Setting altitude

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 Indy 21 Dec 2014
I'd like to be able to calibrate some equipment I've got but am finding it almost impossible to find any height reference data for a given point.

Ideally I'd like to use my phone to google a place name and get a reference height in return. Having tried this approach with a well known place (Trafalgar Square, London) yet I've not been able to get a height figure.
Anyone have any ideas on how to get height data via a phone/Google/website.
Thanks

 KellyKettle 21 Dec 2014
In reply to Indy:

You could try getting an accurate fix with phone gps, then calculating the actual altitude from the deviation between the referance elipsoid and the actual earth.


This site has both a calculator tool, and examples of how to derive that corrected altitude:
http://www.unavco.org/education/resources/educational-resources/tutorial/ge...
 ablackett 21 Dec 2014
In reply to Indy:

I'm not sure what you are asking, if you tell us why you want to do this it might become clearer.

If you want to calibrate some equipment, how accurate do you need your reference to be? If you just need one reference then Scafell Pike is 978m, nip up there, calibrate your equipment and bobs your uncle.

Will that do it?
 Chris Craggs Global Crag Moderator 21 Dec 2014
In reply to Indy:

I'm not sure how accurate you want to be but Google Earth gives it as 67' - between the fountains,


Chris
 KellyKettle 21 Dec 2014
In reply to Indy:

In terms of getting the data you need to derive the correct altitude out of your phone, this proves useful:

https://play.google.com/store/apps/details?id=com.eclipsim.gpsstatus2&h...
OP Indy 21 Dec 2014
In reply to ablackett:

> I'm not sure what you are asking, if you tell us why you want to do this it might become clearer.

I'd like to get meaningful figures when using various kit like Garmin's Edge & Forerunner. The Edge 1000 reported an elevation of -109ft while stood in Trafalgar Square this morning. Was cycling and running in the french Alps last month and it seems silly to have the function yet not feel the data is of any use. Yes it might be relative but would still prefer as accurate as possible.

Not looking for inch perfect data but being able to calibrate kit before use via a phone seems the easiest way.

Alowen 21 Dec 2014
In reply to KellyKettle:

Good app, thanks.

Al
 Mr Trebus 21 Dec 2014
In reply to Indy:

Any Trig point would do.
In reply to Indy:

Almost impossible??? Britain and its maps are covered with trig points!
 Glug 21 Dec 2014
In reply to Indy:

You won't get good quality height data from a single GPS receiver. I've turned on the one I use for hill walking and it's currently showing +12m to the true height above Ordnance datum. If I switched it on again in a few hours time it may be showing -12m, although that's good enough for many purposes. If you're calibrating something that relies on air pressure then OS maps give spot heights as well as contours that you could use. Google Earth is useful, if you hover the cursor over an area of land it's usually within a few metres. With both Google and the OS maps it's best to pick a relatively flat area so your x,y position isn't too critical.
 Rick Graham 21 Dec 2014
In reply to Indy:
Try searching on "bench marks your location " .

Worked well in Coniston, OS bench marks and their exact height above Newlyn Datum.

Will save a walk up a hill and exact to a mm.
Post edited at 18:06
 Rick Graham 21 Dec 2014
In reply to Rick Graham:

Try this one.

www.bench-marks.org.uk
 Richard Wilson 21 Dec 2014
Take a trip to the seaside.

Stand on the beach at the waters edge.

Set device to sea level.
In reply to Rick Graham:

Actually, they're not exact to a mm, or often even to 10s of mms. In fact most were last checked decades ago, and they're no longer maintained by the OS. They are, however, good enough for the purpose required here if the one you're looking for isn't buried, covered in vegetation, or been destroyed.
 Brass Nipples 22 Dec 2014
In reply to Indy:

Look at a map and then enter the height as one of your reference points on your GPS? Make your house one if those reference points so it always calibrates when you turn it on.
 chrisbaggy 22 Dec 2014
In reply to Rick Graham:

Thats assuming they haven't shifted/ moved as they are no longer maintained as with trig points.

Ive know a Benchmark being 400mm out..... it was in a wall and the wall had been taken down and rebuilt!

accurate enough but not quite to the mm

Chris
 nniff 22 Dec 2014
In reply to Indy:

Not exacltly sure of my ground here (dredging up my reading of the detailed Magellan manual many years ago when all this was new-fangled), but....

