XPLMMapScaleMeter
XPLM_API float XPLMMapScaleMeter(
XPLMMapProjectionID projection,
float mapX,
float mapY);
Returns the number of map units that correspond to a distance of one meter at a given set of map coordinates.
Only valid from within a map layer callback (one of XPLMMapPrepareCacheCallback_f, XPLMMapDrawingCallback_f, XPLMMapIconDrawingCallback_f, or XPLMMapLabelDrawingCallback_f.)