+The Roos Posted January 31, 2005 Share Posted January 31, 2005 Hi, I`ve been doing map and compass work for years but am pretty new to GPS`s. Anyway, I`ve tried to place a couple of caches (my first two) recently and found the following. My GPS and map show the cache location as identical in Lat/Long and OSGB. However, when I enter the Lat/Long on the geocache site I get an OSGB conversion over 1km away! Am I doing something wrong or are Geocache.com converting wrongly? If it`s Geocache in the wrong how do I change the cache page co-ordinates for the OSGB? (I know how to change the Lat/Long). Any advice greatly appreciated. Quote Link to comment
Big Black Bunny Posted January 31, 2005 Share Posted January 31, 2005 Hi I am not sure on all the details but this has been asked before. It's got to do with different grid systems. Try searching the forums under "conversions" or "lat. and lon." Also try this thread. Click Quote Link to comment
60North Posted January 31, 2005 Share Posted January 31, 2005 Is it the WGS84 issue, where you are not changing the datum to OSGB? Quote Link to comment
+minstrelcat Posted January 31, 2005 Share Posted January 31, 2005 Are you using the correct map datum in your GPSr? It should be WGS84, not GRB36 - though I may not have got the names exactly right as it is off the top of my head! Lisa Quote Link to comment
+wildlifewriter Posted January 31, 2005 Share Posted January 31, 2005 Anyway, I`ve tried to place a couple of caches (my first two) recently and found the following. My GPS and map show the cache location as identical in Lat/Long and OSGB. However, when I enter the Lat/Long on the geocache site I get an OSGB conversion over 1km away! Am I doing something wrong or are Geocache.com converting wrongly? First thing: there doesn't appear to be anything wrong with the conversion displayed on the GC.com web pages. It isn't overly accurate, but pretty close, nonetheless. If this OP relates to your cache GCMME0 near Hambledon, there DOES seem - on the face of it - to be a problem with the co-ordinates. Follow this link to Ian's excellent UK trigpointing site which may illustrate what has happened. HOW it happened, we can only guess - the discrepancy seems larger than could be accounted for by an incorrect GPS datum setting. Hth -Wlw. Quote Link to comment
+The Forester Posted February 1, 2005 Share Posted February 1, 2005 As WLW has correctly pointed out, this does not appear to be a WGS84/OSGB36 issue. Your Latitude should have been somewhere between 51° 08.828 and 51° 08.832. Your Longitude should have been either 0° 36.199 or 0° 36.200. If you tell me the approximate compass bearing from the man-made feature mentioned in the encrypted clue to the natural one, I'll give you the exact co-ordinates in Lat/Long. The grid coords shown underneath the Lat/Long on the GC.com webspage for the cited cache are only out by a few metres, with respect to the listed geog position. The grid Eastings are a fair conversion from the stated Geog (Lat/Long) position, but the grid Northings at that location overread by about 5 metres. The actual location of the cache is, I think, in the apparently clear area just North of the "d" of "Hydon's Ball" in this sketch. Cheers, The Forester Quote Link to comment
+Bob Smith Posted February 1, 2005 Share Posted February 1, 2005 Get a copy of Grid Inquest from this site http://www.qgsl.com/software/gridiq.php It's free and I find it very useful for conversions. It replaces the old Excel spreadsheet converter which used to be on the OS website. Quote Link to comment
+wildlifewriter Posted February 1, 2005 Share Posted February 1, 2005 (edited) HOW it happened, we can only guess - the discrepancy seems larger than could be accounted for by an incorrect GPS datum setting. OK... I have now worked out how the incorrect co-ords for GCMME0 were arrived at, and - with apologies to Mr. & Mrs. Roo - I'll set out the sequence here. It's a classic worked example of what can happen when one departs from the geocaching "way of doing things" - and how incorrect GPS and other settings can result in a complete tangle... The Roos must have had their units set to "British Grid" - probably working to an OS map when on the ground. They placed their cache, and recorded grid ref: SU 97799 39588. But GC.com doesn't use British Grid for new cache reports... It then seems that they changed the display format on the unit to dd-mm-ss.s leaving the datum as OSGB36. On a Garmin GPS, this will now show N51º 08 48.0, W0º 36 06.0. This was entered onto the "Report a new cache" form - whose default format is dd-mm.mmm -as N51º 08.480, W0º 36.060 (Which is the position listed, over half a kilometer away from the physical cache.) Although events may not have happened in exactly that order, no other combination of mistakes fits the result. An interesting problem in navigational detective work, and a cautionary tale.... -Wlw [Edited for spacing] Edited February 1, 2005 by wildlifewriter Quote Link to comment
