+Teasel Posted April 4, 2005 Share Posted April 4, 2005 Recently there's been a flurry of posts from or about people who get confused between the different datums available on their GPS. Now, IMHO it'd be nice if GC.com only used OS coordinates on UK/Irish caches, but sadly we're stuck with WGS84 and that's that. The biggest problem seems to be when people set caches where some of the clues refer to OS coordinates. Half the time, it seems that the person who set the cache doesn't fully understand datums themselves, so what chance for the poor person trying to find it?! Is it time that we made a UK-specific rule (we're allowed to do that still, aren't we?) that only WGS84 coordinates are allowed in clues? I'm not suggesting that cache descriptions shouldn't include OS translations of WGS84 cache coordinates. Indeed, one of the main features of GeocacheUK is that OS grid references are automatically inserted into cache descriptions. But shouldn't all coordinates and calculations which are required to actually find the cache be in WGS84 H DDD MM.MMM format? Quote Link to comment
Alan White Posted April 4, 2005 Share Posted April 4, 2005 shouldn't all coordinates and calculations which are required to actually find the cache be in WGS84 H DDD MM.MMM format? Yes, absolutely. IMO, with all respect to your excellent work on G:UK, the OSGB coords on the GC.com cache pages merely serve to confuse. It would be better if they weren't there. Quote Link to comment
+Eckington Posted April 4, 2005 Share Posted April 4, 2005 I'm not suggesting that cache descriptions shouldn't include OS translations of WGS84 cache coordinates. Indeed, one of the main features of GeocacheUK is that OS grid references are automatically inserted into cache descriptions. But shouldn't all coordinates and calculations which are required to actually find the cache be in WGS84 H DDD MM.MMM format? I also agree with Teasel's sentiments. I find the reference, on the cache page, for OS very useful in using my Anquet mapping software to get an idea of the location of a cache and, in conjunction with the streetmap link and the benchmarks.org mapping links, for checking out location when I am reviewing. However, increasingly I am finding, when checking out locations, that the mapped location does not quite agree with the cache description. When I consult with the cache setter it often transpires that the co-ords had been obtained using the incorect datum. Quote Link to comment
Remote Part Posted April 4, 2005 Share Posted April 4, 2005 I don't think there is any need for any rule on this. Instead maybe we should have some sort of document / FAQ clearly explaining the different postion formats / datums and when it is appropriate to use each. Yes, there are some newbies who do have problems with position formats / datums, but there are also a number of more 'experienced' cahers who seem to think using anything other then H DDD MM.MMM / WGS84 is totally wrong. Personally, I will continue to use British Grid / OSGB (and find and hide caches just fine). Quote Link to comment
Alan White Posted April 4, 2005 Share Posted April 4, 2005 (edited) Instead maybe we should have some sort of document / FAQ That already exists here. For geocaching, there is only one datum. The reason I agree with Ian's suggestion is that it's one less thing for newbies to worry about. Experienced cachers such as yourself are, of course, perfectly at liberty to convert to whatever datum and format they like. It's the inexperienced we're trying to help. And, by helping the inexperienced we both give them a better introduction to the sport and reduce the time we experienced cachers need to spend in answering the newbies questions Also consider foreign visitors. OSGB will be meaningless to them, so quoting it on a cache page is even more unhelpful to them than to UK cachers. Edit: add last para Edited April 4, 2005 by Alan White Quote Link to comment
Lactodorum Posted April 4, 2005 Share Posted April 4, 2005 Personally, I will continue to use British Grid / OSGB (and find and hide caches just fine). Just remember, that if anyone has their GPSr set this way and decides to place a cache, they need to set it back to "normal" before doing so. I wholeheartedly endorse what Teasel has suggested although I am reluctant to start imposing another "rule". I think we have enough of those already. I will however request that any clues given in OS format are supplemented with another clue in normal WGS84/H DDD MM.MMM format Quote Link to comment
