A technical question about handheld GPS units.

[zahc]

I downhill skateboard when it's a lot warmer than it is now. I've used a borrowed handheld eTrex GPS to measure speed before, and I'm happy with it, but the physicist in me can't help but wonder...

When bombing, of course, you are traveling downhill. Therefore, the speed on the GPS is probably only the horizontal projection of your actual velocity, which is doubtlessly very close to your actual speed, as these hills are'nt very steep in geometric terms. Now, with a few sats, I know GPS can also tell your altitude, so the GPS unit itself could be actually returning your true speed--I just doubt that it does. Perhaps a way to tell would be to get in an elevator and see if the 'speed' value goes above zero. What do you think?

[Manedwolf]

I doubt you're going to get GPS reception inside a metal elevator car in the core of a building?

A glass elevator on the outside of a building, sure.

BTW, lots of people like the Garmin Forerunner for more active things, since it's wrist-worn.

[Tallpine]

I think you better be looking where you are going instead of at the damn GPS 

[AJ Dual]

I think you better be looking where you are going instead of at the damn GPS 

You can go back and review your track when your done. Most will give you maximum speed.

It is a good question, but I'd guess that only a GPS optimized for aircraft use would combine the horizontal and vertical vector sums for a true velocity figure.

I'd go Google for a GPS forum, I'm sure there's one out there. Otherwise try a geocaching discussion, they'd be "GPS Junkies" who are more likely to know for sure.

[roo_ster]

It is my understanding that the capability to get 3-D locations (x, y, z relative to the center of hte earth) and velocity exists in the GPS system.  That said, the particular handheld unit may or may not have the capability to make use of it.  Also, more satelites give more accurate locations and more accurate velocities.  In some cases, I am sure it is the horizontal projection, as not enough satellites can be referenced.

If the desire is for instantaneous velocity, other methods would give more accurate measurements.