+Lizzzzeeeee Posted February 1, 2005 Share Posted February 1, 2005 I've just spotted this thread having just been the FTF on the cache mentioned. The coords I came up with when I averaged my GPS for a good 5 minutes (over 200 readings) were N51°08.825' W0°36.200' which are as near as damit those that Forester came up with. Fortunately Forester's note on the cache page prompted me to read the clue and I used that alone to find the cache - I was a bit suspicious when I put the published coords into Memory Map and came up with the middle of an apparently clear field! The description about views etc also seemed strange! I'm pretty sure wildlifewriter's sequence of events is probably accurate, I thought it would probably be a WGS84/OSGB36/decimal point in the wrong place - what I don't understand when these errors occur is how the cachers have already managed to find so many caches with their GPS set to the wrong thingy - maybe I should try caching without a GPS more often - for one thing you look much less suspicious! Quote Link to comment
+wildlifewriter Posted February 1, 2005 Share Posted February 1, 2005 I'm pretty sure wildlifewriter's sequence of events is probably accurate, I thought it would probably be a WGS84/OSGB36/decimal point in the wrong place - what I don't understand when these errors occur is how the cachers have already managed to find so many caches with their GPS set to the wrong thingy! Because, as I established (to my slight embarrassment) in this experiment - if a waypoint is downloaded from the site, and transferred (from PC) to a GPSr - it DOESN'T MATTER what datum or format the thing is set to: it'll still track to the position. However, as we have seen, it sure as hell matters when one is placing a cache! Well done on finding it - I reckon I should be awarded a find too , just for working all this stuff out... Quote Link to comment
60North Posted February 1, 2005 Share Posted February 1, 2005 I have a similar problem with a lot of the waypoints in my Garmin GPS 2+ Having been in the military for a few years I have always worked with OSGB. When I got the GPS I knew nothing about datums so I left the thing on WGS84, but had the position setting set to British Grid. I put the co-ordinates into the GPS from the Geocaching page manually and had entered all of the ones from this area into it. I then learnt about datums (or rather what it should be set to) and changed the GPS so that both were matched properly. Recently, when I went to an adopted cache, I drove past the cache, got out of the car, then wandered in completely the wrong direction. I knew that the location of the cache from the hint was in or around a plantiecrub and the only one that I had seen was the one that I had driven past. I had my Jornada with me with the web page downloaded for this cache. I checked the grid reference on the GPS and sure enough it was wrong. When I punched in the correct grid it took me back towards the car and thence to the cache. This is a very long-winded way of saying that as far as I can see, if you entered an OS grid ref in whilst you had position set to British Grid and datum set to WGS84, then you subsequently changed one of those to the correct one to pair with the other i.e. BritGrid AND OrdSrvyGB OR HDDD.MM.MMM AND WGS84, it would display a different set of figures to what you put in. I am just going to nip outside and test this ……… …….. biting NW wind tonight up here! …. OK, I apologise to those that this is obvious to, but I will get to the point soon I promise! With the GPS set to BritGrid AND OrdSrvyGB my front door is at HU45794 41866. If I leave position set to BritGrid and change the datum to WGS84 my front door becomes HU45879 42331. And the point of all this blurb I suppose is to say that anything you have marked with your GPS when the settings were wrong will be hundreds of yards away from where the GPS says it is. On the eTrex Camo model (and presumably all eTrex models) changing the position to British Grid automatically changes the datum to OrdSrvyGB. Its people like me that benefit the most when things are idiot proofed! I think that I prefer it when WLW explains things. It makes much more sense! Quote Link to comment
