XPLMMapGetNorthHeading
XPLM_API float XPLMMapGetNorthHeading(
XPLMMapProjectionID projection,
float mapX,
float mapY);
Returns the heading (in degrees clockwise) from the positive Y axis in the cartesian mapping coordinate system to true north at the point passed in. You can use it as a clockwise rotational offset to align icons and other 2-d drawing with true north on the map, compensating for rotations in the map due to projection.
Only valid from within a map layer callback (one of XPLMMapPrepareCacheCallback_f, XPLMMapDrawingCallback_f, XPLMMapIconDrawingCallback_f, or XPLMMapLabelDrawingCallback_f.)