RFC: Explicit OBJ Height

	Revision History
		4/15/10 - Initial Draft		 

X-Plane places OBJs into the scenery system by aligning the origin of the OBJ model with a point on the surface of the terrain mesh whose latitude and longitude match coordinates specified in a DSF file. The elevation is a function of the underlying base DSF, which may not come from the same DSF as the OBJ itself.

This can present a problem for authors who are attempting to model natural phenomena with OBJs, or who are converting real-world 3-d data that is calibrated to a specific datum. The author cannot know what offset X-Plane will use, and in fact the offset may change.

This extension addresses that problem. A new OBJ attribute allows an author to tag an object as explicitly placed in the vertical dimension. When the object is placed, the OBJ specifies its vertical height.

ATTR_explicit_height 100

Explicit heights are in meters relative to mean sea level.

Note: this extension is not meant to allow an OBJ to be used multiple times at multiple heights for "constructive" modeling; it is only meant to provide calibration for large complex models that refer to real world entities with real locations.

Note: this extension changes an authors set of problems when interacting with a base mesh; it does not completely alleviate them. Authors using explicit height do not have to worry about multiple explicitly elevated objects being misaligned, but the author does have to worry that the base mesh and object may not interact correctly.