), 10 > fabs(GetPosAlt() - WaypointAlt(WP__BASELEG)))" wp="_BASELEG"/>
     </block>
     <block name="final">
-      <exception cond="ground_alt + 10 > GetPosAlt()" deroute="flare"/>
+      <exception cond="GetAltRef() + 10 > GetPosAlt()" deroute="flare"/>
       <go from="AF" hmode="route" vmode="glide" wp="TD"/>
     </block>
 
diff --git a/conf/messages.xml b/conf/messages.xml
index ebdbe2acc1..d7c1d6f5c2 100644
--- a/conf/messages.xml
+++ b/conf/messages.xml
@@ -83,6 +83,7 @@
      <field name="utm_east"  type="int32" unit="m"/>
      <field name="utm_north" type="int32" unit="m"/>
      <field name="utm_zone" type="uint8"/>
+     <field name="ground_alt" type="float" unit="m"/>
    </message>
 
    <message name="NAVIGATION" id="10">
@@ -2718,6 +2719,7 @@
     <field name="lat"  type="float" unit="deg"></field>
     <field name="long"  type="float" unit="deg"></field>
     <field name="alt" type="float" unit="m"></field>
+    <field name="ground_alt" type="float" unit="m"/>
  </message>
 
   <message name="SURVEY_STATUS" id="31">
diff --git a/sw/airborne/firmwares/fixedwing/nav.c b/sw/airborne/firmwares/fixedwing/nav.c
index 047bbf991c..bbeded6c73 100644
--- a/sw/airborne/firmwares/fixedwing/nav.c
+++ b/sw/airborne/firmwares/fixedwing/nav.c
@@ -452,7 +452,7 @@ void nav_periodic_task(void) {
 
 static void send_nav_ref(void) {
   DOWNLINK_SEND_NAVIGATION_REF(DefaultChannel, DefaultDevice,
-      &nav_utm_east0, &nav_utm_north0, &nav_utm_zone0);
+      &nav_utm_east0, &nav_utm_north0, &nav_utm_zone0, &ground_alt);
 }
 
 static void send_nav(void) {
diff --git a/sw/airborne/firmwares/fixedwing/nav.h b/sw/airborne/firmwares/fixedwing/nav.h
index 8a2af7b995..d7263d7431 100644
--- a/sw/airborne/firmwares/fixedwing/nav.h
+++ b/sw/airborne/firmwares/fixedwing/nav.h
@@ -199,6 +199,7 @@ bool_t nav_approaching_xy(float x, float y, float from_x, float from_y, float ap
 #define GetPosX() (stateGetPositionEnu_f()->x)
 #define GetPosY() (stateGetPositionEnu_f()->y)
 #define GetPosAlt() (stateGetPositionUtm_f()->alt)
+#define GetAltRef() (ground_alt)
 
 #define SEND_NAVIGATION(_trans, _dev) { \
     uint8_t _circle_count = NavCircleCount(); \
diff --git a/sw/airborne/firmwares/rotorcraft/navigation.c b/sw/airborne/firmwares/rotorcraft/navigation.c
index ae345c8005..1692dbdb94 100644
--- a/sw/airborne/firmwares/rotorcraft/navigation.c
+++ b/sw/airborne/firmwares/rotorcraft/navigation.c
@@ -58,8 +58,6 @@ struct EnuCoor_i nav_last_point;
 
 uint8_t last_wp __attribute__ ((unused));
 
-int32_t ground_alt;
-
 /** Maximum distance from HOME waypoint before going into failsafe mode */
 #ifndef FAILSAFE_MODE_DISTANCE
 #define FAILSAFE_MODE_DISTANCE (1.5*MAX_DIST_FROM_HOME)
@@ -149,7 +147,6 @@ void nav_init(void) {
   }
   nav_block = 0;
   nav_stage = 0;
-  ground_alt = POS_BFP_OF_REAL(GROUND_ALT);
   nav_altitude = POS_BFP_OF_REAL(SECURITY_HEIGHT);
   nav_flight_altitude = nav_altitude;
   flight_altitude = SECURITY_ALT;
@@ -380,13 +377,11 @@ static inline void nav_set_altitude( void ) {
 /** Reset the geographic reference to the current GPS fix */
 unit_t nav_reset_reference( void ) {
   ins_reset_local_origin();
-  ground_alt = POS_BFP_OF_REAL(state.ned_origin_f.hmsl);
   return 0;
 }
 
 unit_t nav_reset_alt( void ) {
   ins_reset_altitude_ref();
-  ground_alt = POS_BFP_OF_REAL(state.ned_origin_f.hmsl);
   return 0;
 }
 
diff --git a/sw/airborne/firmwares/rotorcraft/navigation.h b/sw/airborne/firmwares/rotorcraft/navigation.h
index 3658ad0864..1bbafb23c5 100644
--- a/sw/airborne/firmwares/rotorcraft/navigation.h
+++ b/sw/airborne/firmwares/rotorcraft/navigation.h
@@ -47,9 +47,6 @@ extern void nav_run(void);
 
 extern uint8_t last_wp __attribute__ ((unused));
 