+Bill D (wwh) Posted April 4, 2005 Share Posted April 4, 2005 Yes, I'd like to see WGS84 only. I use OSGB via GeocacheUK a great deal for finding the locations on OS maps, and indeed I wish there were a utility for the Palm to convert multi stages in the field, but as to clues on cache pages WGS84 is in my view how they should be. I do though understand Lactodorum's reluctance to add to the "rules". Quote Link to comment
Remote Part Posted April 4, 2005 Share Posted April 4, 2005 Instead maybe we should have some sort of document / FAQ That already exists here. For geocaching, there is only one datum. But that is a very brief explanation, with no mention of position formats etc.Also, it's not UK specific. Many geocachers may want to refer to an OS map while caching, so will want to use British Grid / OSGB some of the time. And, by helping the inexperienced we both give them a better introduction to the sport and reduce the time we experienced cachers need to spend in answering the newbies questions If we had an FAQ, we could just direct any newbies towards it when the ask about it.Also consider foreign visitors. OSGB will be meaningless to them, so quoting it on a cache page is even more unhelpful to them than to UK cachers. These foreign visitors may also want to use OS maps, so will want to know something about British Grid. Personally, I will continue to use British Grid / OSGB (and find and hide caches just fine). Just remember, that if anyone has their GPSr set this way and decides to place a cache, they need to set it back to "normal" before doing so. Or convert the waypoints (correctly) after loading them onto your PC. Quote Link to comment
+-Phoenix- Posted April 4, 2005 Share Posted April 4, 2005 hm..... I can easilly see where there are problems, and confusion, and there is no way I could stand on the side that would say we go all OSGB and discard WGS84, but, I like OSGB, having OSGB figures means I can easilly reference my maps. I cant help but feel that simply discarding OSGB is catering to the lowest common denominator, sure the rest of the world has crap maps, but here in the UK, we dont, and one of the cornerstones to this is OSGB. Quote Link to comment
+kph100 Posted April 4, 2005 Share Posted April 4, 2005 I must say that I find it really helpful to have OS co-ords for plotting on a map. Without this its really difficuly to find out where a cache is. For the actual caching I use the standard Geocaching.com set up. I definately vote for keeping it exactly as it is. It works ..... why change. Ian. Quote Link to comment
+Peat_Bog Posted April 4, 2005 Share Posted April 4, 2005 I find the reference, on the cache page, for OS very useful in using my Anquet mapping software to get an idea of the location of a cache and, in conjunction with the streetmap link and the benchmarks.org mapping links, for checking out location when I am reviewing. I also find the use of both datums useful and would prefer for things to remain unchanged. Quote Link to comment
+Learned Gerbil Posted April 4, 2005 Share Posted April 4, 2005 (edited) I can't agree with only using an obscure UK only format. Not only would it confuse the hell out of visiting yanks (maybe reconsider the value of that argument) but it would confuse the hell out of me! I have always used and loved maps, but I abandoned OS formats as soon as I adopted caching as I could see they were useless. I have had almost no reason to use them since as the mapping producs I use to go paperless, may display OS maps, but do so without using any OS grids or datum. If streetmap etc are used properly (see the threads on automating GSAK and webpage display of streetmap 1:50,000 maps) then anyone can locate any position on any OS map without using any OS grid. Edited April 4, 2005 by Learned Gerbil Quote Link to comment
Alan White Posted April 4, 2005 Share Posted April 4, 2005 Also, it's not UK specific. That's exactly the point. Geocaching is a worldwide sport, so a datum which works worldwide is required. Quote Link to comment
+2202 Posted April 4, 2005 Share Posted April 4, 2005 What a load of ********. there is only one datum and that it WGS84. You know where you are anywhere in the world. Talk of using OS datum is a load of tosh and should be ignored. Can you just see a pilot or a sailor saying, oh god we are in/over the UK, we must go onto OS datum. Absolute pish!! Quote Link to comment
