Titanological pedantry.

Weinbaum's version of Titan is mostly covered in moving mountains of ice, basically very high dunes of ice crystals, not frozen together because it's too cold and dry, driven by 100-MPH winds. However, one real rock mountain is mentioned. I would like to describe it in terms of its height, but since there is no fixed surface equivalent to "sea level" to refer to I'm not sure how to do so. Should I just say e.g. 5000 ft and hope that nobody asks "relative to what?"