diff --git a/src/blackmisc/aviation/aircraftsituation.cpp b/src/blackmisc/aviation/aircraftsituation.cpp
index 5da214392..19b4699cf 100644
--- a/src/blackmisc/aviation/aircraftsituation.cpp
+++ b/src/blackmisc/aviation/aircraftsituation.cpp
@@ -202,13 +202,12 @@ namespace BlackMisc
             return { 0, nullptr };
         }
 
-        CAltitude CAircraftSituation::getCorrectedAltitude(const CLength &cgAboveGround) const
+        CAltitude CAircraftSituation::getCorrectedAltitude() const
         {
-            if (!this->hasGroundElevation()) { return this->getAltitude(); }
-            const CAltitude groundElevation(cgAboveGround.isNull() ?
-                                            this->getGroundElevation() :
-                                            CAltitude(this->getGroundElevation() + cgAboveGround, CAltitude::MeanSeaLevel));
-            return (groundElevation <= this->getAltitude()) ? this->getAltitude() : groundElevation;
+            if (this->getGroundElevation().isNull()) { return this->getAltitude(); }
+            if (this->getAltitude().getReferenceDatum() != CAltitude::MeanSeaLevel) { return this->getAltitude(); }
+            if (this->getGroundElevation() < this->getAltitude()) { return this->getAltitude(); }
+            return this->getGroundElevation();
         }
 
         void CAircraftSituation::setCallsign(const CCallsign &callsign)
diff --git a/src/blackmisc/aviation/aircraftsituation.h b/src/blackmisc/aviation/aircraftsituation.h
index 52dc2b384..89088cb24 100644
--- a/src/blackmisc/aviation/aircraftsituation.h
+++ b/src/blackmisc/aviation/aircraftsituation.h
@@ -132,8 +132,8 @@ namespace BlackMisc
             //! Get altitude
             const BlackMisc::Aviation::CAltitude &getAltitude() const { return this->m_position.geodeticHeight(); }
 
-            //! Get altitude under consideration of ground elevation and CG (if available)
-            BlackMisc::Aviation::CAltitude getCorrectedAltitude(const PhysicalQuantities::CLength &cgAboveGround = { 0, nullptr }) const;
+            //! Get altitude under consideration of ground elevation
+            BlackMisc::Aviation::CAltitude getCorrectedAltitude() const;
 
             //! Set altitude
             void setAltitude(const BlackMisc::Aviation::CAltitude &altitude) { this->m_position.setGeodeticHeight(altitude); }
diff --git a/src/blackmisc/simulation/interpolator.cpp b/src/blackmisc/simulation/interpolator.cpp
index 57f84bc23..36d7eff05 100644
--- a/src/blackmisc/simulation/interpolator.cpp
+++ b/src/blackmisc/simulation/interpolator.cpp
@@ -8,11 +8,13 @@
  */
 
 #include "interpolator.h"
-#include "blackmisc/aviation/callsign.h"
 #include "blackmisc/simulation/interpolationhints.h"
+#include "blackmisc/aviation/callsign.h"
+#include "blackmisc/pq/length.h"
 #include <QDateTime>
 
 using namespace BlackMisc::Aviation;
+using namespace BlackMisc::PhysicalQuantities;
 
 namespace BlackMisc
 {
@@ -68,7 +70,7 @@ namespace BlackMisc
             const auto soonestLanding = std::find_if(partsNewer.begin(), partsNewer.end(), [](auto && p) { return p.isOnGround(); });
 
             // our clairvoyance is limited by the time offset
-            const double significantPast = 5.0;
+            const double significantPast = 5.0; // \fixme 20170121 KB would it make sense to centrally define the update time (5secs), in case it changes. I see a lot of 5.0 hardcoded here
             const double predictableFuture = soonestLanding == partsNewer.end() ? 5.0 : std::min(5.0, static_cast<double>(soonestLanding->getTimeOffsetMs()) / 1000.0);
 
             const double secondsSinceTakeoff = latestTakeoff == partsOlder.end() ? 5.0 : (currentTimeMsSinceEpoch - latestTakeoff->getAdjustedMSecsSinceEpoch()) / 1000.0;
@@ -107,10 +109,10 @@ namespace BlackMisc
 
