diff --git a/src/blackmisc/aviation/aircraftsituation.cpp b/src/blackmisc/aviation/aircraftsituation.cpp
index 9d94fa303..1b650f238 100644
--- a/src/blackmisc/aviation/aircraftsituation.cpp
+++ b/src/blackmisc/aviation/aircraftsituation.cpp
@@ -241,17 +241,15 @@ namespace BlackMisc
 
         CLength CAircraftSituation::getHeightAboveGround() const
         {
+            if (this->getAltitude().isNull()) { return { 0, nullptr }; }
             if (this->getAltitude().getReferenceDatum() == CAltitude::AboveGround)
             {
                 // we have a sure value explicitly set
                 return this->getAltitude();
             }
             const CLength gh(this->getGroundElevation());
-            if (!gh.isNull() && !getAltitude().isNull())
-            {
-                return this->getAltitude() - gh;
-            }
-            return { 0, nullptr };
+            if (gh.isNull()) { return { 0, nullptr }; }
+            return this->getAltitude() - gh;
         }
 
         CAltitude CAircraftSituation::getCorrectedAltitude() const
diff --git a/src/blackmisc/aviation/aircraftsituation.h b/src/blackmisc/aviation/aircraftsituation.h
index c5ceffa78..0f1f09690 100644
--- a/src/blackmisc/aviation/aircraftsituation.h
+++ b/src/blackmisc/aviation/aircraftsituation.h
@@ -73,9 +73,10 @@ namespace BlackMisc
             //! Reliability of on ground information
             enum OnGroundReliability
             {
-                OnGroundByGuessing,
+                OnGroundByInterpolation,  //!< strongest
+                OnGroundByElevationAndCG,
                 OnGroundByElevation,
-                OnGroundByInterpolation,
+                OnGroundByGuessing,       //!< weakest
                 OnGroundReliabilityNoSet
             };
 
diff --git a/src/blackmisc/simulation/interpolator.cpp b/src/blackmisc/simulation/interpolator.cpp
index 36f9a696b..6926c8d4d 100644
--- a/src/blackmisc/simulation/interpolator.cpp
+++ b/src/blackmisc/simulation/interpolator.cpp
@@ -303,15 +303,23 @@ namespace BlackMisc
                 return;
             }
 
-            // no value by factor, guess on elevation
-            const CLength heightAboveGround(situation.getHeightAboveGround());
-            const CLength cgAboveGround(hints.getCGAboveGround());
-            if (!heightAboveGround.isNull())
+            // on elevation and CG
+            if (!situation.getGroundElevation().isNull())
             {
-                const bool og = cgAboveGround.isNull() ?
-                                heightAboveGround.value(CLengthUnit::m()) < 1.0 :
-                                heightAboveGround <= cgAboveGround;
-                situation.setOnGround(og ? CAircraftSituation::OnGround : CAircraftSituation::NotOnGround, CAircraftSituation::OnGroundByElevation);
+                CLength offset(hints.isVtolAircraft() ? 0.0 : 1.0, CLengthUnit::m()); // offset from ground
+                CAircraftSituation::OnGroundReliability reliability = CAircraftSituation::OnGroundByElevation;
+                if (!hints.isVtolAircraft() && !hints.getCGAboveGround().isNull())
+                {
+                    offset = hints.getCGAboveGround();
+                    reliability = CAircraftSituation::OnGroundByElevationAndCG;
+                }
+
+                Q_ASSERT_X(situation.getGroundElevation().getReferenceDatum() == CAltitude::MeanSeaLevel, Q_FUNC_INFO, "Need MSL elevation");
+                const CAircraftSituation::IsOnGround og =
+                    situation.getHeightAboveGround() <= offset ?
+                    CAircraftSituation::OnGround : CAircraftSituation::NotOnGround;
+                situation.setOnGround(og, reliability);
+                return;
             }
 
             // for VTOL aircraft we give up