[ORD C#] TerrainSurface units


I am trying to see if our features have been placed over a TerrainSurface. To do this, I need to read the geometry of the surface.

It all works, however there seems to be a units issue. The steps to reproduce are below.

1. Open drawing with TerrainSurface in. When I open the drawing it is in "US Survey Feet". 

2. Run the following code. Range (and returned verticies too) returns Min X of 2560.

TerrainSurface Surface = GetFirstSurfaceFromDrawing();
DTM bentleyTerrain = Surface.DTM;
var range3d = bentleyTerrain.Range3d;

3. Change the Units of the drawing via the Utilities Ribbon to Metres.

4. Re-run the above code. The values returned are still the same. 

When I look at the geometry that we are testing against, the Range / geometry changes when the units change. The surface range/ geometry does not.

Is there any way to find the insertion units of the Surface or can anyone suggest an alternative way of doing things?

Many Thanks,