A GPS device requires one more satellite to be in view to calculate height (4 IIRC) comnpared to calculating location (3). In a built-up area such as London the probability of this is diminished as your horizon is high (ie the true horozon is obscured by the buildings). Similalrly, you have the same problems in valleys and can encounter it too in mountains where your line of sight is constrained by the bulk of mountain itself. IOnce you get to the top, you're fine and your device will tell you something you already know.

For working level calibration, go to the Stanfords or the shop in Victoria whose name I always forget and get a 1:10,000 map of somewhere convenient and go and find an identifiable location on a contour line. Or 1:25,000 on Bing maps

 BStar 22 Dec 2014
In reply to Richard Wilson:

And at which tide would you do that?
 Gone 22 Dec 2014
In reply to Indy:

NASA have an elevation database that they made by flying the space shuttle around the planet bouncing radar off the earth, and it has been available to the public for some years with measurements taken every 90m on the ground, and they are now upgrading the data to every 30m. Pretty cool huh, knocks trig points into the shade.

http://www.sciencedaily.com/releases/2014/09/140923154150.htm
OP Indy 23 Dec 2014
In reply to Gone:

Thanks everyone for the suggestions. Cross referencing some of them with each other shows some alarming descrepencies. Its odd in this world of informarion overload that such a basic bit of informartion is so hard to find to any sort of accuracy.

Am going down to Bath tomorrow with the wife and was looking to cycle from there to Devizes after all the work is done and the women folk have their natter. Not sure how much time I'm going to have so might start closer to Devizes (Batheastern, Box?) but where ever I do would be good to get a correct elevation number as the ride has quite a bit of climbing.
 d_b 23 Dec 2014
In reply to wurzelinzummerset:

In theory you can get an accurate measurement by averaging out the noise in the signal over a long period of time. Most consumer GPS kit doesn't have the software for it as it's more designed for getting a reasonable fix now.

The underlying chipsets are pretty much the same though, so you could probably do something with a phone and an averaging app or a normal walkers GPS and a laptop.

Throw DGPS into the mix and things get a lot more accurate, but you still probably want to average it if you want to be fanatical about accuracy.
 Damo 23 Dec 2014
In reply to Indy:
> I'd like to be able to calibrate some equipment

I'm not sure how accurate you want to get, but…

Horizontally, most late-model consumer outdoor handheld GPS units do the averaging if you leave them on for a while and should be accurate to within a couple of metres. i.e. probably get 5m accurately straight away, 3m if you leave it on the spot a few minutes, possibly less than 1m if you sit there half an hour.

Vertically, most of these are pretty rubbish and are regularly out 20m-30m vertically, despite what it says on the box. Leaving it turned on there will help a bit but I wouldn't expect better than 5m accuracy vertically, if that.

If you want sub-metre accuracy in the vertical plane i.e. exact altitude, you need a DGPS and process the results.

I'm sure some of the OS trig points are very accurate. Some not so much.

Google Earth is amazing but the heights can be WAY out, depending where you're looking, the quality of the imagery etc.

SRTM (space shuttle) data is good, but has gaps and other vagaries similar to Google Earth and may not be in a form useable to you.
Post edited at 12:15
In reply to Indy:

GPS altitude is about 3x as inaccurate as horizontal position, and the error varies randomly. So there's little point trying to correct it with a fixed value.
In reply to captain paranoia:

And when I say 'varies randomly', I mean with a fairly short timescale, as it varies with satellite constellation, with each satellite passing through the sky about twice per day, with a usable visibility of about four hours each time it passes. You'll generally see about 8 satellites, moving through their orbits, which should give you some idea of the dynamics of the constellation, and thus the error; the constellation can change quite a bit in 15-30 minutes.

Then there are the true random errors, caused by atmospheric and local multipath conditions...

(For any geodesists reading; I'm trying to keep it simple, and use terms most people will be familiar with...)
 nniff 24 Dec 2014
In reply to Indy:

If you start at Box, the cross roads on top of the tunnel has a spot height of 143m according to OS 1:25000. There's a 37m spot height on the road on the other side of the canal from the George in Bathampton.

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