[Talk-GB] accurate GPS
Dave F
davefoxfac63 at btinternet.com
Wed Oct 9 11:03:57 UTC 2019
2cm? I'm intrigued, what model are you using?
What were the atmospheric conditions on the day you took your reading?
DaveF
On 09/10/2019 11:05, Simon Ritchie wrote:
> I've been working with some GPS equipment that claims to be accurate to
> 2cm. To test it, I've been visiting local OS trig points, taking position
> measurements and checking if they are correct.
>
> Unfortunately I've discovered that the data I'm getting from the OS is not
> nearly as accurate as my equipment claims to be, which is wrecking my
> testing.
>
> We tend to assume (well, I do anyway) that OS trig points are very accurate
> position markers, but compared with modern equipment, that's no longer so.
> I thought people might be interested in knowing how accurate they are.
>
> A related issue is this: GPS devices don't work in terms of OS map
> references. If your tracker device gives you a position in that form, it's
> done a conversion. How accurate is that?
>
> The GPS device in a typical tracker is accurate to maybe three metres, so
> the position you see on the screen will always be a bit wrong. If you get
> it to display your position in OS map reference form, it will need to do a
> conversion, which introduces an extra error, so the result will be even
> more wrong. Not good if you are trying to produce an accurate map.
>
> The OS published a spreadsheet giving the positions of their trig points in
> OS map references. This is available from them as a spreadsheet and Ian
> Harris has used that data to create the web site:http://trigpointing.uk
>
> The OS also offer a web page that can convert this to other forms including
> Cartesian, which is one of the forms that my GPS device gives me:
> https://www.ordnancesurvey.co.uk/gps/transformation/
>
> To test my equipment, I take the OS map ref of a trig point, convert it to
> Cartesian form, visit the trig point, get the position in Cartesian form
> from my device and compare the two.
>
> The results are typically out by at least half a metre. Is my equipment
> faulty, or is the OS data wrong. How accurate is the published position of
> the trig point and, when I use the OS web page to convert that to Cartesian
> form, how accurate is thatt?
>
> This OS document was very enlightening:
> https://www.ordnancesurvey.co.uk/legacy/docs/gps/updated-transformations-uk-ireland-geoid-model.pdf
> It explains how the Cartesian coordinates work, which is useful. It
> reminds me that OS maps pretend that the Earth is flat, which introduces an
> error, but that's tiny, and for my purposes itcan be ignored. It explains
> how accurate you can expect the published measurements of trig point
> positions to be - they can be out by as much as 60 cm! In general, the
> document stresses that there is no sure-fire way to convert a position from
> one system to another. The result will always be inaccurate.
>
> So now I know that the published positions of the trig points are a bit
> wrong, but how accurate is the conversion from OS map ref to Cartesian form?
>
> OS map references plus height above sea level and Cartesian coordinates
> both specify a position using a 3D coordinate system. The origin and the
> direction of the axes are different in each system so you can't compare thm
> directly. However, the distances between two points should be the same
> regardless of which system you use. If you have two points in the same
> coordinate system (a1,b1,c1) and (a2,b2,c2) and the difference along each
> axis is a,b and c then the distance between them is
>
> the square root of (a squared plus b squared plus c squared) by
> Pythagoras
>
> If you have two points in a different coordinate system representing the
> same two positions, the distance between them should be the same.
>
> So I can test the accuracy of the conversion from OS map references to
> Cartesian. In the table below, on the left, we have the trig points at Box
> Hill and Leith Hill in OS map reference form, the difference along each
> axis and below that the resulting distance. On the right we have the same
> calculation but using the Cartesian coordinates from the OS conversion page.
>
> Below that I do the same comparison, this time using the trig point at
> Mickleham Down and the one at Leith Hill.
>
> In both cases, the distances are out by over two metres.
>
> So, I'm trying to test equipment which is supposed to be accurate to two cm
> using data that is out by at least two metres. That's not going to work.
> I need something more accurate to compare my results with.
>
>
> OS Map Ref
> Cartesian
>
> Box Hill Leith Hill Difference Box Hill Leith Hill
> Difference
> easting 517971.06 513949.28 4021.78 x 4000676.63 4006902.33
> -6225.70
> northing 151163.16 143161.71 8001.45 y -21724.35 -25963.72
> 4239.37
> height above 171.97 307.00 -135.03 z 4950992.32 4946141.89
> 4850.43
> sea level
>
> distance 8956.35 8958.70
>
>
> Mickleham Leith Hill Difference Mickleham Leith
> Hill Difference
> easting 517891.74 513949.28 3942.46 x 3998820.07
> 4006902.33 -8082.26
> northing 153518.13 143161.71 10356.42 y -21739.43
> -25963.72 4224.29
> height above 142.73 307.00 -164.27 z 4952444.39
> 4946141.89 6302.49
> sea level
>
> distance 11082.66 11085.53
>
>
>
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