Thanks TIG. I slept on it, and your answer this morning made a lot of sense.

I seem to have determined the units of the ModelTranslation values - inches. My problem now is to reset the z-axis of the model to make it easier to calculate elevations. For instance, I want to show the impact of a flood which reached a certain height. That height is given in AHD (Australian Height Datum) relative values. I presume that the location data elevations are also AHD-relative. I can represent a flood by drawing a rectangle centred on 0,0,0 and pulling the surface up to the height of the flood. The idea works well, apart from some apparent inaccuracies in the terrain data. However, it is far easier to simply pull the "water" to a given height in metres, than to convert the height to inches, subtract the Z translation in inches, then reconvert the difference to metres to calculate a level relative to 0,0,0 to pull to. Flood level