Re: Distance calculation from trackpoints. Sanity check Please.

JF Mezei wrote:
QUESTION:

Could the 90 second track recording interval explain such discrepancies (basically cutting curves too much and hence recording smaller distances ?)

Since the absolute difference between arc length and chord increases approximately as the cube of the distance, you could expect approximately three times the error at 90 sec vs 60 sec. We'd need to know either the radius of the curves or the speed and net change in heading. The later is easily obtained from the track log.

Is it more likely that it was due to the bad smapling rate that I got such a high error rate (2.3%) ? Or should I test my software to ensure the formulas are applied properly ? (I took them from the aviation formulary).

So did I for mine. :-) Which algorithm did you use and what language did you implement it in? What GPS do you have? Can you produce a .GPX file?

I have written functions in APL2 to do these sort of calculations so I can make the following two suggestions:
1) You could send me the track file, preferably as a .GPX file or, if you are using a Garmin GPS and MapSource, a .GDB file. I would then run the calculation and you could see the results. (It should be pretty obvious how to get my correct e-mail address.)
2) If you have (or have access to) APL2, I could send you my functions to play with.

Elevation: in raw vertical elevation, I'd say max 2km total change (up+down). So on a road with max 10% angles, it would be far less than 2km diference. (and on the track section, elevation was only about 150m up and 150m down), it was the rest of the road that was hilly.

At 10% grade, 2Km total elev change would occur over 20Km horizontal travel. Since (1-Cos .1 Atan2 1) is ~0.005, you should accumulate less than 99.3 meters of horizontal error. Since it is highly unlikely that the slope was always that much, it's pretty clear that isn't the problem.

I would conclude that either the bicycle computer, your software or the track point interval is the problem.

On a 320Km trip through the BC mountains last September, I recorded the track on my Garmin GPS-V using the auto setting on Auto-Highand accumulated 1786 points. My software will extract x,y,z position (as well as other stuff from a .GPX file.
+/Pyth 2-/[1]x
320522.6873
+/Pyth 2-/[1]2{take}[2]x
320361.824
As you can see, there was only about 161m difference between the 3D and 2D track lengths. The GPS-V spaced 1786 points such that
between 0 100 200 300 400 500 600 700 800 meters
# of pts 494 693 309 148 86 43 12 1

Ted
.

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