Try "units imperial".
At first look that seems to be the obvious answer, but (without doing any tests) I notice the therion book lists this as an option within a layout, so in principle it only affects the output (forces conversion of metric data to imperial units is how I have used it in the past), and does not specify what the input data units are. Or am I mistaken? The other place it is listed is as an entry in the therion initialisation file. Does this usage specify both unit system for both input and output perhaps? And perhaps it can be used in the thconfig file to the same effect? The above affects the entire dataset though, which is only convenient in some situations. Can "units [imperial/metric]" be used within a survey or centreline though? As for specifying the type of input data, this is the place where it would seem logical to put it. I can imagine that in the US, some caves might comprise both metric and imperial source data, and in NZ we might for example have some 1960's data to include with modern data. Clarification anyone? Bruce