map reference conversion

General discussion about trigpoints, the TrigpointingUK website etc

map reference conversion

Postby simon.ritchie » Thu Jul 18, 2019 6:03 pm

This site uses OS map references to specify the positions of the trig points. Unfortunately, some GPS devices use longitude and latitude, so we need a way to convert from one to the other.

Initially I used the Movable Type web page, but I found that it uses a very simple algorithm and can be fairly inaccurate: https://www.movable-type.co.uk/scripts/ ... idref.html

I then discovered a web page run by the Ordnance Survey, which is much more accurate and handles all the formats that I know about, converting from one to all the others: https://www.ordnancesurvey.co.uk/gps/transformation/

I'd like to be able to do this conversion using a program. If anybody knows of a library that can do it accurately, preferably one that can be used from C or C++, I'd be grateful to know about it.

By the way, I also found this site useful. Given an OS map reference it displays a local map - very useful for figuring out how to get to a trig point: https://gridreferencefinder.com/
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Re: map reference conversion

Postby agentmancuso » Thu Jul 18, 2019 11:05 pm

Wouldn't it be easier to change the GPSr coordinate setting to use BNG?
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Re: map reference conversion

Postby clochandighter » Thu Jul 18, 2019 11:52 pm

Is this of any help?

http://www.fieldenmaps.info/cconv/cconv_gb.html

To be honest, I agree with agentmancuso. If you have an interest in trigs, their referenced positions were all calibrated on the National Grid. Is it possible your GPSr is capable of working on the British grid? I am aware that there are others here using the Lat/Long system, possibly navigating with car SatNav system.

Out in the field, I still navigate with paper maps! :-)


Rgds,

G.
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Re: map reference conversion

Postby stripybadger » Fri Jul 19, 2019 8:29 am

Hi Simon, to try to answer your question about a library to do it, I've previously used proj4js (which is actually a javascript port of the c++ library 'proj'). I've generally found that the inaccuracy introduced by the conversion does not make things worse than the inaccuracy in the original reference, but I don't have any stats to back that up.

However you also have to consider the accuracy of the thing you're using to display those coords. Whether you're displaying them on an online map or entering them into a gps device, the coords probably get transformed into yet another format, which will potentially introduce yet more inaccuracy - and if using a handheld gps, you've then got yet more inaccuracy from the actual gps location to add to this, which I'd guess would be much greater than any inaccuracy in your original transformation.
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Re: map reference conversion

Postby simon.ritchie » Fri Jul 19, 2019 12:49 pm

> Wouldn't it be easier to change the GPSr coordinate setting to use BNG?

The OS web page takes a set of coordinates in any form and converts it to all the other forms, so you can do whatever is convenient. I'm using the trigs to calibrate my GPS equipment. I choose a suitable trig, get the map reference, convert it to longitude. latitude and ellipsoidal height, and note down the result. Then I take the equipment to the trig, get the position and compare it with the expected result. Doing it that way means that I don't have to cart around a computer to do the conversions at the trig.

Of course, an accurate GPS device that produced an OS map reference would be much handier. That's why I would like to know about any programmatic solutions.

Regards

Simon
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Re: map reference conversion

Postby agentmancuso » Fri Jul 19, 2019 1:10 pm

Isn't it likely that the transformational error in converting coords in one format to another is at least as great as the error in satellite-derived position, assuming the GPS equipment we're talking about is something more substantial than an etrex?
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Re: map reference conversion

Postby simon.ritchie » Fri Jul 19, 2019 6:15 pm

Concerning accuracy, a GPS device like an eTrex is only accurate to about four metres(?) so, yes, the inaccuracy in the conversion would just be in the noise.

The equipment I'm testing is the new U-Blox ZED-F9P which claims to be much more accurate, and the inaccuracy in the conversion starts to matter. The Movable Type page wasn't up to the job.

Once I've finished my testing, I'll be able to say just how accurate this device is in practice.
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Re: map reference conversion

Postby agentmancuso » Fri Jul 19, 2019 7:28 pm

I see, it starts to make sense now! I used to use the BGS site for batch conversion of coordinates, but I'm afraid programme language stuff is beyond me. Good luck finding something more suitable; it would be interesting to hear how you get on in the end.
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