Mittellegi Posted April 4, 2005 Share Posted April 4, 2005 ...having OSGB figures means I can easilly reference my maps. I cant help but feel that simply discarding OSGB is catering to the lowest common denominator, sure the rest of the world has crap maps, but here in the UK, we dont, and one of the cornerstones to this is OSGB. That's not quite true: The Cypriots have excellent maps (but the Brits did all the surveying ), and they are, to all intents and purposes, OS maps. Except they use UTM on the ED50 datum. The Swiss also have excellent maps (and will sell you 25m DEM data). They use their own oblique TM projection on the CH-1903 datum. Rather than pander to the LCD, it is better to educate: Think Football The Datum is the shape and size of your Ball. Whether that be WGS84, OSGB36, ED50 or CH-1903. The Position Format is the Net you keep it in. Whether that be DD MM.MMM, British Grid, UTM or Swiss Grid. You just need to get the Ball in the right Net. Get it right and you're a winner, get the Ball in the wrong Net and you'll score an own goal. BB Quote Link to comment
+-Phoenix- Posted April 4, 2005 Share Posted April 4, 2005 I do love lowest common denomiator thinking, its poor compromises that have kept engineers in work for generations.... not only do we get the work in the first place, but we also get the work to rework the original inadequate job we did because we were forced into poor compromises, by individuals who were convinced that they knew better. Quote Link to comment
Remote Part Posted April 4, 2005 Share Posted April 4, 2005 Also, it's not UK specific. That's exactly the point. Geocaching is a worldwide sport, so a datum which works worldwide is required. I was referring to the explanation you referred to, not the datum in general. I think a UK specific datum / position format explanation / FAQ would be useful, due to the specific needs and understanding of cachers in the UK. I'm going to start writing up such a document soon. I noticed that seems to be some confusion between datums and position format, including in this thread. In reply to Learned Gerbil and others, as to the advantages of using British Grid / OSGB: it's used on nearly all maps in this country it is a lot simpler to understand, as it is a simple grid it is metric, in that each grid square represents a 1km / 100m / 10m / 1m etc this makes working out the distance between 2 points much simpler, as you can use simple arithmetic and Pythagoras As bexybear said, grid based systems are popular around the world, though they don't work over long distances, so aren't used by pilots / sailors. Maybe the more logical solution would be for everyone to change to UTM (with your datum of choice)... Quote Link to comment
+Teasel Posted April 4, 2005 Author Share Posted April 4, 2005 (edited) Can you just see a pilot or a sailor saying, oh god we are in/over the UK, we must go onto OS datum. Pilots fly, sailors sail. EVERYONE who walks in Britain and Ireland uses OS grid references. Oh, apart from an small bunch of geocachers who use an American site to log their finds. (And then express indignation when the UK emergency services don't understand their weird coordinate system when they get in trouble!) The sad truth is that we are sacrificing simplicity for thousands of UK geocachers, simply for the convenience of a handful of foreign tourists who can't be bothered to change their GPS datum when they land at Heathrow. Hell they reset their watches, why not their datums? Every schoolchild in the land learns about grid references, as does every Scout, Guide, rambler, etc etc. It's the only grid shown on OS maps, and the only grid shown on Harveys maps. Oh, and if you're driving, it's the OS grid that's used on the AA road atlas, and the A-Z street atlas, and the Philips Street atlas, and... (get the picture?!). The only reason geocachers are even aware of WGS coordinates is that providing familiar local coordinates wasn't high on Jeremy's todo list. (Look at early UK caches to see what came naturally to UK geocachers before people got used to suffering the hitherto unknown WGS84). Anyone writing a new geocaching listings site from scratch should use local coordinate systems, not the lazy WGS84 global "lowest common denominator"! Edited April 4, 2005 by Teasel Quote Link to comment
MCL Posted April 4, 2005 Share Posted April 4, 2005 (edited) I'm rather against the imposition of a rule banning the use of OSGB coordinates, since at least two caches I have done actually rely on calculations done using them. Were such a rule to be imposed youwould be forcing at least two dadgum good caches to be archived. I'm sure there are many others but the two I am referring to are Huga's "OSX" and Pharisee's "Triangles, Trigonometry and treachery" although the latter was so treacherous I wouldn't actually mind seeing the thing flung into the evelasting pits of hell but that's another story. To impose such a rule is to effectively remove one entire sphere of cache-solving. If you don't like that sort of cache you don't have to do it, but I would like to have the choice please. Edited April 4, 2005 by MCL Quote Link to comment
