+Quadbarrel Posted September 21, 2008 Share Posted September 21, 2008 (edited) 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 Quote Link to comment
+Simply Paul Posted September 21, 2008 Share Posted September 21, 2008 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? Quote Link to comment
+Just Roger Posted September 21, 2008 Share Posted September 21, 2008 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. Quote Link to comment
+Quadbarrel Posted September 21, 2008 Author Share Posted September 21, 2008 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? Quote Link to comment
+Quadbarrel Posted September 21, 2008 Author Share Posted September 21, 2008 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? Quote Link to comment
+Simply Paul Posted September 21, 2008 Share Posted September 21, 2008 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. Quote Link to comment
+Just Roger Posted September 21, 2008 Share Posted September 21, 2008 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. Quote Link to comment
+Quadbarrel Posted September 21, 2008 Author Share Posted September 21, 2008 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. Quote Link to comment
+Quadbarrel Posted September 21, 2008 Author Share Posted September 21, 2008 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!!! Quote Link to comment
+Simply Paul Posted September 21, 2008 Share Posted September 21, 2008 (edited) 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 Quote Link to comment
+drdick&vick Posted September 21, 2008 Share Posted September 21, 2008 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. Quote Link to comment
+careygang Posted September 21, 2008 Share Posted September 21, 2008 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. Quote Link to comment
+Quadbarrel Posted September 21, 2008 Author Share Posted September 21, 2008 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!! Quote Link to comment
Garmin8888 Posted September 23, 2008 Share Posted September 23, 2008 Why don't UK Geocachers use the OSGB data then we can see it on OSGB maps! Quote Link to comment
big_bob Posted September 23, 2008 Share Posted September 23, 2008 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. Quote Link to comment
+Team Sieni Posted September 23, 2008 Share Posted September 23, 2008 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. Quote Link to comment
Edgemaster Posted September 23, 2008 Share Posted September 23, 2008 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. Quote Link to comment
+walkergeoff and wife Posted September 23, 2008 Share Posted September 23, 2008 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. Quote Link to comment
+Amberel Posted September 24, 2008 Share Posted September 24, 2008 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 Quote Link to comment
Deceangi Posted September 24, 2008 Share Posted September 24, 2008 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 Quote Link to comment
+Team Sieni Posted September 25, 2008 Share Posted September 25, 2008 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. Quote Link to comment
+lodgebarn Posted September 25, 2008 Share Posted September 25, 2008 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! Quote Link to comment
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