This text was supplied by Michel Verheughe firstname.lastname@example.org
"Unofficial" texts are member contributions that have not yet been discussed or approved by the ESP community.
The ENV format.
X-Plane scenery is made of a succession of "tiles" called ENV files.
Each file represents one degree of latitude by one degree of longitude.
This area is divided in 59 by 79 node points and corresponding polygons.
The reason there is 59 by 79 nodes is that the files are based on the USGS 1 minute DEM data. It means that there is one elevation (node) per minute of longitude.
World Maker display the ENV file as a square. This is unfortunate because this is only true at the equator. At latitude 60 degrees north, for example, a degree of latitude is 60 NM but a degree of longitude is only 30 NM (cosine 60 = 0.5).
Considering this, one polygon of ground texture in X-plane is only a square at about latitude 42. (arc cosine (59 / 79)).
In the ENV file, each node point is listed with its position, elevation and a "flag" indicating which texture and which rotation is to be applied to the upper right polygon corresponding to the node.
Then comes a list of objects, both custom and obstacles.
At the end, there is a list of all the names of the textures allocated for the slots in World Maker.
This is why a custom textured ENV file will display white polygons if the textures are not to be found in the custom textures folder.