Garmin8888 Posted April 5, 2005 Share Posted April 5, 2005 Well, I think that we should as we are walkers whilst Geocaching in the UK think of ourselves as the yanks do to themselves.... I say old chaps! let us only have OSGB why, because if ......say one is hurt one can... via their mobiles call for help and give the OSGB info to the Authorites as they use OSGB maps and even Ambulances now have G.P.S. systems in their vehicles...... Plus as some one said we are walkers not Pilots flying to a Geocach! we are in GB and we love our Ordnance as we love maps and our G.P.S.'s now.......! This is a great site! Quote Link to comment
+2202 Posted April 5, 2005 Share Posted April 5, 2005 The only reason geocachers are even aware of WGS coordinates is that providing familiar local coordinates wasn't high on Jeremy's todo list. (Look at early UK caches to see what came naturally to UK geocachers before people got used to suffering the hitherto unknown WGS84). Anyone writing a new geocaching listings site from scratch should use local coordinate systems, not the lazy WGS84 global "lowest common denominator"! I never thought I would again see such xenophobic crap on this forum. Get real and live in the world! Quote Link to comment
Lactodorum Posted April 5, 2005 Share Posted April 5, 2005 A very interesting discussion which has brought out several points I, as a reviewer, hadn't considered. As a result of the comments made I'm beginning to tend towards a particular conclusion but I need to discuss it with Eckington before doing anything. However one thing occurs to me, again which I'd not considered. As has been pointed out, UK caches have the published co-ordinates expressed in H DDD MM.MMM format AND British Grid format. I decided to do a bit of fiddling around on Memory Map and it looks like the British Grid display format still uses the WGS84 datum. Maybe somebody more technically competent than me would like to check this. If OS maps are based on British Grid format and OSGB datum (again someone please confirm this), I can forsee possible problems when trying to use co-ords expressed in a "pseudo-British/OS" format with a paper OS map. Or am I worrying unnecessarily? In case of any doubt, I'm asking a question, not making a point Quote Link to comment
+The J J Noodle Fan Club Posted April 5, 2005 Share Posted April 5, 2005 (edited) However one thing occurs to me, again which I'd not considered. As has been pointed out, UK caches have the published co-ordinates expressed in H DDD MM.MMM format AND British Grid format. I decided to do a bit of fiddling around on Memory Map and it looks like the British Grid display format still uses the WGS84 datum. Maybe somebody more technically competent than me would like to check this. If OS maps are based on British Grid format and OSGB datum (again someone please confirm this), I can forsee possible problems when trying to use co-ords expressed in a "pseudo-British/OS" format with a paper OS map. From what I've tried in the past the OS co-ords given on gc.com are true OS co-ords and locate the cache correctly on an OS map (at least sufficiently to find a cache). The problem isn't the use of OS co-ords, it's that the lat/long used by the OSGB datum is offset (and I think skewed) from the WGS datum. gc.com translates the lat/long from the WGS datum to the lat/long of the OSGB datum (don't know how accurate gc.com do this, I expect they use the same approximation for the whole of the UK, in reality the offsets change across the country) then converts this second lat/long to OS co-ords. That's why the streetmap.co.uk link on the cache always shows the cache at the correct place on the map. If it just converted the WGS datum lat/long to OS co-ords then the position would be wrong on an OS map. Yes - OS maps use British grid and OSGB (OSGB 36 to be precise - there are others!) In summary - yes - I think you are worrying unnecessarily Just a note to throw into the pot: Traditionally sea charts for the coast of the UK have been based on OSGB 36 - starting 2 or 3 years ago charts newly updated are being issued based on WGS 84. Could this happen for our land maps? Jon. Edited April 5, 2005 by The J J Noodle Fan Club Quote Link to comment
