I don't recall reading this in any of my ATPL notes. But since the DOC includes an altitude as well as a range (e.g. BNN is 60nm/25000 ft amsl) it would only really make sense if the range was a ground distance.
Also, since the DOC applies to both the VOR and the DME where they are co-located, and slant distance is not relevant to a VOR, this would tend to back up my theory.
FFF
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