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/