diff --git a/src/blackmisc/aviation/aircraftparts.cpp b/src/blackmisc/aviation/aircraftparts.cpp
index 27a5cbaf7..e24f4d5d8 100644
--- a/src/blackmisc/aviation/aircraftparts.cpp
+++ b/src/blackmisc/aviation/aircraftparts.cpp
@@ -14,6 +14,7 @@
 #include "aircraftsituationchange.h"
 #include "blackmisc/comparefunctions.h"
 #include "blackmisc/stringutils.h"
+#include "blackmisc/verify.h"
 #include "blackconfig/buildconfig.h"
 
 #include "QStringBuilder"
@@ -152,9 +153,16 @@ namespace BlackMisc
 
             if (situation.hasGroundElevation())
             {
+                const CLength aboveGnd = situation.getHeightAboveGround();
+                if (aboveGnd.isNull() || std::isnan(aboveGnd.value()))
+                {
+                    BLACK_VERIFY_X(false, Q_FUNC_INFO, "above gnd.is null");
+                    return parts;
+                }
+
                 const double nearGround1Ft = 300;
                 const double nearGround2Ft = isLikelyTakeOffOrClimbing ? 500 : 1000;
-                const double aGroundFt = situation.getHeightAboveGround().value(CLengthUnit::ft());
+                const double aGroundFt = aboveGnd.value(CLengthUnit::ft());
                 static const QString detailsInfo("above ground: %1ft near grounds: %2ft %3ft likely takeoff: %4 likely landing: %5");
 
                 if (details) { *details = detailsInfo.arg(aGroundFt).arg(nearGround1Ft).arg(nearGround2Ft).arg(boolToYesNo(isLikelyTakeOffOrClimbing), boolToYesNo(isLikelyLanding));  }
@@ -325,7 +333,7 @@ namespace BlackMisc
 
         void CAircraftParts::guessParts(const CAircraftSituation &situation, const CAircraftSituationChange &change, const CAircraftModel &model)
         {
-            *this = guessedParts(situation, change, model);
+            *this = CAircraftParts::guessedParts(situation, change, model);
         }
     } // namespace
 } // namespace
diff --git a/src/blackmisc/aviation/aircraftsituation.cpp b/src/blackmisc/aviation/aircraftsituation.cpp
index 7ea85dac2..501f29728 100644
--- a/src/blackmisc/aviation/aircraftsituation.cpp
+++ b/src/blackmisc/aviation/aircraftsituation.cpp
@@ -241,8 +241,12 @@ namespace BlackMisc
 
             if (!situation.isNull())
             {
-                const double distanceOldNewM = (distance.isNull() ? oldSituation.calculateGreatCircleDistance(newSituation) : distance).value(CLengthUnit::m());
                 const double distanceSituationNewM = situation.calculateGreatCircleDistance(newSituation).value(CLengthUnit::m());
+                if (distanceSituationNewM < 5.0) { return newSituation.getGroundElevationPlane(); }
+
+                const double distanceOldNewM = (distance.isNull() ? oldSituation.calculateGreatCircleDistance(newSituation) : distance).value(CLengthUnit::m());
+                if (distanceOldNewM < 5.0) { return oldSituation.getGroundElevationPlane(); }
+
                 const double distRatio = distanceSituationNewM / distanceOldNewM;
 
                 // very close to the situations we return tehir elevation
@@ -785,15 +789,30 @@ namespace BlackMisc
 
         CLength CAircraftSituation::getHeightAboveGround() const
         {
-            if (this->getAltitude().isNull()) { return { 0, nullptr }; }
+            if (this->getAltitude().isNull()) { return CLength::null(); }
             if (this->getAltitude().getReferenceDatum() == CAltitude::AboveGround)
             {
                 // we have a sure value explicitly set
                 return this->getAltitude();
             }
+
             const CLength gh(this->getGroundElevation());
-            if (gh.isNull()) { return { 0, nullptr }; }
-            return this->getAltitude() - gh;
+            if (gh.isNull()) { return CLength::null(); }
+
+            // sanity checks
+            if (std::isnan(gh.value()))
+            {
+                BLACK_VERIFY_X(false, Q_FUNC_INFO, "nan ground");
+                return CLength::null();
+            }
+            if (std::isnan(this->getAltitude().value()))
+            {
+                BLACK_VERIFY_X(false, Q_FUNC_INFO, "nan altitude");
+                return CLength::null();
+            }
+
+            const CLength ag = this->getAltitude() - gh;
+            return ag;
         }
 
         const CLengthUnit &CAircraftSituation::getAltitudeOrDefaultUnit() const
diff --git a/src/blackmisc/geo/elevationplane.cpp b/src/blackmisc/geo/elevationplane.cpp
index feb65a364..75f69acd7 100644
--- a/src/blackmisc/geo/elevationplane.cpp
+++ b/src/blackmisc/geo/elevationplane.cpp
@@ -52,7 +52,9 @@ namespace BlackMisc
 
         CElevationPlane::CElevationPlane(double latDeg, double lngDeg, double altitudeMSLft, const CLength &radius) :
             CCoordinateGeodetic(latDeg, lngDeg, altitudeMSLft), m_radius(radius)
-        { }
+        {
+            Q_ASSERT_X(!std::isnan(altitudeMSLft), Q_FUNC_INFO, "elv.nan");
+        }
 
         void CElevationPlane::setRadiusOrMinimum(const CLength &radius)
         {
@@ -93,7 +95,7 @@ namespace BlackMisc
         bool CElevationPlane::isWithinRange(const ICoordinateGeodetic &coordinate) const
         {
             if (coordinate.isNull()) { return false; }
-            if (isNull()) { return false; }
+            if (this->isNull()) { return false; }
             const CLength d = this->calculateGreatCircleDistance(coordinate);
             const bool inRange = (m_radius >= d);
             return inRange;
diff --git a/src/blackmisc/pq/physicalquantity.cpp b/src/blackmisc/pq/physicalquantity.cpp
index 436d06b87..ea993ef2f 100644
--- a/src/blackmisc/pq/physicalquantity.cpp
+++ b/src/blackmisc/pq/physicalquantity.cpp
@@ -53,7 +53,9 @@ namespace BlackMisc
         template <class MU, class PQ>
         CPhysicalQuantity<MU, PQ>::CPhysicalQuantity(double value, MU unit) :
             m_value(unit.isNull() ? 0.0 : value), m_unit(unit)
-        { }
+        {
+            Q_ASSERT_X(!std::isnan(value), Q_FUNC_INFO, "nan value");
+        }
 
         template <class MU, class PQ>
         CPhysicalQuantity<MU, PQ>::CPhysicalQuantity(const QString &unitString) :
diff --git a/src/blackmisc/simulation/interpolator.cpp b/src/blackmisc/simulation/interpolator.cpp
index efff22cbb..6c9cca1c8 100644
--- a/src/blackmisc/simulation/interpolator.cpp
+++ b/src/blackmisc/simulation/interpolator.cpp
@@ -311,6 +311,7 @@ namespace BlackMisc
                 }
 
                 // check if model has been thru model matching
+                Q_ASSERT_X(!m_lastSituation.isNull(), Q_FUNC_INFO, "null situations");
                 parts.guessParts(m_lastSituation, m_pastSituationsChange, m_model);
                 this->logParts(parts, 0, false);
             }