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
