Thanks Richard Your info is interesting and scary!
Please see my generalised reply, just published. Graham > On 6 May 2020, at 21:21, Hillen Richard via use-livecode > <use-livecode@lists.runrev.com> wrote: > > Hello Graham, > > 20 years ago I started to use GPS-technique and found the same mysteries as > you do now. > > I learned that there a two main reason for the variation of location data: > > Atmosperic density variations cause short term runtime variations and > Different relativ positions of satellites cause long term triangulation > errors. > > So I sampled at a constant location over some days the position data > delivered from a gps-receiver taken every 10 seonds. > > First I plotted the data and found that there were local shorttime-variations > around a center, which was wandering slowly over the plot-area. The > shorttime-fluctuations were most in a range of +-5m, the longtime-area showed > - if I remember right - a radius from +-20m. > > After that I started to calculate time-series of mean-values over different > minute-intervalls and learned by plotting the result again, how to smooth my > data best. > > Nowadays you will of course get smaller short-time variations using the more > intelligent gps-receiver of an iphone, but it probably gives you an > impression about what is going on there. > > Richard. > > > >> >> Message: 17 >> Date: Wed, 6 May 2020 15:09:16 +0200 >> From: Graham Samuel <livf...@mac.com> >> To: How to use LiveCode <use-livecode@lists.runrev.com> >> Subject: Re: Has anyone experience of GPS on iPhone? >> Message-ID: <074980d0-f69d-45af-9891-5ceb351f3...@mac.com> >> Content-Type: text/plain; charset=utf-8 >> >> Bill, I think you are confirming that there is some mystery here. There are >> a lot of apps that seem to get location, and measures derived from location, >> almost completely right, whereas I am having trouble doing so with what must >> be the same essential data. >> >> Take the problem of measuring the length of a country walk (I mean a walk >> not in a straight line). My basic approach is to process locationChanged >> messages, which unsurprisingly are triggered every time the GPS-measured >> location changes. So as not to get overwhelmed with very small, frequent >> changes, I only process a locationChanged message every 3 seconds - I know >> at least one other app that does this. As a person probably walks up to two >> metres a second, this fits in with what we know about accuracy, I think. The >> method is very simple. Every time we respond to a locationChanged message, >> we work out the straight line distance delta as in >> >> delta = (where we were 3 seconds ago) - (where we are now) >> >> Ignoring the sign of the result, of course. This can be done by Haversine or >> similar algorithms for measuring short distances on the Earth?s surface - >> it?s essentially a Pythagoras calculation. Then we add up all the deltas and >> we know how far we walked on the trip with a fair if not complete degree of >> accuracy - easy! >> >> Only there are complications. Of course if any delta is zero, it doesn?t >> contribute to the trip; but what if it?s **nearly** zero - is it sensible to >> ignore very small deltas on the grounds that they are due to GPS wobble, or >> should we put them all in? >> >> Here?s what happened when I tried to do it: first I calculated the deltas to >> two decimal places, and I found that I was badly underestimating the >> distance walked; so then I pushed up the accuracy of the calculation to 5 >> decimal places. Sure enough, the measured route got longer in kilometers, >> until I noticed that if I simply put the phone on the grass and left it, so >> it wasn?t moving at all, in about 45 minutes I?d accumulated a completely >> spurious half a kilometre of walking! The small variations in the GPS signal >> (what I call the wobble) must have been responsible, since there was no >> other source of data but the GPS reading. >> >> How then to avoid either under- or over-estimating the trip distance? Plenty >> of apps have done it but I just can?t see how, although I keep tinkering >> with the parameters. Of course I can never forget that my scripting might >> just be plain wrong, but so far my incremental method hasn?t worked >> sufficiently well, in the sense that if run the app and choose to walk in an >> exact straight line, I can compare a single measure of distance from the >> starting point with my integral approach. So far the result is not even >> close. As you say, intensive Internet searches are called for. >> >> I wish all this were easier. >> >> Graham >> > > > _______________________________________________ > use-livecode mailing list > use-livecode@lists.runrev.com > Please visit this url to subscribe, unsubscribe and manage your subscription > preferences: > http://lists.runrev.com/mailman/listinfo/use-livecode _______________________________________________ use-livecode mailing list use-livecode@lists.runrev.com Please visit this url to subscribe, unsubscribe and manage your subscription preferences: http://lists.runrev.com/mailman/listinfo/use-livecode
