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This confuses me a bit. My phone shows a height of around 240 ft outside my front door - which is about 4 ftabove sea level - and this is constant both day to day and with different phones.
Is that due to the GPS seeing the earth as a perfect sphere whereas it is not in reality?
then when I go up into my flat 70 ish feet above - it shows a similar number? Is it assuming I am on the ground?
Does this mean when out in the hills that I should disregard height for navigation purposes?
The location horizontally seems pretty accurate
Are you just quite tall?
Thats not how gps works.
It requires 4 signals minimum for a 3d location of the receiver (phone etc) its the distance fomr the beacons that calculates its position from nothing to do with the globe. Heights in gps are really innacurate more so than the horizontal positioning.
Also sea level is probably not what you think it is which not going to account for that discrepancy but still.
Step off your high horse, now and again.... 🙂
Never used my phone for GPS, Garmin VA4 is however tremendously accurate, I check it against the map regularly, and it's never more than 5m out, which is very reassuring in a haar on an unfamiliar pathless hill.
This is a very deep rabbit hole your venturing down here, if your really interested then the OS have a good introduction to co-ordinate systems and GPS that may throw some light on what is happening.
The other week I tucked my phone in a dry-bag inside my wetsuit when I went windsurfing on my local lake. Strava showed an elevation gain of about 70 metres. My dismounts didn't account for more than about 2 metres, and I could have sworn there wasn't a hill.
What is this "feet" thing.
You may recall that the Munro Society set about measuring some marginal Munros and Corbetts a few years ago (including Foinaven). To do this they had to hire some very expensive equipment. They didn't just wander up with a cheap phone from Carphonewarehouse.
If you are inside, your phone is probably using wifi and mobile phone networks to figure out the location. So this could be even less accurate than GPS, especially for height.
What is this “feet” thing.
A sensible way of measuring height - after all a munro is 3000 ft! 914.4 m is just not the same
Feet here too, run and cycle in miles!
Craig - my "inside" has only timber and slates between me and the outside world. It does pick up sattelites I am fairly sure.
Why does it show 240 ft consistently at my front door? the world being an oblate spheroid rather than a sphere - does the GPS take that into account? Its measuring the distances from the satellites to the receiver is it not?
if the error was not consistent I could understand it being inaccurate. I suppose I had better check against a known height like Arthurs seat
Horizontally it can tell pretty accurately not just which room I am in in the house but which chair in the room I am sat in
Step off your high horse, now and again
🙂 🙂 🙂
What app are you using to get the height? Different apps may figure it out in different ways.
Even if you have GPS, your phone may be using wifi locations as well, depends on the app. And some phones have a barometer, which can be inaccurate in other ways.
For the shape of the earth, yes depends on what datum/geoid you are using. Some apps let you configure this, some don't. Can be quite complicated to accurately transform between them.
Ta.
Your phone probably knows where you are on a coordinate system
It'll be referring to a map set for height data
Your flat wasn't surveyed as part of the height data.
Strava had an interesting article on height accuracy. How the height data points on maps can be quite coarse, how weather affects it, how they use data from other riders on the same trail using a device with a barometric sensor to correct the height data for GPS only devices, and using that data when they detect your GPS trace has wondered off the trail and therefore deviating from the correct altitude of the trail.
trailrat - but the road outside my flat is at 4 ft and the phone gps thingy says 240 ft consitently. You have to travel miles to get to 240 ft.
Forget oblate spheroids, your problem is simply that GPS is very poor at calculating height. It's an estimate at best. The "accurate" Garmin someone mentioned earlier has a discrete altimeter sensor in it which your phone almost certainly doesn't have.
If you're out and about and require an accurate height reading for navigation purposes, you need a better dedicated GPS device with that functionality rather than using your phone.
What app are you using to get the height? Different apps may figure it out in different ways.
Unlikely, they just get height from the GPS Silicon in the phone, the apps don't get to see the individual path timings and so can't do their own maths.
GPS height isn't very accurate as the geometry has quite a high inherent error due to the lack of height variability in each path to the satellite.
GPS can also be worse in towns / cities, you get a canyon effect where your handset can only see a few satellites directly overhead wheras in the country side it can see the whole hemisphere and potentially get 10+ satellites which help improve accuracy. In places like New York, with tall buldings all round it can fail to lock eg car satnavs can struggle to locate at all.
The AlpineQuest app has a setting to adjust elevations to EGM96 geoid. That seems to make it more accurate for me, about 50m difference.
Seems it depends on the GPS chipset in the phone as well. Some just report WGS84 altitude, others also give altitude above MSL.
https://github.com/barbeau/gpstest/issues/296
I find these discussions of GPS altitude quite interesting - I had an Etrex (the old, old, yellow one) and accepted the view that, as it didn’t have a barometric altimeter, altitude readings were likely to be unreliable. I felt, however, that getting a position to within a few metres meant that a glance at the map should give me a pretty good estimate. What I did find, oddly, was the whenever I did that, the Etrex was within 5-10 metres too.
Prior to that I had an Avocet altimeter, which was used as an adjunct to observation/guesswork to determine location, and that was a lot more likely to be inaccurate during a day's walking in changeable weather. The constant checking/resetting at known heights was irritating.
I had an Avocet. As you say, it was pretty variable and I'd have to reset it repeatedly on any day that had fronts coming through. Not as bad as my mates Suuto though - that was comically bad.
There are two different problems, 1, altitude error from GPS is 1.5-3x long & lat error. 2, the model in use, see https://www.esri.com/news/arcuser/0703/geoid1of3.html
eta; I'm not sure if android allows apps to expose the NMEA data from the GPS chipset, but ifit does, take a look at the GPGGA message
TJ - we canoed all the way down the river Wye over a few days, still managed to record quite significant elevation gains at times 😀 elevation on GPS = pinch of salt.
I always assumed that pretty much everyone in Leith was high as a kite most of the time.
Seems that the technology agrees.
Not far out, approx 13' at a fair old height.
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