Photoshop :: Converting Bird's Eye View From Game To Elevation Map

I would like to convert in-game terrain (from whatever game) to an elevation map, so that I can use it in a program like Vue to generate a 3D terrain. Basically, I'd be flying above the game terrain and taking snapshots, and would like to know what's the usual approach to convert these snapshots into elevation data in Photoshop.

I understand the basic concept of how a grayscale image translates to elevation, but am having difficulties grasping how the grayscale version of my snapshots would translate in proper elevation information.