British Grid v Desgrees and Minutes Reference Errors

[+Quadbarrel]

Ok. Apologies in advance for a newbie question. I have tried looking round the forums but can't see an obvious (in language I understand) answer to this.

 

Quite new to geocaching but loving it.

 

But. I use Garmin Etrex which is great. When I use the British Grid system, the references I get for the geocaches are spot on. But when I use the Degrees and Minutes, there seems to be some problem with the Westing reference.

 

For example. Went to "Floak" yesterday. GCXVMG.

 

British Grid is NS50602 51888. Stood at the cache I got NS 50603 51890. So just about spot on.

 

The Degrees and Minutes Reference is N 55 44.239 W 004 22.866. I got N 55 44.240 W 004 22.785.

 

So the Northing was fine but the Westing is out sufficiently to make it just about impossible to find a cache using this reference. I check ed 2 others yesterday and they both were a similar error on the Westing with the Northing fine (having found the cache using the British Grid).

 

I have a feeling there will be an obvious answer and I will feel stupid. But I am happy to feel stupid if someone can help!!

 

Thanks in advance to all the celeverer people out there!

 

QB.

Edited September 21, 2008 by Quadbarrel

[+Simply Paul]

Sounds like a Spheroid/Datum thing to me. 300ft is the usual difference. When you switch from DM.M to BG, does it swap from WGS84 to something else?

[+Just Roger]

Your problem is in the datum you are using. OS grid references use the OSGB datum while all geocaching lat and Long data uses WGS 84. You must change over to the correct datum when you change the coordinate system you are using.

[+Quadbarrel]

Sounds like a Spheroid/Datum thing to me. 300ft is the usual difference. When you switch from DM.M to BG, does it swap from WGS84 to something else?

 

Hmmm. Now that sounds like a good question, but my dumb reply is along the lines of how would I know?

[+Quadbarrel]

Your problem is in the datum you are using. OS grid references use the OSGB datum while all geocaching lat and Long data uses WGS 84. You must change over to the correct datum when you change the coordinate system you are using.

 

And dumb questions continuing, is that change done in my GPS unit?

[+Simply Paul]

Yep, in the menu somewhere. I don't have my etrex to hand to talk you through it, and yours may be different anyway, but it's in there under Datum, I think. Just select WGS84 and you'll be laughing. It's the same thing which stops your GPS saying W000 00.000 when stood on the 'line in Greenwich.

[+Just Roger]

Your problem is in the datum you are using. OS grid references use the OSGB datum while all geocaching lat and Long data uses WGS 84. You must change over to the correct datum when you change the coordinate system you are using.

 

And dumb questions continuing, is that change done in my GPS unit?

 

On my Etrex Venture it is on the main Menu - SetUp - Units. The same place as you switch from DDD MM.mmm to OS Grid. The next box down lets you select the datum in use from a dropdown list.

[+Quadbarrel]

Ok, just done a wiki search on OSGB following Simply Paul and Just Roger's replies. Gives a great description of the differences between British Grid OSGB and WGS84. Just need to see if I can change the settings in my GPS.

[+Quadbarrel]

Ok, just done a wiki search on OSGB following Simply Paul and Just Roger's replies. Gives a great description of the differences between British Grid OSGB and WGS84. Just need to see if I can change the settings in my GPS.

 

Thanks guys. Seems to be simple and I will go and try it out this afternoon!!!

[+Simply Paul]

If you struggle too much, here's the PDF of the yellow/cammo current etrex manual on Garmin's website.

 

Edit: Yay

Edited September 21, 2008 by Simply Paul

[+drdick&vick]

Must be model orientated as just checked on both mu Vista Etrex's (Legend HCx & Vista Cx) and the both change the Datum automatically when I switch between British Grid and Degrees and Minutes.

[+careygang]

The good old faithful eTrex Yellow has to be done manually through the menus. If there is a miss match between the 2 then the error can be several 100 ft.

 

See pages 35/36 of the document that Simply Paul posted.

[+Quadbarrel]

Must be model orientated as just checked on both mu Vista Etrex's (Legend HCx & Vista Cx) and the both change the Datum automatically when I switch between British Grid and Degrees and Minutes.

