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Errors In Some Os Refs On Geocaching Site

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I have noticed a problem in the Geocaching site's conversion from LAT/LONG to the British OS Grid reference.


The Geocaching algorithm knocks off any leading zeros. This means that with any place with a leading zero the British OS map references don't reflect accurately the LAT/LONG


The correct British Grid format should be AA 11111 11111 (2 letters followed by 2 sets of five digits). But the Geocaching algorithm wipes out any leading zero at the front of either of the two sets of five digits.


For example


For waypoint GCJV0P Geocaching incorrectly gives the British OS Grid reference :


SJ 5238 65821


It should read


British Grid: SJ 05238 65821




Similarly, for waypoint GCHW76 the Geocaching British Grid reference shows:


TF 62075 1762


It should read as


British Grid: TF 62075 01762


I've written to GEOCACHING HQ about this but only had a "got your email" reply so far. This is a serious problem that will need fixing soon. But at least you know why it screws up your OS map reference.

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I only started geocaching this year and to begin with I was fooled by this stripping of leading zeros in the geocaching algorithm but soon cottoned on to what was happening.


Even so, I agree with Happy Landins that it should be a simple error to correct and I am surprised that it hasn't been sorted before now as I'm certain we are not the only ones to have noticed this glitch!?


I use these British Grid co-ords all the time ... first to find areas on Multimap and OS websites and secondly in the field when taking compass readings on OS maps.


I'm willing to bet I am not alone in using OS maps when necessary....and even knowing about this glitch....I have still been caught out on occassion.



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Thanks for reporting this "feature". I've not noticed it as I never use OS co-ordinates (I use Memory Map on the PC/PDA). However I am sure many people use normal OS paper maps for navigating their way around so this could be a problem.


I'll push the problem "upstairs" and let you know what happens.


Cheers, Lactodorum

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I use the grid ref and map to find out roughly where the cache is in relation to area and parking etc. then use Lat / Lon to find the exact cache site. Some caches only give Lat / Lon. Although some times I wish you could swap between the two more easily, and didn't have to keep changing the GPS settings. Kev. :P

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I frequently use the OS Grid references, and this is why and how . . .


I have found this useful link on the OS site




You can punch in the OS ref (leaving no blank spaces) e.g. SJ1234576543


and it will give you a 1:25000 high res printable map showing the exact location of the cache. If you choose the 1:250000 scale this lets you see the roads in the area so you can find where the 1:25000 is located (again centred on the cache)


This is a handy hint because it gives you the exact part of the map you want, at the best available scales and, best of all, at no cost!


Thank you Mr OS Map maker!

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The "Geo" conversion is also slightly different from that done by my eTrex Legend, and also by the OS document which gives precise formulae (obtained this in hard copy format many years ago, and have yet to discover if they supply now as a pdf (I also have a similar document from the Swiss mapping agency, which uses an oblique Mercator transformation, and which also does not appear to be available officially as a pdf))


I think it all depends on whether you use a rough and ready transformation, a 3-parameter shift, or a 7-parameter shift. Not surprisingly, the Swiss go the whole hog and use 7!


If there is any interest, I might be persuaded to scan and pdf either or both of these documents for the usual fee: hint, think multiples of 568ml.



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