+Chris n Maria Posted April 5, 2005 Share Posted April 5, 2005 I like the way it is now, all that confusion and complexity means that UK cachers soon learn much more about mapping systems, coordinates and datums than cachers in the rest of the world. All part of the fun Don't they have similar problems in the US with most of their maps being a different NAD and in Aus don't they use UTM ? Chris Quote Link to comment
+The J J Noodle Fan Club Posted April 5, 2005 Share Posted April 5, 2005 The problem isn't the use of OS co-ords, it's that the lat/long used by the OSGB datum is offset (and I think skewed) from the WGS datum. I don't want to be responsible for starting one of those urban myth type thingies so can I just say I've looked it up and the offsets are NOT skewed - not for OSGB at least. Jon. Quote Link to comment
NickPick Posted April 5, 2005 Share Posted April 5, 2005 Ah, Map projections. At times, the bane of my life. Even the WGS84 isn't exactly the shape of the earth, so there are many regional variations of it. Get the wrong one when overlaying different datasets, and everything goes pear shaped. Still, I think the best option is to give waypoints in WGS84. That's what has been set as the convention for the past 5 years or so, and it is a Global Positioning System A GPSr calculates its position in WGS, then if you have it set to another co-ord system, it converts it for you. I don't know if all GPSrs use the same calculation for this, so there might be problems with Magellan / Garmin / Other when the setter is using a different make to the searcher. I don't know whether we should ban people from giving clues in OSGB36, as long as they make it clear in the clue. Cache setters should make sure that they're using the right datum though. If I need to convert from one to the other, I tend to use Grid Inquest. My Etrex won't let me set co-ord system to WGS without resetting the datum aswell. Do other GPSrs allow you to mix the two that easily? As far as WGS goes, there are also WGS 60, 66, and 72. Are they intending to do another World Geodetic Survey soon? and will our GPSrs have to change to match it? And I believe (although it's a couple of years since I did much GIS work) that the OS have updated their grid systems for new digital mapping, because using GPS for surveying has shown that there are some errors in the grid. I think this sort of error is only noticable at large scales (1:2500), so probably won't affect the smaller scale maps we tend to use. Quote Link to comment
+Learned Gerbil Posted April 5, 2005 Share Posted April 5, 2005 Yes - I would be very surprised if OS maps were not transfered to WGS84 soon. Some of us are old enough to remember the expose on Watchdog when the OS threw out our beloved one mile to the inch maps and replaced them with these new fangled kilometre thingies. Somehow we coped! Quote Link to comment
+Teasel Posted April 5, 2005 Author Share Posted April 5, 2005 I decided to do a bit of fiddling around on Memory Map and it looks like the British Grid display format still uses the WGS84 datum. No, the grid reference on GC.com correctly uses the OSGB36 datum. Coordinates will agree with maps to within a couple of m or so. If OS maps are based on British Grid format and OSGB datum (again someone please confirm this) OS (and most other UK) maps are based on a British Grid format, yes. Until very recently they were based on the OSGB datum, yes. (I believe they are currently based on the ETRS89 datum, but let's not get into the nitty gritty about how to compensate for a 2.2cm/yr tectonic drift rate!) Traditionally sea charts for the coast of the UK have been based on OSGB 36 - starting 2 or 3 years ago charts newly updated are being issued based on WGS 84. Could this happen for our land maps? What grid do they use? As noted above, even OS maps no longer use the OSGB36 datum (ETRS89 is actually based on WGS84 ), but we still have gridlines a set number of km apart (rather than a set number of degrees apart). The few sea charts I've seen don't actually have a grid. Possibly because ships generally move in long, straight lines, often in defined shipping lanes, often from one country to another, so it's a very different style of navigation. I'd be surprised if many walkers or drivers would choose to go from a square grid (1km x 1km for walkers, bigger squares for drivers) to something rectangular. It's currently very easy to look at a route on a map and estimate how long it is - you'd lose that if you went to a lat/long grid. Even the "D3" style of grid used on road atlasses would look a bit funny if the grid wasn't square. Quote Link to comment
