Re: Course lengths and GPS.

Sericinus hunter wrote:
Elflord wrote:

On 2008-09-06, Sericinus hunter <serhunt@xxxxxxxxx> wrote:


OK. My question stands though. What is it going to show? Zero?


Not quite zero. *If* it couldn't measure height at all, it still would not be zero. It would show about the same thing that it would show if you ran around the
local playground for 20 minutes.

I believe that they can measure height accurately enough that a ~380m altitude gain
(which is pretty substantial) would register (though it may be off by 20m or so).

The problem is that in more typical use, the altitude gains are much smaller
(often less than 50m), and the noise is substantial. Noise could be around 20m
or so. That would be fine if it was consistently off by 20m for the entire
duration of your run. But this is not just a 20m offset -- this is somewhat
random noise, which exhibits "drop out" or "step" effects that lead to
discontinuous jumps. The bottom line is that if your intention is to produce a
useful altitude chart, GPS technology isn't adequate, which is why GPS units are
often fitted with barometric altimeters.


Right. Well, I don't know much about GPS

Please go read the links I already posted, which includes several tutorials. You might also look at Topofusion.com for some discussions on getting decent elevation profiles.


but why would altitude accuracy
be different from position accuracy, given we have enough sattelites in view?
My understanding is that the more sattelites, the better. Four is minimum to
measure altitude.

Getting good satellite view of enough satellites in optimal posn (ie. 3 satellites in a line is not good for position) *is* an issue, sometimes a major issue. This is why the newer gps units are so much better than the older ones (like FR x05 series vs FR x01 series).


In the devices with downloaded maps they could have taken a different
approach. Simply having altitude already there, in the map database. I think
this would be the best in accuracy even with interpolation when needed.

I've looked at the garmin maps on CD, but never bothered loading them on my gps. Granted the DEMs are probably better than the visual (visual shows my usual rolling hills as essentially flat, a term no one in real life would use to describe them), but they still leave much to be desired and not sure if they are in the maps when loaded to CD (IOW, not sure if the info is there or not). After several years of trying gps and maps, I gave up and went to barometric altimeter on my hrm for elevation profiles - then to gps with new chip and barometric altimeter, which is better still. It still has some inaccuracies (like different elev when I end vs where I start), but much better than anything I've used before. And the profiles I get from that are *much* better than what I've gotten from overlaying gps route on a map and using DEMs. (I never posted the RP elevation profile from Natl Geographic last year since it was so noisy compared with elevation profile generated from the actual gps data, which reflected the trail *much* better. I'm using perfils.exe now so I can put waypoints on there.)

Dot

--
"Magic rocks and roots - the ones that trip you but you can never find afterwards" - Matt Carpenter

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