I'm sure the GPS method works with precise flying and patience (as it was recommended to me by Transport Canada Flight Test department), but with the client watching the mounting bill for flying hours, the simpler method answered the question faster!
It's worth mentioning that there is not "a GPS method", there are several, and all require good understanding of the maths to make them work well.
Either the May or June issue of
Aeronautical Journal will have a paper that goes into this in considerable depth and may be worth interested flight testers reading.
G