The file A830_Progress_2dem.tif contains a linear stretch of the DEM.
To reconstruct the z-axis of the DEM in units of mm, 
   1) multiply TIFF by     0.0270137
   2) Add      -6.88850
The x & y scales are 0.030 mm / pixel.
Maximum value for DEM, corresponding to TIFF value of 255:       0.00000
Minimum value for DEM, corresponding to TIFF value of 0:      -6.88850