XPLM_API XPLMMapLayerID XPLMCreateMapLayer( XPLMCreateMapLayer_t * inParams);
This routine creates a new map layer. You pass in an XPLMCreateMapLayer_t structure with all of the fields set in. You must set the structSize of the structure to the size of the actual structure you used.
Returns NULL if the layer creation failed. This happens most frequently because the map you specified in your XPLMCreateMapLayer_t::mapToCreateLayerIn field doesn’t exist (that is, if XPLMMapExists() returns 0 for the specified map). You can use XPLMRegisterMapCreationHook() to get a notification each time a new map is opened in X-Plane, at which time you can create layers in it.