         void IInterpolator::setGroundElevationFromHint(const CInterpolationHints &hints, CAircraftSituation &situation)
         {
-            if (hints.getElevation().isNull()) return;
-            if (situation.hasGroundElevation()) return;
-            if (!hints.isWithinRange(situation)) return;
-            situation.setGroundElevation(hints.getElevation().geodeticHeight());
+            if (situation.hasGroundElevation()) { return; }
+            const CAltitude elevation = hints.getGroundElevation(situation);
+            if (elevation.isNull()) { return; }
+            situation.setGroundElevation(elevation);
         }
 
         bool IInterpolator::InterpolationStatus::allTrue() const
diff --git a/src/blackmisc/simulation/interpolatorlinear.cpp b/src/blackmisc/simulation/interpolatorlinear.cpp
index 2fc9c3d62..5407556d1 100644
--- a/src/blackmisc/simulation/interpolatorlinear.cpp
+++ b/src/blackmisc/simulation/interpolatorlinear.cpp
@@ -100,8 +100,8 @@ namespace BlackMisc
             // take hint into account to calculate elevation and above ground level
             if (!hints.getElevation().isNull())
             {
-                setGroundElevationFromHint(hints, oldSituation);
-                setGroundElevationFromHint(hints, newSituation);
+                IInterpolator::setGroundElevationFromHint(hints, oldSituation);
+                IInterpolator::setGroundElevationFromHint(hints, newSituation);
             }
 
             CAircraftSituation currentSituation(oldSituation);
@@ -135,8 +135,8 @@ namespace BlackMisc
             currentSituation.setPosition(currentPosition);
 
             // Interpolate altitude: Alt = (AltB - AltA) * t + AltA
-            const CAltitude oldAlt(oldSituation.getCorrectedAltitude(hints.getCGAboveGround()));
-            const CAltitude newAlt(newSituation.getCorrectedAltitude(hints.getCGAboveGround()));
+            const CAltitude oldAlt(oldSituation.getCorrectedAltitude()); // avoid underflow below ground elevation
+            const CAltitude newAlt(newSituation.getCorrectedAltitude());
             Q_ASSERT_X(oldAlt.getReferenceDatum() == newAlt.getReferenceDatum(), Q_FUNC_INFO, "mismatch in reference"); // otherwise no calculation is possible
             currentSituation.setAltitude(CAltitude((newAlt - oldAlt)
                                                    * simulationTimeFraction
@@ -149,11 +149,12 @@ namespace BlackMisc
                 const double groundFactor = hints.getAircraftParts().isOnGroundInterpolated();
                 if (groundFactor > 0.0)
                 {
-                    const auto &groundElevation = hints.getElevationProvider();
-                    if (groundElevation)
+                    const CAltitude groundElevation = hints.getGroundElevation(currentSituation);
+                    Q_ASSERT_X(groundElevation.getReferenceDatum() == CAltitude::MeanSeaLevel, Q_FUNC_INFO, "Need MSL value");
+                    if (!groundElevation.isNull())
                     {
                         currentSituation.setAltitude(CAltitude(currentSituation.getAltitude() * (1.0 - groundFactor)
-                                                               + groundElevation(currentSituation) * groundFactor,
+                                                               + groundElevation * groundFactor,
                                                                oldAlt.getReferenceDatum()));
                     }
                 }
diff --git a/src/plugins/simulator/fsx/simulatorfsx.cpp b/src/plugins/simulator/fsx/simulatorfsx.cpp
index 764451021..b5db7e3ea 100644
--- a/src/plugins/simulator/fsx/simulatorfsx.cpp
+++ b/src/plugins/simulator/fsx/simulatorfsx.cpp
@@ -1000,7 +1000,7 @@ namespace BlackSimPlugin
             SIMCONNECT_DATA_INITPOSITION position;
             position.Latitude = situation.latitude().value(CAngleUnit::deg());
             position.Longitude = situation.longitude().value(CAngleUnit::deg());
-            position.Altitude = situation.getCorrectedAltitude(hints.getCGAboveGround()).value(CLengthUnit::ft());
+            position.Altitude = situation.getAltitude().value(CLengthUnit::ft()); // already corrected in interpolator if there is an underflow
             position.Heading = situation.getHeading().value(CAngleUnit::deg());
             position.Airspeed = situation.getGroundSpeed().value(CSpeedUnit::kts());