-/** ground reference altitude in meters << #INT32_POS_FRAC */
-extern int32_t ground_alt;
-
 extern uint8_t horizontal_mode;
 extern uint8_t nav_segment_start, nav_segment_end;
 extern uint8_t nav_circle_centre;
@@ -209,13 +206,14 @@ bool_t nav_check_wp_time(uint8_t wp_idx, uint16_t stay_time);
 
 #define navigation_SetFlightAltitude(x) { \
   flight_altitude = x; \
-  nav_flight_altitude = POS_BFP_OF_REAL(flight_altitude) - ground_alt; \
+  nav_flight_altitude = POS_BFP_OF_REAL(flight_altitude - state.ned_origin_f.hmsl); \
 }
 
 
 #define GetPosX() (stateGetPositionEnu_f()->x)
 #define GetPosY() (stateGetPositionEnu_f()->y)
-#define GetPosAlt() (stateGetPositionEnu_f()->z+ground_alt)
+#define GetPosAlt() (stateGetPositionEnu_f()->z+state.ned_origin_f.hmsl)
+#define GetAltRef() (state.ned_origin_f.hmsl)
 
 
 extern void navigation_update_wp_from_speed(uint8_t wp, struct Int16Vect3 speed_sp, int16_t heading_rate_sp );
diff --git a/sw/ground_segment/cockpit/gcs.glade b/sw/ground_segment/cockpit/gcs.glade
index 7f00cf86ed..1bc9bebfe5 100644
--- a/sw/ground_segment/cockpit/gcs.glade
+++ b/sw/ground_segment/cockpit/gcs.glade
@@ -171,7 +171,7 @@
                                         <child>
                                           <widget class="GtkEventBox" id="eventbox14">
                                             <property name="visible">True</property>
-                                            <property name="tooltip" translatable="yes">Above Ground Level (m), climb indicator, vertical speed (m/s)</property>
+                                            <property name="tooltip" translatable="yes">Above Ground Level from SRTM if available, ground reference otherwise (m), climb indicator, vertical speed (m/s)</property>
                                             <child>
                                               <widget class="GtkDrawingArea" id="drawingarea_agl">
                                                 <property name="visible">True</property>
diff --git a/sw/ground_segment/cockpit/live.ml b/sw/ground_segment/cockpit/live.ml
index 001c53950a..dacb0ad9d4 100644
--- a/sw/ground_segment/cockpit/live.ml
+++ b/sw/ground_segment/cockpit/live.ml
@@ -1288,10 +1288,12 @@ let listen_waypoint_moved = fun () ->
     let wp_id = Pprz.int_assoc "wp_id" vs in
     let a = fun s -> Pprz.float_assoc s vs in
     let geo = { posn_lat = (Deg>>Rad)(a "lat"); posn_long = (Deg>>Rad)(a "long") }
-    and altitude = a "alt" in
+    and altitude = a "alt"
+    and ground_alt = a "ground_alt" in
 