+Lizzzzeeeee Posted February 1, 2005 Share Posted February 1, 2005 OK, I think I get it; although I swap constantly between D MM.MMM and British Grid and my Garmin eMap, as 60North pointed out, swaps the datum from WGS84 to OSGB, because I always set MY OWN waypoints with it set to D MM.MMM WGS84 I've never had a problem. The problem would only happen if I went out setting a cache with it set to British Grid. Am I right? If not I'm really confused about something that I've not had a problem with in over two years. Probably best not to reply if I am wrong!!!! Quote Link to comment
60North Posted February 1, 2005 Share Posted February 1, 2005 (edited) The problem would only happen if I went out setting a cache with it set to British Grid. Am I right? If not I'm really confused about something that I've not had a problem with in over two years. Probably best not to reply if I am wrong!!!! No not really, if I have read your words correctly. The problem happens if you use British Grid with WGS84. There is no reason why you cant use OS grid refs, but some of the Geocaching community seem to frown on it, but just make sure that if you are doing this the datum is set to OrdSrvyGB. When you upload the co-ords take them from the GPS as HDDD.MM.MMM with the WGS84 datum set. As you say this is automatic on your GPS, but don't assume that this will always be the case on all GPS units. Edited February 1, 2005 by 60North Quote Link to comment
+wildlifewriter Posted February 1, 2005 Share Posted February 1, 2005 (snip) On the eTrex Camo model (and presumably all eTrex models) changing the position to British Grid automatically changes the datum to OrdSrvyGB. Its people like me that benefit the most when things are idiot proofed! The 'kicker' is, of course, that it doesn't work the other way round - not on Garmin handhelds, anyway. Change the display format and, yes, the datum setting alters automagically... BUT, if you then go down the menu and alter a datum, the display format stays the same... ... at which point, new caches start to move and all sorts of horrible things can happen. I think that I prefer it when WLW explains things. It makes much more sense! I'll let you know when the book comes out. Quote Link to comment
+Team Ullium Posted February 2, 2005 Share Posted February 2, 2005 (edited) (snip) On the eTrex Camo model (and presumably all eTrex models) changing the position to British Grid automatically changes the datum to OrdSrvyGB. Its people like me that benefit the most when things are idiot proofed! The 'kicker' is, of course, that it doesn't work the other way round - not on Garmin handhelds, anyway. Change the display format and, yes, the datum setting alters automagically... BUT, if you then go down the menu and alter a datum, the display format stays the same... ... at which point, new caches start to move and all sorts of horrible things can happen. I think that I prefer it when WLW explains things. It makes much more sense! I'll let you know when the book comes out. Just to clarify what you mean by "The 'kicker' is, of course, that it doesn't work the other way round - not on Garmin handhelds, anyway." On my Garmin GPS 76 if I change the co-ordinates from Lat & Long to Ord Srvy (and vice versa) the datum changes automatically ... but if I change the datum the co-ords do not change automatically. Also, on my wife's Garmin Etrex if you change either the co-ords or the datum nothing changes automatically... so that one is forced to manually select the correct datum for whatever co-ordinate system you have chosen. And I think I have the most up to date system running on both GPSr's Edited February 2, 2005 by Team Ullium Quote Link to comment
+wildlifewriter Posted February 2, 2005 Share Posted February 2, 2005 On my Garmin GPS 76 if I change the co-ordinates from Lat & Long to Ord Srvy (and vice versa) the datum changes automatically ... but if I change the datum the co-ords do not change automatically. Just so. Also, on my wife's Garmin Etrex if you change either the co-ords or the datum nothing changes automatically... so that one is forced to manually select the correct datum for whatever co-ordinate system you have chosen. For the Rev.2 (non-mapping) models, I concede the point. The basic (yellow) eTrex doesn't seem to have had a firmware revision since before the old King died. Quote Link to comment
+The Roos Posted February 5, 2005 Author Share Posted February 5, 2005 PROBLEM SOLVED Thanks to all who replied to this problem. With your help I`ve now got the GPS set on WGS which puts all my locations in the right place. HURRAH!! Up until now I`ve had to rely on map and compass so at least I`ll have a back up if the batteries in my GPS go flat. Thanks again to all. The Roos Quote Link to comment
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