I noticed that it was basicly an negative of a elevation map. If you load it to a paint program, take a nagitive to reverse it, then load into a program like leveller, you will have something of a 3d view of the world. It's not totally correct as the roads seem to all have the same level of grey and they should fade between light/dark based on elevation.
Also I say negative because normally when storing elevation data as a grey scale map, black is low and white is high.