     try
       let w = ac.fp_group#get_wp wp_id in
+      w#set_ground_alt ground_alt;
       w#set ~altitude ~update:true geo
     with
         Not_found -> () (* Silently ignore unknown waypoints *)
diff --git a/sw/ground_segment/tmtc/aircraft.ml b/sw/ground_segment/tmtc/aircraft.ml
index ec5d564782..334b2961c3 100644
--- a/sw/ground_segment/tmtc/aircraft.ml
+++ b/sw/ground_segment/tmtc/aircraft.ml
@@ -121,6 +121,7 @@ type aircraft = {
   mutable climb   : float;
   mutable nav_ref : nav_ref option;
   mutable d_hmsl : float;
+  mutable ground_alt : float; (* ground alt ref if no SRTM data *)
   mutable desired_pos    : Latlong.geographic;
   mutable desired_altitude    : float;
   mutable desired_course : float;
@@ -169,7 +170,7 @@ let new_aircraft = fun id name fp airframe ->
     unix_time = 0.; itow = Int32.of_int 0;
     roll = 0.; pitch = 0.;
     gspeed=0.; airspeed= -1.; course = 0.; heading = 0.; alt=0.; climb=0.; agl = 0.;
-    nav_ref = None; d_hmsl = 0.;
+    nav_ref = None; d_hmsl = 0.; ground_alt = 0.;
     desired_pos = { Latlong.posn_lat = 0.; posn_long = 0. };
     desired_course = 0.; desired_altitude = 0.; desired_climb = 0.;
     cur_block=0; cur_stage=0;
diff --git a/sw/ground_segment/tmtc/aircraft.mli b/sw/ground_segment/tmtc/aircraft.mli
index 85a7a2ad35..36528997df 100644
--- a/sw/ground_segment/tmtc/aircraft.mli
+++ b/sw/ground_segment/tmtc/aircraft.mli
@@ -87,6 +87,7 @@ type aircraft = {
     mutable climb : float;
     mutable nav_ref : nav_ref option;
     mutable d_hmsl : float; (* difference between geoid and ellipsoid *)
+    mutable ground_alt : float; (* ground alt ref if no SRTM data *)
     mutable desired_pos : Latlong.geographic;
     mutable desired_altitude : float;
     mutable desired_course : float;
diff --git a/sw/ground_segment/tmtc/fw_server.ml b/sw/ground_segment/tmtc/fw_server.ml
index 23bbffb302..610094afc3 100644
--- a/sw/ground_segment/tmtc/fw_server.ml
+++ b/sw/ground_segment/tmtc/fw_server.ml
@@ -113,7 +113,7 @@ let log_and_parse = fun ac_name (a:Aircraft.aircraft) msg values ->
           a.course  <- norm_course ((Deg>>Rad)(fvalue "course" /. 10.));
           if !heading_from_course then
             a.heading <- a.course;
-          a.agl     <- a.alt -. float (try Srtm.of_wgs84 a.pos with _ -> 0);
+          a.agl     <- a.alt -. (try float (Srtm.of_wgs84 a.pos) with _ -> a.ground_alt);
           if a.gspeed > 3. && a.ap_mode = _AUTO2 then
             Wind.update ac_name a.gspeed a.course
         end
@@ -128,7 +128,7 @@ let log_and_parse = fun ac_name (a:Aircraft.aircraft) msg values ->
       a.course  <- norm_course ((Deg>>Rad)(fvalue "course" /. 10.));
       if !heading_from_course then
         a.heading <- a.course;
-      a.agl     <- a.alt -. float (try Srtm.of_wgs84 a.pos with _ -> 0);
+      a.agl     <- a.alt -. (try float (Srtm.of_wgs84 a.pos) with _ -> a.ground_alt);
       a.gps_mode <- check_index (ivalue "mode") gps_modes "GPS_MODE";
       if a.gspeed > 3. && a.ap_mode = _AUTO2 then
         Wind.update ac_name a.gspeed a.course
@@ -152,12 +152,14 @@ let log_and_parse = fun ac_name (a:Aircraft.aircraft) msg values ->
       a.desired_climb <- (try fvalue "climb" with _ -> fvalue "desired_climb");
       begin try a.desired_course <- norm_course (fvalue "course") with _ -> () end
     | "NAVIGATION_REF" ->
-      a.nav_ref <- Some (Utm { utm_x = fvalue "utm_east"; utm_y = fvalue "utm_north"; utm_zone = ivalue "utm_zone" })
+        a.nav_ref <- Some (Utm { utm_x = fvalue "utm_east"; utm_y = fvalue "utm_north"; utm_zone = ivalue "utm_zone" });
+        a.ground_alt <- fvalue "ground_alt"
     | "NAVIGATION_REF_LLA" ->
       let lat = ivalue "lat"
       and lon = ivalue "lon" in
       let geo = make_geo (float lat /. 1e7) (float lon /. 1e7) in
-      a.nav_ref <- Some (Geo geo)
+      a.nav_ref <- Some (Geo geo);
+      a.ground_alt <- fvalue "ground_alt"
     | "ATTITUDE" ->
       let roll = fvalue "phi"
       and pitch = fvalue "theta" in
@@ -316,7 +318,7 @@ let log_and_parse = fun ac_name (a:Aircraft.aircraft) msg values ->
         a.course  <- norm_course (fvalue "course" /. 1e3);
         if !heading_from_course then
           a.heading <- a.course;
-        a.agl <- a.alt -. float (try Srtm.of_wgs84 a.pos with _ -> 0);
+        a.agl <- a.alt -. (try float (Srtm.of_wgs84 a.pos) with _ -> a.ground_alt);
         a.bat <- fvalue "vsupply" /. 10.;
         a.energy <- ivalue "energy" * 100;
         a.throttle <- fvalue "throttle";
diff --git a/sw/ground_segment/tmtc/rotorcraft_server.ml b/sw/ground_segment/tmtc/rotorcraft_server.ml
index dded7be570..07c1a904dd 100644
--- a/sw/ground_segment/tmtc/rotorcraft_server.ml
+++ b/sw/ground_segment/tmtc/rotorcraft_server.ml
@@ -30,8 +30,6 @@ module LL = Latlong
 module U = Unix
 module Dl_Pprz = Pprz.Messages (struct let name = "datalink" end)
 
