[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?


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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