refs #865, based on discussion https://dev.vatsim-germany.org/issues/865#note-12

* do not call XP`s elevation provider if not needed in interpolator
* some renaming/utility functions

src/blackmisc/geo/elevationplane.h

@@ -42,6 +42,9 @@ namespace BlackMisc
             //! Radius
             void setRadius(const BlackMisc::PhysicalQuantities::CLength &radius) { m_radius = radius; }
 
+            //! Altitude when within radius, else null
+            const Aviation::CAltitude &getAltitudeIfWithinRadius(const ICoordinateGeodetic &coordinate) const;
+
             //! Existing value
             bool isNull() const;