-let nav_ref_alt = ref 0.
-let nav_ref_hmsl = ref 0.
 
 (* FIXME: bound the loop *)
 let rec norm_course =
@@ -168,7 +166,7 @@ let log_and_parse = fun ac_name (a:Aircraft.aircraft) msg values ->
         and vnorth = foi32value "vnorth" /. speed_frac in
         a.gspeed  <- sqrt(vnorth*.vnorth +. veast*.veast);
         a.climb   <- foi32value "vup" /. speed_frac;
-        a.agl     <- a.alt -. float (try Srtm.of_wgs84 a.pos with _ -> 0);
+        a.agl     <- a.alt -. (try float (Srtm.of_wgs84 a.pos) with _ -> a.ground_alt);
         a.course  <- norm_course ((Rad>>Deg) (foi32value "psi" /. angle_frac));
         a.heading <- norm_course (foi32value "psi" /. angle_frac);
         a.roll    <- foi32value "phi" /. angle_frac;
@@ -200,6 +198,7 @@ let log_and_parse = fun ac_name (a:Aircraft.aircraft) msg values ->
       let nav_ref_ecef = LL.make_ecef [| x; y; z |] in
       a.nav_ref <- Some (Ltp nav_ref_ecef);
       a.d_hmsl <- hmsl -. alt;
+      a.ground_alt <- hmsl;
     | "ROTORCRAFT_NAV_STATUS" ->
       a.block_time <- ivalue "block_time";
       a.stage_time <- ivalue "stage_time";
diff --git a/sw/ground_segment/tmtc/server.ml b/sw/ground_segment/tmtc/server.ml
index fe7afc012b..af1b52da70 100644
--- a/sw/ground_segment/tmtc/server.ml
+++ b/sw/ground_segment/tmtc/server.ml
@@ -294,7 +294,8 @@ let send_moved_waypoints = fun a ->
          "wp_id", Pprz.Int wp_id;
          "long", Pprz.Float ((Rad>>Deg)geo.posn_long);
          "lat", Pprz.Float ((Rad>>Deg)geo.posn_lat);
-         "alt", Pprz.Float wp.altitude] in
+         "alt", Pprz.Float wp.altitude;
+         "ground_alt", Pprz.Float (try float (Srtm.of_wgs84 geo) with _ -> a.ground_alt)] in
       Ground_Pprz.message_send my_id "WAYPOINT_MOVED" vs)
     a.waypoints
 
diff --git a/sw/lib/ocaml/mapCanvas.ml b/sw/lib/ocaml/mapCanvas.ml
index 5e380f07e2..9077564795 100644
--- a/sw/lib/ocaml/mapCanvas.ml
+++ b/sw/lib/ocaml/mapCanvas.ml
@@ -645,7 +645,7 @@ object (self)
 
 
 class widget =  fun ?(height=800) ?(srtm=false) ?width ?projection ?georef () ->
-  let srtm = GMenu.check_menu_item ~label:"SRTM" ~active:srtm () in
+  let srtm = GMenu.check_menu_item ~label:"display SRTM alt" ~active:srtm () in
   let lbl_xy = GMisc.label ()
   and lbl_geo = GMisc.label ()
   and lbl_alt =  GMisc.label ()
@@ -676,6 +676,7 @@ class widget =  fun ?(height=800) ?(srtm=false) ?width ?projection ?georef () ->
       let bg_menu = my_check_menu_item "Background" ~active:true ~callback:self#switch_background ~packing:self#file_menu#append () in
 
