UKC

Out with a friend in Skye at the weekend and I started up OS Locate to help decide when was the right moment to dodge rightwards to get on to Bruach na Frithe’s NW ridge. Friend started talking about settings and got his phone out as well. What we noticed was that my phone was showing 540-ish metres altitude whereas his said 600-ish. You learn that the app doesn’t average the altitude data it gets from the satellites, the numbers always jump about a bit. We studied the map (and the ground) a bit more and concluded that mine was roughly right and he had about a 50m error.

Later, sitting in the Glenbrittle Memorial hut, we  were chatting about this to another club member who promptly flourished his phone which said 62metres altitude. Mine said 12, which from the map is more convincing.

Friend 2 said he had emailed the OS about this and their explanation (or excuse) was that mean sea level must be different where we are.
Can anyone make any sense of this? A 50m discrepancy in some circumstances could have consequences and it leaves you with reduced trust in the app.

I've found the height readings on OS Locate to be so inaccurate I routinely ignore it, it seems to over read by around 50 metres

That was what was interesting. Both my friends’ apps said around 45-50m more than mine, whilst in both locations we were able to interrogate the lie of the land and conclude that mine was about right whilst theirs were wrong. Same app. Or does it behave better on IPhone?(mine)

Possibly, both my wife and I have the same issue, both using Android.

GNSS VDOP is about 3x worse than HDOP. All due to satellite constellation geometry. In other words, it's the basic GNSS fix error, not OS Locate. Try comparing with another mapping app that reports GNSS altitude. You can also use a DEM altitude, provided you have DEM data (e.g. SRTM data), but that is likely to be interpolated from the 3 arc second (~90m) resolution data (1 arc second if you're lucky).

Most of the time, you need a 2D position. For land navigation. If you're airborne, an accurate 3D fix can be useful...

Then there's the geoid to consider; WGS84 vs Airy (OSGB36). That might be the 'sea level' thing the OS mentioned.

^ What CP said 😃

It is well documented that the heights on OS locate are different from what you would expect (note, I didn't say wrong...).

Essentially uses a different (simpler) model of the earth's shape to what is used when we measure altitude (heights above sealevel are relative to a theoretical baseline). There is a previous thread about this on ukc). Basically we should ignore the altitude given on os locate - they do say they are working on it.

I have three different apps on my phone incl. OS locate, and they are all different every time and don't even agree with the map when checked on the summit of Fairfield. So the lesson if there is is one is use the map first and if location and altitude is very difficult to determine then fall back with an educated guess with the apps. Grid Refs. however appear to record more accurately.

Who said, "Don't work with animals, kids or electronics?"

https://shop.ordnancesurvey.co.uk/os-locate-faq/#heightacc

Did you have mobile data and they didn't? According to the FAQ, without data you get height from GPS (as referenced above, ± 50 m), with you get ± 5 m from the OS data set.

I don't think the data connection works / has been omplemented yet....

Even with a strong signal I'm sitting at home with a very good data connection and a 50m height discrepancy (android phone).

This is interesting, thanks for joining in. I agree that it’s better to use more traditional ways of determining your altitude rather than staring at a screen, and as I mentioned, interrogating the map and landscape suggested that my iPhone readout was reasonably accurate. Any more iPhone users got any observations?

I've just turned on my GNSS on my phone. Fix acquired in less than a minute (with AGPS). The two apps I have that display 10 figures (Orux and Grid Reference) agreed to the metre, exactly. The third (GPS Status), which gives 8 figures, agreed to the last digit, exactly.

The coordinates wandered a bit as the GNSS receiver improved its fix, and the constellation changed, but all three apps gave the same position at the same time, and were within a few metres of the mapped location. And that's standing by the window, so half the sky partially obscured.

What phone, what apps are you using?

Sean said that grid refs were acceptably accurate. It was altitude readings that were at odds with each other. 

I’ve just opened the app and noticed that the altitude figure also gives error margins (for both position and altitude), which are going to be important to look at if you are ever really relying on the reading. Presumably in the example you gave your friend’s phone would have also been telling them that the altitude figure given had an error margin of at least +-50m.

So with regard to your comment about reduced trust in the app, it seems to offer you a quantitative assessment (which we hope is itself accurate or, if not, suitably conservative) of how much you can trust it at any given time which should aid your decision making.

You're right, he did. But I've already addressed the altitude issues with GNSS.

If you have an accurate 2D fix, and you're land navigating (i.e. on the ground), I really don't see why you would be concerned about altitude.

Yeah, I agree. I’ve never felt the need to consult an altitude reading, although I’m far from a master navigator!

I could see there being some value in an altitude reading if all you had was map, compass and altimeter. But it’s not likely I’ll ever be in that position as any device I have that can tell me altitude can also give me a grid reference or show my position on a digital map, either of which would be far more useful.

Indeed. An altimeter can be a useful navigational tool, if you don't have something that gives you a 2D fix to ~3m. It makes me wonder if there's a bit of ludditism going on, trying to use GNSS purely for altitude, in support of old skool navigation.

Of course, pressure measurement altimeters also suffer from significant accuracy problems, especially short term drift.

No error range is shown on my android os locate. Last update was Oct 2021. I don't have os maps on my phone.

I think they should just remove the altitude.

Does iphone app behave differently?

That’s interesting. I’m using an iPhone and get a +- error figure in small characters just below the main readings. The last app update was 7 months ago, so around February 2022. Maybe it was added then.

I think I have the latest Android version - last updated 12 Oct 2021, so maybe you're right.
(Oh - edit to add while I'm at it - currently says my altitude is 45m higher than it actually is.)

Just installed os locate to try this out and yeah, it reports ~40m higher than every other GPS app I have. Android phone.

Edit: and it's wrong. Just checked the paper map and the lower answer is correct.

iPhone 11 Pro Max user. Have both OSMaps and Locate  

Currently standing on a 130 metre grid line. OS locate when first opened stated +/- 27 m accuracy but leaving the app open and phone static that fluctuates and has come down to +/- 9 metres  

reporting height as 132.1 metres  

accuracy and height are dynamically moving while phone is static presumably due to GPS lock changing  

apologies for grammar non capitals, iPhone is ace but not for typing into uk climbing forum  

Ditto. Samsung Android.

GR is a perfect match.

Altitude is +40m, compared with other GNSS apps, SRTM DEM and OS 1:25k mapping.

Can't find a feedback option...

At least it has an altitude readout. OS Maps doesn't,  and its GR display method is very poor. There's no 'dashboard' to display any position, height, speed, distance, error, etc. Nice maps, but that's all.

altitude matches 1:25k at my home (oslocator/android) but no compass (my old moto e4 doesn't have magnetometer and looking to upgrade to one that has but seems some of their latest models dont either eg g9 plus. (thinking of something like the g7 power with long battery life- (what do you reckon?))