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+--- Thread: Reported velocity does not agree with manual calc from distance and lat/long? (/showthread.php?tid=1632)
Reported velocity does not agree with manual calc from distance and lat/long? - MaughanJamesR@gmail.com - 10-12-2022
In one experiment I've done, I use the Location (GPS) feature to record the velocity of my canoe as I paddle it up to a fixed speed then let it drift to a stop. (The rate of deceleration is a measurement of the drag.) The export of Speed every second looks credible and I'm inclined to believe it but in checking the reported speed against a manual speed calculation based on a) the change of recorded distance and b) the distance from change in latitude and longitude I get different results. The latter two agree except they appear to be offset by one interval. The travel is in a relatively straight line, so I don't think it's that.
I have version 1.1.11, build 10011, file format 1.15 on a iPhone 13.1, iOS 14.4.2
Original data file with my added calculations and graphs attached.
Any insight would be welcome. Thanks again.
RE: Reported velocity does not agree with manual calc from distance and lat/long? - Jens Noriʇzsɔɥ - 10-13-2022
Thanks for spotting the difference between the velocities by d vs. lat-/longitude. We are looking into this. I am with you that it appears counterintuitive.
The Location service directly provides us a velocity. Technically, it could be measured by the GPS module, however, we are not sure how exactly this is handled in smartphones. The systematics when your canoe slows down appear to favor velocity as an independent measure.
At the end the distance traveled (integral of the velocities) differs just by a few meters, so quite below GPS uncertainties. Here we would need more data to check if the systematics are … systematic.