       let tooltips = GData.tooltips () in
+      tooltips#set_tip srtm#coerce ~text:"Display SRTM alt at pointer position (will request for download if not available)";
 
       let b = GButton.button ~packing:toolbar#add () in
       ignore (b#connect#clicked (fun _ -> bg_menu#activate ()));
@@ -777,7 +778,9 @@ class widget =  fun ?(height=800) ?(srtm=false) ?width ?projection ?georef () ->
 
     method display_alt = fun wgs84 ->
       if srtm#active then
-    lbl_alt#set_text (sprintf " SRTM:%dm"(self#altitude wgs84))
+        lbl_alt#set_text (sprintf "  SRTM:%dm"(self#altitude wgs84))
+      else if not (Srtm.available wgs84) then
+        lbl_alt#set_text (sprintf "  SRTM: N/A")
 
     method display_group = fun s ->  lbl_group#set_text s
 
diff --git a/sw/lib/ocaml/mapWaypoints.ml b/sw/lib/ocaml/mapWaypoints.ml
index bd2f8dd45e..38c5f820e0 100644
--- a/sw/lib/ocaml/mapWaypoints.ml
+++ b/sw/lib/ocaml/mapWaypoints.ml
@@ -74,6 +74,7 @@ object (self)
   val label = new CL.widget ~name:name ~color:"white" s 0. wpt_group
   val mutable name = name (* FIXME: already in label ! *)
   val mutable alt = alt
+  val mutable ground_alt = 0.
   val mutable moved = None
   val mutable deleted = false
   val mutable commit_cb = None
@@ -145,7 +146,7 @@ object (self)
     ea#set_adjustment adj;
     ea#set_value alt; (* this should be done by set_adjustment but seems to fail on ubuntu 13.10 (at least) *)
 
-    let agl = alt -. float (try Srtm.of_wgs84 initial_wgs84 with _ -> 0) in
+    let agl = alt -. (try float (Srtm.of_wgs84 initial_wgs84) with _ -> ground_alt) in
     let agl_lab  = GMisc.label ~text:(sprintf " AGL: %4.0fm" agl) ~packing:ha#add () in
     let plus10= GButton.button ~label:"+10" ~packing:ha#add () in
     let change_alt = fun x ->
@@ -202,7 +203,7 @@ object (self)
       try
         set_coordinates ();
         let wgs84 = self#pos in
-        let agl  = ea#value -. float (try Srtm.of_wgs84 wgs84 with _ -> 0) in
+        let agl  = ea#value -. (try float (Srtm.of_wgs84 wgs84) with _ -> ground_alt) in
         agl_lab#set_text (sprintf " AGL: %4.0fm" agl)
       with _ -> ()
     in
@@ -282,6 +283,7 @@ object (self)
           if update then updated ()
       | (None, false) | (Some _, true) -> ()
       | Some _, false -> self#reset_moved ()
+  method set_ground_alt ga = ground_alt <- ga
   method delete () =
     deleted <- true; (* BOF *)
     wpt_group#destroy ()
diff --git a/sw/lib/ocaml/mapWaypoints.mli b/sw/lib/ocaml/mapWaypoints.mli
index 25d9ec5fa5..09c18514f8 100644
--- a/sw/lib/ocaml/mapWaypoints.mli
+++ b/sw/lib/ocaml/mapWaypoints.mli
@@ -52,6 +52,7 @@ class waypoint :
     method move : float -> float -> unit
     method name : string
     method set : ?altitude:float -> ?update:bool -> Latlong.geographic -> unit
+    method set_ground_alt : float -> unit
     method set_name : string -> unit
     method xy : float * float
     method zoom : float -> unit
diff --git a/sw/lib/ocaml/srtm.ml b/sw/lib/ocaml/srtm.ml
index 672744f070..22d3b7a1ae 100644
--- a/sw/lib/ocaml/srtm.ml
+++ b/sw/lib/ocaml/srtm.ml
@@ -79,6 +79,9 @@ let of_wgs84 = fun geo ->
 let of_utm = fun utm ->
   of_wgs84 (Latlong.of_utm WGS84 utm)
 