+Learned Gerbil Posted April 5, 2005 Share Posted April 5, 2005 There is nothing to stop you abolishing OS grid whilst maintaining 1km lines on the map. I use OS maps al lthe time, but because I use Streetmap and Fugawi, I never have to use OS grid. But I do still use those funny lines to estimate stuff. Quote Link to comment
+Chris n Maria Posted April 5, 2005 Share Posted April 5, 2005 Surly they would leave the grid as it is and replace the lat/long round the very edge with one based on WGS84? Quote Link to comment
SlytherinAlex Posted April 5, 2005 Share Posted April 5, 2005 I can forsee possible problems when trying to use co-ords expressed in a "pseudo-British/OS" format with a paper OS map. Paper maps? What are those? Whatever will they think of next? Quote Link to comment
Prof. Y. Lupardi Posted April 5, 2005 Share Posted April 5, 2005 Why does not someone put a webpage on the net where everyone can read how to set his type of GPSr to British Grid and back again. Every geocacher can then stuff a link on his cachepage to this document when searchers need to use OSGB In the Netherlands we have a likewise situation with the Dutch Grid complicated by the fact that many GPSr's do not know about Dutch Grid and users have to set all the parameters for the User Grid/Datum. For foreigners we have the document in English: http://www.xs4all.nl/~atarist/geo/gps_rd.htm Lately I get many questions like: "I have autonavigation with GPS in my car. How do I get those strange numbers called coordinates into this machine? I want to find a cache." Quote Link to comment
60North Posted April 5, 2005 Share Posted April 5, 2005 To All The first thing that I was told about a GPS was never to rely on it, always carry paper maps as a back up system. Having a Garmin GPS2+ I have quite often had the datum set wrong and ended up 300 yards or so from the cache on the wrong side of a hill. I have since learnt to match the datum to the position format or use my eTrex!. As I always use paper maps to get a rough idea of where the cache is before I set off I tend to work in OSGB and then convert to the "other" flavour. When laying a cache I take a reading in both formats, one for my OSGB records and one for the rest (almost) of you. As ex-forces I had to suffer 6400 mils to a circle instead of 360 degrees. For the sort of work I was involved in I really didn't need that sort of accuracy. There was too many rules there too. Lets hope that we don't end up having to parade every morning on GC.com in green clothes! Quote Link to comment
Garmin8888 Posted April 5, 2005 Share Posted April 5, 2005 So which is more accurate for geocaching in the UK........WGS84 H DDD MM.MMM or OSGB as someone on the net said they are 100meters differant and I now believe that they are, why, because I did a Trig test with a Garmin Map60c and I found it by shere curosity: "wot is up that hill I thought" after not finding it where my G.P.S said it should be......... an voila it was there......... Quote Link to comment
+The J J Noodle Fan Club Posted April 5, 2005 Share Posted April 5, 2005 So which is more accurate for geocaching in the UK........WGS84 H DDD MM.MMM or OSGB as someone on the net said they are 100meters differant and I now believe that they are, why, because I did a Trig test with a Garmin Map60c and I found it by shere curosity: "wot is up that hill I thought" after not finding it where my G.P.S said it should be......... an voila it was there......... WGS84 as that's what all the caches are listed as - except - that the OSGB given is also accurate. It's the issue of having your GPS set to the same datum as was used to take the co-ordinates. In your example the 100m difference was simply because of the datum. This is exactly what we found on our very first caching trip. I put in the N & W from gc.com but the position was obviously on private property. After head scratching I put in the OS grid co-ords and went to the cache. The problem was I'd have the GPS set to OSGB datum all along. After that we read up about datums, stuck it at WGS and apart from one multi where the final co-ords were in OS grid & OS datum that's where it's stayed. To be clear N52°00.000 W0°00.000 on the WGS datum IS A DIFFERENT LOCATION to N52°00.000 W0°00.000 on the OS datum. If you enter the right N & W figures but get the datum wrong you won't be in the right place. Jon. Quote Link to comment
Garmin8888 Posted April 5, 2005 Share Posted April 5, 2005 OSGB is the way to go in uk so that we can check it out on maps as the other way maybe on some ones elses land and that will never do......that will get geocaching in uk outlawed.... Quote Link to comment
