The coverage values determines how much "valid" data is stored in the heightmap sample: A coverage of
0 means that no data is stored at all; a coverage of
1 means that the sample data if fully valid. Usually, one these two values is used. Intermediate values result from interpolation and are used for proper merging of heightmap data.
The normalized elevation value of the heightmap sample. A value of
0 represents zero elevation (e.g. on the surface of the ellipsoid); a value of
+/-1 represents maximum/minimum elevation along the terrain-space up vector.