+let available = fun geo ->
+  try ignore(of_wgs84 geo); true with _ -> false
+
 let area_of_tile = fun tile ->
   let area = open_compressed "srtm.data.bz2" in
   let rec _area_of_tile = fun () ->
diff --git a/sw/lib/ocaml/srtm.mli b/sw/lib/ocaml/srtm.mli
index c3742b41bf..8c0a9becc7 100644
--- a/sw/lib/ocaml/srtm.mli
+++ b/sw/lib/ocaml/srtm.mli
@@ -33,11 +33,14 @@ exception Tile_not_found of error
 
 val error : error -> string
 
+val available : Latlong.geographic -> bool
+(** [available wgs84_pos] Returns true if srtm tile is available *)
+
 val of_utm : Latlong.utm -> int
 (** [of_utm utm_pos] Returns the altitude of the given UTM position *)
 
 val of_wgs84 : Latlong.geographic -> int
-(** [of_utm utm_pos] Returns the altitude of the given geographic position *)
+(** [of_wgs84 wgs84_pos] Returns the altitude of the given geographic position *)
 
 val area_of_tile : string -> string
 
diff --git a/sw/simulator/nps/nps_fdm_jsbsim.c b/sw/simulator/nps/nps_fdm_jsbsim.c
index 19702fea82..b0d711d5e7 100644
--- a/sw/simulator/nps/nps_fdm_jsbsim.c
+++ b/sw/simulator/nps/nps_fdm_jsbsim.c
@@ -385,6 +385,9 @@ static void init_jsbsim(double dt) {
   //initRunning for all engines
   FDMExec->GetPropulsion()->InitRunning(-1);
 
+  // LLA initial coordinates (geodetic lat, geoid alt)
+  struct LlaCoor_d lla0;
+
   JSBSim::FGInitialCondition *IC = FDMExec->GetIC();
   if(!jsbsim_ic_name.empty()) {
     if ( ! IC->Load(jsbsim_ic_name)) {
@@ -394,6 +397,12 @@ static void init_jsbsim(double dt) {
       delete FDMExec;
       exit(-1);
     }
+
+    lla0 = {
+      FDMExec->GetPropagate()->GetLongitude(),
+      FDMExec->GetPropagate()->GetGeodLatitudeRad(),
+      FDMExec->GetPropagate()->GetAltitudeASLmeters()
+    };
   }
   else {
     // FGInitialCondition::SetAltitudeASLFtIC
@@ -421,17 +430,18 @@ static void init_jsbsim(double dt) {
       exit(-1);
     }
 
-    // compute offset between geocentric and geodetic ecef
-    struct LlaCoor_d lla0 = { RadOfDeg(NAV_LON0 / 1e7), gd_lat, (double)(NAV_ALT0+NAV_MSL0)/1000. };
-    ecef_of_lla_d(&offset, &lla0);
-    struct EcefCoor_d ecef0 = {
-      MetersOfFeet(FDMExec->GetPropagate()->GetLocation().Entry(1)),
-      MetersOfFeet(FDMExec->GetPropagate()->GetLocation().Entry(2)),
-      MetersOfFeet(FDMExec->GetPropagate()->GetLocation().Entry(3))
-    };
-    VECT3_DIFF(offset, offset, ecef0);
+    lla0 = { RadOfDeg(NAV_LON0 / 1e7), gd_lat, (double)(NAV_ALT0+NAV_MSL0)/1000. };
   }
 
+  // compute offset between geocentric and geodetic ecef
+  ecef_of_lla_d(&offset, &lla0);
+  struct EcefCoor_d ecef0 = {
+    MetersOfFeet(FDMExec->GetPropagate()->GetLocation().Entry(1)),
+    MetersOfFeet(FDMExec->GetPropagate()->GetLocation().Entry(2)),
+    MetersOfFeet(FDMExec->GetPropagate()->GetLocation().Entry(3))
+  };
+  VECT3_DIFF(offset, offset, ecef0);
+
   // calculate vehicle max radius in m
   vehicle_radius_max = 0.01; // specify not 0.0 in case no gear
   int num_gear = FDMExec->GetGroundReactions()->GetNumGearUnits();
diff --git a/sw/simulator/sim.ml b/sw/simulator/sim.ml
index 059afe960e..44228c7e49 100644
--- a/sw/simulator/sim.ml
+++ b/sw/simulator/sim.ml
@@ -195,7 +195,7 @@ module Make(AircraftItl : AIRCRAFT_ITL) = struct
               Some s ->
                 begin
                   try s.Gps.alt -. float (Srtm.of_wgs84 s.Gps.wgs84) with
-                      _ -> s.Gps.alt
+                      _ -> s.Gps.alt -. !alt0
                 end
             | None -> 0. in
       FM.state_update !state FM.nominal_airspeed (!wind_x, !wind_y, !wind_z) agl fm_period