[ClockWatcher]

Using Maps\empty2a and a 255 pixel long, 1 pixel wide strip filled with an R=G=B gradient from 0 to 255, no dither, in Gimp the height changes are NOT linear as the RGB value increases. The scale, roughly is the following:

0 - 600m: 1 RGB increment in value = 3.75m: RGB range 0 to 160
600 - 1100 m: 1 RGB increment in value = 15.6m: RGB range 160 to 192
1100 - 2080m: 1 RGB increment in value = 29.7m: RGB range 192 to 225
2080 - 4030m: 1 RGB increment in value = 65.0m: RGB range 225 to 255

ie, if
RGB = 2, height = 2 x 3.75m = 7.5m
RGB = 160, height = 160 x 3.75m = 600m
RGB = 180, height = (160 x 3.75m) + (20 x 15.6m) = 912m

Curiously, when using photoshop, even with dither off, the gradient produces a wobbly slope!