Has anyone experience of GPS on iPhone?
livfoss at mac.com
Wed May 6 09:09:16 EDT 2020
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.
> On 5 May 2020, at 15:58, Prothero-ELS via use-livecode <use-livecode at lists.runrev.com> wrote:
> This makes me wonder how the Google and Apple Maps get their accuracy. When driving or walking, trying to figure out how to get to an address, it shows me walking along a street, certainly more accurate than 20meters. Yet, when I’m wondering where my wife is, and load up “Find My iphone”, the location does jump around (easily 20 meters) for awhile, then seems to settle down. Hmm... In the olden days, NASA used to add error into publicly available gps readings, for security reasons. But now errors are not added and geo scientists are measuring very small fault motion with gps, but it’s comparing relative motion between two stations so wouldn’t apply in your case. I think some serious internet searching may be in order.
> You must have tried Google Maps with satellite view, to see where it puts you relative to your house. You might be interested in this demo of getting google maps from Livecode.
> I made a basic version of this and a hermann (hh), who helped me figure out the boundary coordinate calculation, added to it. I think it’s also in the livecode repository. It’s a couple of years old, so ....
> William Prothero
>> On May 5, 2020, at 2:30 AM, Graham Samuel via use-livecode <use-livecode at lists.runrev.com> wrote:
>> Thanks Bill for your two messages. I see from the link you gave that the LC system is (as expected) simply a wrapper for the iOS API, and indeed the term “horizontal accuracy” is actually used internally in iOS. So it’s unlikely that LC itself is adding to my problems.
>> I don’t have access to an entirely different GPS system - all mine are on iPhones and I’m in lockdown. Google Earth reckons my reference point is at 43.276693, 2.160029. (The curious can see where I live).
>> Here is a little test done by simply sitting on a bench in my garden - as far as possible, this is what Google Earth is looking at.
>> Another app - “MyAltitude” on my phone. Location 43.2766, 2.1601
>> Built-in “Compass” app. Location 43.2766, 2.1600
>> My app - successive calls to mobileCurrentLocation a few seconds apart differ only beyond the fourth decimal place, a typical one is 43.276732 (so 43.277), 2.160163 (so 2.1602). During this set of about 20 measurements, horizontal accuracy varied from a minimum of 5 to a maximum of 23, tending to settle down to the lower end - but on other days I’ve seen much higher maxima, certainly above 60 and very occasionally a lot more.
>> So the readings agree, at least to three decimal places. Presumably the accuracy is supposed to tell me the amount of wobble to expect, although it’s measured in metres, whereas longitude and latitude are not so straightforward, since the grid covers a spherical earth. Where I live (England and the South of France), the North-South distance of a second of arc (or .000278 in decimal measurement) is about 20 metres, which is perhaps acceptable in short journeys such as walks, so I need to take account of say four or five decimal places if I’m going to hope for reasonable accuracy. This certainly looks feasible - my only real problem therefore is the wild variation in horizontal accuracy that I’m seeing.
>> All this makes me think that I should not have a target accuracy, since I may never hit the target and my user will be waiting forever to start his or her trip. I am therefore about to modify the app so it only looks for consistency - as soon as the horizontal accuracy settles down to a value without say more than five percent of variation, I will allow the trip to start. Seems weird but it’s the best I can do. Without access to anyone else’s coding for an iOS app, I can’t see a way to improve on that strategy. Obviously I will now run tests to see how my app compared with others that measure on-the-ground trip distance, particularly for short distances.
>> In case anyone else thinks this is interesting (!), I’ll report back.
>>> On 4 May 2020, at 16:02, Prothero-ELS via use-livecode <use-livecode at lists.runrev.com> wrote:
>>> I would want to be verifying the coordinates with another gps device. I’d also check it against apps that give lat/Lon.
>>> William Prothero
>>>>> On May 4, 2020, at 5:07 AM, Graham Samuel via use-livecode <use-livecode at lists.runrev.com> wrote:
>>>> I’m using GPS on iPhone using the various commands and functions available in LC. Most of this works, but I am having difficulty with the quantity “horizontal accuracy”. This can be queried via an array which is returned when mobileCurrentLocation() is executed.
>>>> Looking at other iOS apps that use GPS and allow one to examine their data, I find that in my part of the world, accuracy for them is of the order of 5 metres: but when I query it using LC I get much higher numbers, indeed some of them look like nonsense, being hundreds of metres!
>>>> I am aware that it takes time for a GPS signal to settle down after mobileStartTrackingSensor “location” has been called, but even if I script the examination of accuracy to wait for it to settle down (I look for a run of similar or identical readings against a ‘reasonable’ criterion, such as 5 metres), it still sometimes settles on very large numbers comparatively speaking, for example it might stabilise at 65 metres. With that level of inaccuracy any cumulative use, as in working out the number of kilometres in a trip, would not be reliable.
>>>> I wonder if my expectation is wrong, or if there is something different about LC’s encapsulation of these iOS outputs, or if there is some other explanation.
>>>> In the short term, I’m simply going to have to relax my search for a reasonable accuracy of the order of 5 metres, and just go for the first run of stable readings. But I would love to hear any further insight from anyone who’s already tried it.
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