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Wgs84 Or Osgb For Gps In The Uk?


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Hi all.

 

Although we've now used our yellow Etrex GPS three times without any problems, I noticed when messing around with it last night that it is set to use the WGS84 map datums. As we live in the UK I'm wondering whether it would be better set at Ordnance Survey GB - or does this apply only to more sophosticated units that show map details etc?

 

Thanks in advance,

 

Steve.

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As the wizard says, all coordinates on GC.com are listed in WGS84 format, and most multicaches in the country will use the same. The only time I ever switch to OSGB is in the firld, if I have an OS map with me and want to plot some calculated coordinates on it. And even then, if you only need a rough idea, newer 1:25,000 scales cetainly have Lat Long on them as well.

 

Stick to WGS84!

 

Dave

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As purple_pineapple says you should only change it if you want to use GB Grid References (usually easier to work with esp. with an OS map), in which case you need to change the display format and the datum.

Display format: variation of lat/long - Datum: WGS84

Display format: British National Grid - Datum: OSGB36

Display format: Irish Grid - Datum: Irish '65

 

The only exception if you want to use the lat/long grid on the side of OS maps then you do need the datum set to OSGB36. (about the only place you will see lat/long on the OSGB36 datum)

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Which will explain why some of the markers in the UK for the Greenwich Meridian are in the wrong place,I know of some placed to osgb and some placed to wgs84, in nearby villages.

I guess someone with a gps volenteered to do the siting without looking into it properly.

 

See this for some reasoning behind the meridian differences.

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The only exception if you want to use the lat/long grid on the side of OS maps then you do need the datum set to OSGB36. (about the only place you will see lat/long on the OSGB36 datum)

 

So are the Lat/Long numbers on an OS map based on the OSGB datum??? I always assumed they were based on the International datum. Two thoughts occur - 1) that might explain a feplotting problems I've had in the past and 2) WHY?? Surely if one is using the OSGB datum, one is likely to be working in OSGB grid format, and vice versa...

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I've had a lot of trouble at work with staff putting an seemingly-incorrect OS grid reference on some of our emergency telephones.

The references are in the two letter, six digit format (e.g. TL345123) and I find digit three is one out and digit six is 4 or 5 out (so find something like TL344127).

 

In every case, the GPSr that has been used is the Garmin eTrex yellow.

Main reason appears to be that it defaults to WGS84 and this datum stays when they switch to Ordnance Survey GB. Once they manually set the datum to OSGB36, all is fine.

Other Garmin models I have used (e.g Venture) seem to automatically change the datum to suit, but the yellow one doesn't, so you have to know what you're doing!

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Thanks Wizard.

 

Those settings don't make much difference then?

 

Steve.

 

Just to clarify, those settings DO make a difference - the amount of difference varies across the country, but if you're using the wrong format you could be up to a few hundred metres away from where you think you are.

 

There are websites out there that explain the differences, but you don't really need to know...just remember that WGS84 is the one to use for geocaching, and most other things <_<

 

HTH

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So are the Lat/Long numbers on an OS map based on the OSGB datum??? I always assumed they were based on the International datum. Two thoughts occur - 1) that might explain a feplotting problems I've had in the past and 2) WHY?? Surely if one is using the OSGB datum, one is likely to be working in OSGB grid format, and vice versa...

 

Yes they are. The reason being that OS maps quite simply predate WGS84 (1984 :unsure: ), and the OS are a very tranditional organisation... Besides if they where to change it there would be two different datums used on maps and lead to further errors.

 

Using lat/long is actully quite a specalist (and count geocaching in that :unsure: ) use, and I guess they assume if you using lat/long then you will know about changing datums :unsure:

 

As for the problems with getting the datums mixed, it can easily mean errors of about 200m, depending on location.

 

Edit: just remembered I did put this document together to hopefully explain things:

http://www.nearby.org.uk/gps_ll_v_gr.html

Edited by barryhunter
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Fair enough. I realise of course that OS maps predate the WGS84 datum, although I hadn't noticed the Lat/Long grid on the maps until recently. I assumed this was due to the rise (?) in use of GPS and the WGS84 datum. Probably wrong on both counts though! :unsure:

 

Anyway, now I now, I'll just have to change to OSGB format and datum if I want to plot on the map. Easy enough. :unsure:

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