Ive been using my Geko 201 for a while now to navigate succesfully on the hills..... (before anyone starts I also carry a map and compass and am proficient in their use also)

Recently though I discovered geocaching and have been doing this every so often when I'm bored and the weather is nice.

I couldnt find my last cache and I know why. Human error....mine to be exact. I met a nice fella on the walk in to the cache who asked what I was up to. He had never heard of geocaching so I ended up giving him my printout from the geocache page to look up when he got home. After a little searching and reading the map a little closer I realised I was in the wrong area and gave up. Anyway I digress........

When I returned home I logged in to the geocache page and confirmed that I had got a digit wrong when entering the coords. I wrote this in on my log and some nice helpfull chap emailed me suggesting that the OSGB coords dont work as they have an inbuilt conversion error? He reckons I should use the WGS 84 datum and deg and dec mins instead of OSGB. I thought that my GPS is set to the WGS 84 datum anyway and the OSGB is just a "display" but now I'm not so sure? can anyone shed any light on this? what datum should I use and will it make any difference changing the OSGB coords to deg and dec mins?

Hi BBF,

I'm not a technical gps guru, but I've been using a Geko 201happily in a number of countries since I bought mine when they first came out.

There are 2 settings that affect how your gps reports its position - Map Datum and Coordinate System.

In the UK our Map Datum is OSGB because we're typically using map data surveyed from the Ordnance Survey. Accompanying that we most normally represent our position in terms of the Bristish (National) Grid system. On the Geko if you set OSGB as the Datum it automatically defaults to British Grid, but you could override this if, for example, you wanted to see your position displayed as Lat/Lon.

Other countries produce maps based on other survey datums such as WGS84 or European 1954, and these are often used in conjunction with (displayed in terms of) the UTM grid system. If you use the wrong datum then your position will display incorrectly - on a trip to Norway in the early days of my gps use I moved from one map to another and suddenly all my UTM gridrefs seemed to be about 100m to 200m out. It turned out that this particular map was older and based on European 1954 datum while my gps was still set to WGS84 from the previous map.

Most paper maps print in the legend somewhere what datum and grid they are using. In the UK OSGB and British Grid would be the defaults. I don't know a lot about geocaching - do you get your locations off a website,and is it international, could the positions be based on something other than OS data? It seems unlikely to me, but possible perhaps.

One other point, as far as an 'inbuilt conversion error', I can only say that I've never experienced any such error in all my gps usage, provided my datum and grid settings tally between the map and the gps unit.

Don't know if that helps much.....? 

Great explanation Matt, but..........

Technically there is an inbuilt conversion error, when switching between OSGB and Lat/Long although it's minimal. The Lat/Long system is based on a 3D Earth, with lines of longitude diverging and coverging dependent on their proximity to the equator or poles. OSGB maps use a flat 2 dimentional grid system with parallel lines that makes conversion amazingly complex.

The Ordnance Survey grid is a Transverse Mercator projection with an origin at  49°N, 2°W based on the Airy 1830 ellipsoid using the OSGB36 datum.  GPS is based on WGS84/GRS80 which can vary from OSGB36 by as much as 120m or 6" or arc .OSGB36/Airy is a better fit to the UK geiod than the geocentric WGS84 which covers the entire world.The OS describe their Co-ordinates system here.

In reality the error is rarely noticeable and although OSGB36 dataum is more accurate for UK use WGSB84 is a worldwide datum and is the default for use in the UK for most GPS receivers.

Presumably the lat long marked round the edge or the map on the thick black border uses OSGB36.

So the answer would be that you should use whatever datum the person who set the cache used, WGS 84 is the most likely.

p.s. if you want to see how much the transformation tweaks the lat/long, have a look at these images:

> So the answer would be that you should use whatever datum the person who set the cache used, WGS 84 is the most likely.

In the UK, I'd expect OSGB36.  Unless they've got their GPS receiver set to WGS84 for some reason known only to themselves...

Right Im @work sitting at my desk with the GPS and the manual.

If I go into the "units" menu its says "Position format = British" and "Map datum=Ord srvy GB" It wont let me into the "Map datum" like this. "Units=metric" "North ref-Grid" "variation=002"

I take it this is the correct way i should have the GPS for Nav on the hills in conjunction with an OS map? I bloody hope so cos thats how I've been using the last year or so and Ive not been lost.

If I change the "Position format" to "Position format= H D M.M" The "Map datum" automatically changes to "Map datum=WGS 84" It now lets me change this field to a anything from a list as long as your arm. WGS 72 is on it but no WGS 36? If I leave it this way i.e. "Position format=H D M.M" and "Map datum=WGS 84" I take this will be the most accurate for geocacheing?

OSGB36 is represented by the text "Ord Srvy GB" on my etrex for both Britsh Grid (where it's the only option as this is the only datum supporting the National Grid) or Lat/Long.

You've chosen the most likely settings there.

> If I go into the "units" menu its says "Position format = British" and "Map datum=Ord srvy GB" It wont let me into the "Map datum" like this.

Okay, that's good; it understands that you're using OSGB grid references for your 'units' (i.e. position display), so it MUST use Ord srvy GB (which is the 1936 Airy geoid, rather than the WGS84 geoid) for the Map Datum.

Using any other datum would cause errors in the re-projection process.  A bit like if you use the Swiss National Grid to re-project the UK mapping data:

Thank you to all those who posted to this article.

I use my Geko 201 with my PDA when walking and mountain biking, I could get it to work on my Anquet maps for off road but wanted it to also work with my TomTom Navigator 6 for road sections, this has helped solved my problem.

Anquet maps on the PDA uses the OSGB datum but with these settings on my geko my TomTom Navigator 6 software was miles out!

Having changed my Geko to WGS 84 datum configured with H .D D° my Tom Tom now works, I just have to remember to swap the config when changing between the two systems! 

Although this was my thread originally asking for help it appears I neglected to thank people. So thank you all

I now regularly switch between the 2 settings as there are parking coord listed sometimes on the cache pages but only in Degs and Mins.