so i'm not sure that i understand you question: so you're asking if a GPS could be used to measure NOT the 2-D distance but the true 3-D "distance travelled"? first, unless you're in a really mountainous area i don't think the elevation difference from 2 points will matter much so just computing from start-end won't work. So what you're really asking is if the GPS can use real-time 3-D measurement to compute "true 3-D distance" travelled? if so, in theory it shouldn't be too hard to do, but i don't know of any GPS that does and of course you have the problem similar to the barometric Altimeter vertical counting devices that you have to define a metric and it makes a big difference whether 5m or 10cm "counts" as vertical change
That's what I was getting at. Could a GPS unit replace an existing bike computer? I know some/all GPS units let you review the course (waypoints) travelled? But I'm not sure if that is an automatic recording, like saving current position every 30 seconds, for example, or if you have to record it manually by pressing a button to record the snapshot?
Bike computers are not as accurate as we'd like ot think they are. If you set the computer with the wheel circumference then that might change a bit when you are sitting are on the bike, and it might also change again depending on tire pressure, though I think the difference in a typical 26" wheel will not be too noticeable?
I see GPS eliminating the calibration/mechanical aspects of existing bike computer distance recording, but it now seems if you opt for GPS, then you introduce different problems into the calculations.