+pieces_of_8 Posted April 5, 2005 Share Posted April 5, 2005 I'm happy to use both. I had teething troubles at first as i'd always used my GPS on OSGB before I got into caching...hadn't realised what a difference WGS84 made. I know some caches that use OSGB co-ords as part of multi-cache clues, its only a couple of key presses to switch datums. When doing multi-caches i often switch between datum so i can mark on an OS map where i need to go and the paths i need to follow. As for tourists..... if they read cache pages first before coming.....it would give them some time to research the OSGB datum which to be honest is quite an easy datum to understand. Mart Quote Link to comment
+Alibags Posted April 5, 2005 Share Posted April 5, 2005 I am happy with things the way they are at present. I do sometimes use OS grid when I have calculated the final location of a multi and I want to find out where it is on the map. In these occasions, having entered the co-ords, I change display preferences to British OS and off I go. The Etrex does not give you the option of displaying BOTH OS and WGS84 at the same time, unless you also have OS set as your default, which means you cannot enter cache co-ords correctly. If I had a GPS that used WGS84 as the default and displayed Britsh OS as well, then I would be a happy bunny. Quote Link to comment
+littlejim Posted April 5, 2005 Share Posted April 5, 2005 It seems to me we have two different sets of users discussing (aka arguing) here: those that use OS maps and those that don't. If you never have to use a (paper) map then the only reason to use the British system is if, as somebody has mentioned previously, you need to relate position information to somebody else. On the other hand, if you are out for the day walking and navigating using a large scale OS map, you will be working in British format. I use both systems interchangeably with my software and Garmin GPSr. I understand that Garmin firmware in GPSrs holds references in an unpublished internal format and applies conversion for all input / output formats/ datums. Thus whatever combination you choose, data will pass through conversion algorithms which I believe are accurate to 1 metre. In fact I sometimes use the GPSr as a converter to get from one system to another by up/downloading data. On my Geko 201 selecting British Format forces OSGB datum and selecting HDMM format defaults to the last selected datum for that mode which is WGS84 in my case - this avoids an incorrect combination (working in HDDMM but to the OSGB datum) which would give the 100m plus offsets some users have experienced. Quote Link to comment
+Kitty Hawk Posted April 5, 2005 Share Posted April 5, 2005 The Etrex does not give you the option of displaying BOTH OS and WGS84 at the same time, unless you also have OS set as your default, which means you cannot enter cache co-ords correctly. If I had a GPS that used WGS84 as the default and displayed Britsh OS as well, then I would be a happy bunny. Cor, that would be useful - does a Garmin Vista do that, and if so, I'd love to know what to do. I'd prefer to keep both methods open. I just wondered though, do you think that if OS was more prominent we'd have more cachers who stick at it beyond a handful - it seems to me that WGS84 may be a hurdle to overcome before a newbie becomes relaxed, whereas most will be relaxed with OS. Adrian Quote Link to comment
+Learned Gerbil Posted April 5, 2005 Share Posted April 5, 2005 You are assuming that newbies used OS maps for navigation. You might be better off using AA Roadmap grid for a more universal appeal. Certainly I knew aboput OS grid, but hardly ever used it for anything prior to caching. Quote Link to comment
+Teasel Posted April 5, 2005 Author Share Posted April 5, 2005 You are assuming that newbies used OS maps for navigation. You might be better off using AA Roadmap grid for a more universal appeal. Certainly I knew aboput OS grid, but hardly ever used it for anything prior to caching. AA road maps use... wait for it... OSGB!!! Admittedly the coordinates are given in page/letter/number format, rather than the "proper" grid letters/eastings/northings format found on OS maps. But even so, it's actually really useful to have an OS grid as it allows us xenophobe luddites to quickly and easily find an OS grid reference using an AA road atlas. Quote Link to comment
