DME
When a DME reads 1 nautical mile, is it reading 6076 feet along the hypotenuse (direct from the DME to the aircraft), or are you 6076 feet from the DME horizontally along the ground?
If the DME measures direct, isn't the "groundspeed" that some DMEs give inaccurate? If I recall a bit of calculus, the rate of change of the hypotenuse isn't the same as that of the triangle's base.