Add convenience function to keep position with just an other elevation.

Modified Files:
	simgear/math/SGGeod.hxx

simgear/math/SGGeod.hxx

@@ -42,6 +42,10 @@ public:
   static SGGeod fromRadM(double lon, double lat, double elevation);
   /// Factory from angular values in degrees and elevation in m
   static SGGeod fromDegM(double lon, double lat, double elevation);
+  /// Factory from an other SGGeod and a different elevation in m
+  static SGGeod fromGeodM(const SGGeod& geod, double elevation);
+  /// Factory from an other SGGeod and a different elevation in ft
+  static SGGeod fromGeodFt(const SGGeod& geod, double elevation);
   /// Factory to convert position from a cartesian position assumed to be
   /// in wgs84 measured in meters
   /// Note that this conversion is relatively expensive to compute
@@ -192,6 +196,20 @@ SGGeod::fromDegM(double lon, double lat, double elevation)
 #endif
 }
 
+inline
+SGGeod
+SGGeod::fromGeodM(const SGGeod& geod, double elevation)
+{
+  return SGGeod(geod._lon, geod._lat, elevation);
+}
+
+inline
+SGGeod
+SGGeod::fromGeodFt(const SGGeod& geod, double elevation)
+{
+  return SGGeod(geod._lon, geod._lat, elevation*SG_FEET_TO_METER);
+}
+
 inline
 SGGeod
 SGGeod::fromCart(const SGVec3<double>& cart)