Alan White Posted April 6, 2005 Share Posted April 6, 2005 AA road maps use... wait for it... OSGB!!! For the underlying datum, yes. As the maps are licensed from OS you wouldn't expect it any other way. But there's no relationship between the OS grid and the letter/number grid used to help users find a place in the atlas. At least, there isn't in this AA 2005 Road Atlas of Britain I have in front of me. To pick a random example, SU 00000 00000 lies just west of Wimborne Minster in Dorset and, as you would expect, on the Landranger map there's a grid intersection at that point. However, there is no corresponding grid intersection on the AA map. The nearest intersection on the AA map is 1.5km east in the centre of Wimborne Minster. And this intersection is, roughly, at SZ 015 998. So, on this AA atlas at least, there's no correspondence between OS grid and the atlas grid. And, also on this AA atlas, the squares aren't even square. They're 25mm NS and 26mm EW. Doesn't sound a lot, but at this scale that's 200m on the ground, so there's no way the atlas grid and the OS grid could ever correspond. (Yes, I know the line itself accounts for 20m of it.) The same applies to the other AA atlas I have to hand: a 2003 version. This has the same scale as the 2005 one but the grid "squares" (still not!) are ten times larger than the 2005 version, and still don't correspond to the OS grid. Quote Link to comment
+Alibags Posted April 6, 2005 Share Posted April 6, 2005 Cor, that would be useful - does a Garmin Vista do that, and if so, I'd love to know what to do. no, as that's still an Etrex... unless the Vista has all sorts of whizzy stuff on it. My friend's GPS ( a none geocacher) was some other older Garmin model. Not sure quite what. Black thing with lots of buttons! Quote Link to comment
+Pharisee Posted April 6, 2005 Share Posted April 6, 2005 Cor, that would be useful - does a Garmin Vista do that, and if so, I'd love to know what to do. no, as that's still an Etrex... unless the Vista has all sorts of whizzy stuff on it. My friend's GPS ( a none geocacher) was some other older Garmin model. Not sure quite what. Black thing with lots of buttons! You can with a Legend, so I suspect you can with a Vista Quote Link to comment
Alan White Posted April 6, 2005 Share Posted April 6, 2005 Cor, that would be useful - does a Garmin Vista do that, and if so, I'd love to know what to do. no, as that's still an Etrex... unless the Vista has all sorts of whizzy stuff on it. My friend's GPS ( a none geocacher) was some other older Garmin model. Not sure quite what. Black thing with lots of buttons! The Legend (unsurprisingly, as it's also an Etrex) also has the option of showing both formats simultaneously. What it won't do, though, is show the OS format in OSGB36 datum, and H DD MM.MMM format in WGS84 datum, presumably because it can't process in two datums simultaneously. This leads to confused users, who see both formats and think they're doing things correctly, until they find their caches are 100m away from where they thought they were. Even more confusingly, though the Legend helpfully changes the datum when you change the format, it doesn't do it the other way round. Thus you can set WGS84 datum and British Grid format, which is meaningless. It's no wonder newbies get confused, and it just convinces me further that there should be only one datum and it should work worldwide. Or, more accurately, that our hobby should use only one datum and format worldwide. Quote Link to comment
Alan White Posted April 6, 2005 Share Posted April 6, 2005 You can with a Legend, so I suspect you can with a Vista Exactly what I mean. Since those coords must be in the same datum, then they are misleading. I assume it's set to OSGB, not least because that's bang on the trigpoint. If (and I'm not suggesting that you would do this) someone sees the DD MM.MMM format then they might well assume that it's in WGS84. And that puts it 70m away from the same point in OSGB Quote Link to comment
+Pharisee Posted April 6, 2005 Share Posted April 6, 2005 (edited) Alan is absolutely correct. The map below shows the error. In the photo above, the Legend IS set to OSGB so the grid ref NS 09804 54751 is correct and as he says, bang on the trig point (I was up there on Sunday ). The mark off to the right is the indicated lat/long plotted as a WSG84 location. You can see how far out it is. Edited April 6, 2005 by Pharisee Quote Link to comment
+Kitty Hawk Posted April 6, 2005 Share Posted April 6, 2005 Thanks gents - I suspect you've just saved me having a bad cache day. Quote Link to comment
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