 

Great. Just been out and found 4 geocaches. DrDick&Vick is right that I need to manually select the datum in my etrex. When I did that I got the coords spot on using both systems so thanks to all for the sound advice.

 

Happy geocaching!!

[Garmin8888]

Why don't UK Geocachers use the OSGB data then we can see it on OSGB maps!

[big_bob]

Why don't UK Geocachers use the OSGB data then we can see it on OSGB maps!

Because Geocaching was invented by Americans and is intended to be an international game. It uses an Internationally recognised system.

[+Team Sieni]

Why don't UK Geocachers use the OSGB data then we can see it on OSGB maps!

 

Deg/min in WGS84 is the normally the default setting for handheld GPS and works consistently all over the world. If each country used its own local system then Geocaching would be pretty unmanageable as the website would have to handle loads of competing systems.

 

I often find it a bit of a fiddle when doing multi caches, when I work out the next destination (in deg/min) then have to switch my GPSr to OSGB to find the GR to locate it on my paper map, and then back again to work out the next point. Serves me right for being a paper-map-using dinosaur I suppose.

 

I sometimes wonder if GS ever considered using UTM as the default co-ordinate system instead of deg/min when they started out.

[Edgemaster]

UTM is even more foreign to us, even though it too is an international system.

At least DMS is fairly straightforward in what the numbers mean.

[+walkergeoff and wife]

Why don't UK Geocachers use the OSGB data then we can see it on OSGB maps!

 

Deg/min in WGS84 is the normally the default setting for handheld GPS and works consistently all over the world. If each country used its own local system then Geocaching would be pretty unmanageable as the website would have to handle loads of competing systems.

 

I often find it a bit of a fiddle when doing multi caches, when I work out the next destination (in deg/min) then have to switch my GPSr to OSGB to find the GR to locate it on my paper map, and then back again to work out the next point. Serves me right for being a paper-map-using dinosaur I suppose.

 

I sometimes wonder if GS ever considered using UTM as the default co-ordinate system instead of deg/min when they started out.

There is an excellent programme for the Palm called Mimee which does this for you.

[+Amberel]

I often find it a bit of a fiddle when doing multi caches, when I work out the next destination (in deg/min) then have to switch my GPSr to OSGB to find the GR to locate it on my paper map, and then back again to work out the next point. Serves me right for being a paper-map-using dinosaur I suppose.

If you have a PocketPC I can send you a conversion program.

 

Rgds, Andy

[Deceangi]

I believe WGS 84 was selected as the majority of GPSr's come straight out of the box as they are set to this by the manufacture.And as a reference the settings are

 

 

WGS 84

 

Position Format: hddd°mm.mmm

 

Map Datum WGS: 84

 

OS GB

 

Position Format: British Grid

 

Map Datum: Ord Srvy GB

 

it's a common mistake when swapping from WGS 84 to OS GB and back on units where both have to be set manually to only change one and not both

[+Team Sieni]

it's a common mistake when swapping from WGS 84 to OS GB and back on units where both have to be set manually to only change one and not both

I did that on a maintenance visit once. I nearly had to log a maintenance DNF. (DNM?) Even more galling as I only have one cache so I should be able to find it without a GPS.

[+lodgebarn]

I did that on a maintenance visit once. I nearly had to log a maintenance DNF. (DNM?) Even more galling as I only have one cache so I should be able to find it without a GPS.

On one of my very early caching adventures I managed to do exactly this in the USA Musicians' Cache #1. I only had an hour or so to find this one (I needed to drop a TB) before my return home and the GPS was unhelpfully pointing to the middle of a lake . I was sure I had the correct answer to the puzzle. Panic mode set in but I managed to find another screen where cache was showing a different spot so went for it. This then involved a dash around the lake in 90 degree temperatures only to find hundreds of items matching the hint. It all ended well when by some miracle I chanced across the actual location . When I got home, a few google searches revealed the reason. Just before I left I had demonstrated the wonders of GPS mapping and how I could switch to OS grid positions at the flick of a button. The device had helpfully switched automatically to OSGB but not back again. Lesson learned!