Jeremy Posted December 10, 2002 Share Posted December 10, 2002 I added British Grid conversion to all UK Cache pages. Since I'm not too familiar with who actually uses this format (other than the UK), let me know the other countries and I'll add them as well. Jeremy Irish Groundspeak - The Language of Location Quote Link to comment
+Chris n Maria Posted December 10, 2002 Share Posted December 10, 2002 Thank you Jeremy - Brilliant! (pushing their luck) any chance of a link to streetmap.co.uk as well? Thanks for listening to our needs. Bear rescues a speciality! London & UK Geocaching Resources: http://www.sheps.clara.net Quote Link to comment
+BigBirdNL Posted December 11, 2002 Share Posted December 11, 2002 How about the Dutch RD grid on Dutch cache pages? BigBirdNL -- there is no spoon -- Quote Link to comment
+Huga Posted December 11, 2002 Share Posted December 11, 2002 Great work Jeremy. Cheers for listening to us whining poms and doing some stuff for us - makes us feel all warma nd fuzzy and wanted By the way, where can I find the algorithm/code for converting from lat/long (WGS84) to OS Grid? I'm writing a program that plonks dots on an OS map, but to do that I need to convert lat-long to grid....any help muchly appreciated Cheers again Jeremy! -- **Mother is the name of God on the lips of all children** Quote Link to comment
Ben Pid Posted December 11, 2002 Share Posted December 11, 2002 quote:Originally posted by Huga:Cheers for listening to us whining poms The Yanks actually call us Limeys, its only Ozzies that call us Poms. Cheers the extra Jeremy, does wonders! Nice one. Pid Ben Piddington http://www.buckscaching.co.uk Quote Link to comment
iryshe Posted December 11, 2002 Share Posted December 11, 2002 There is a converter available on the UK mapping site that is written in Excel. I converted the function into dot net. Re: Dutch RD grid - Send me to a location that has the function to do the conversion both ways and I'll add it. My goal is to duplicate the conversions in the Garmin and Magellan units. The WGS84 to NAD27 datum (and back) is also a particularly nasty beast, but one I'd like to tame. Jeremy Irish Groundspeak - The Language of Location Quote Link to comment
iryshe Posted December 11, 2002 Share Posted December 11, 2002 Chris n Maria, Sure. Not to be lazy, but if you could provide the link format to Streetmap I'll add it to the UK pages as well. Jeremy Irish Groundspeak - The Language of Location Quote Link to comment
+Chris n Maria Posted December 11, 2002 Share Posted December 11, 2002 quote:Originally posted by Jeremy (Admin):Chris n Maria, Sure. Not to be lazy, but if you could provide the link format to Streetmap I'll add it to the UK pages as well. Jeremy Irish Groundspeak - The Language of Location It's in your mail. Just a thought - Will OS Refs be in the GPX format as well? Bear rescues a speciality! London & UK Geocaching Resources: http://www.sheps.clara.net Quote Link to comment
+Bill D (wwh) Posted December 11, 2002 Share Posted December 11, 2002 That's an excellent addition - thanks! Just one small point - using Ordnance Survey's GridInQuest to convert co-ordinates of caches from the Lat/Long on the cache page to OSGB, I'm finding that there are small differences in the last digits of the eastings and northings on the cache pages and in GIQ's results. Typically the easting is one figure different and the northing is around four or five different. For example, E abcd3 N efgh3 on the cache page and E abcd4 N efgh7 from GIQ. Does anyone know why this would be? I do realise it's a tiny difference - I'd just like to know the reason. Quote Link to comment
+The Wombles Posted December 11, 2002 Share Posted December 11, 2002 Brilliant! This will save much confusion for Newbies and useful time for me in adding a Streetmap link within the page of every cache that I write. Jeremy, well done, we feel more loved every minute. All we need now is a cache attribute for the nearest pub and we're sorted! Dave Quote Link to comment
+Gary & Jane Posted December 12, 2002 Share Posted December 12, 2002 Thanks for this useful addition. We've noticed a minor bug when the conversion results in a grid reference less than 10000. When this occurs the leading zeroes aren't displayed. For example N 50°51.482 W 001°56.974 gets converted to SU 3647 6510 instead of SU 03647 06510. Quote Link to comment
+Teasel Posted December 12, 2002 Share Posted December 12, 2002 Fantastic, thanks Jeremy! quote:Originally posted by Bill D:Just one small point - using Ordnance Survey's GridInQuest to convert co-ordinates of caches from the Lat/Long on the cache page to OSGB, I'm finding that there are small differences in the last digits of the eastings and northings on the cache pages and in GIQ's results. Does anyone know why this would be? I do realise it's a tiny difference - I'd just like to know the reason. The quick answer is that OSGB36 is quite old (that's 36 as in 1936) and has some distortions. The simple "Helmert" transformation which is used in the OS spreadsheet (and geocaching.com) is accurate to only 5m or so. There are more accurate transformations which use a "rubber sheet" model, in which the exact transformation changes depending on where in the country you are. For the full, gory details try this Quote Link to comment
+BigBirdNL Posted December 12, 2002 Share Posted December 12, 2002 quote:Originally posted by Jeremy (Admin):Re: Dutch RD grid - Send me to a location that has the function to do the conversion both ways and I'll add it. In CacheMaps I use an excel spreadsheet based on the formula's of Ejo Schrama. His software can be found at http://pocus.geo.tudelft.nl/~schrama/Public/gps/rd2wgs. I will make my converter spreadsheet available as a separate workbook, so you can analyze the coding. I will post it on my site tonight (dutch time). Thanks for all your good work! BigBirdNL -- there is no spoon -- Quote Link to comment
+washboy Posted December 13, 2002 Share Posted December 13, 2002 quote: by Teasel:The simple "Helmert" transformation which is used in the OS spreadsheet (and geocaching.com) is accurate to only 5m or so. So, I guess the question is "which transformation does my GPSr use - simple Helmert of rubber sheet"? The former, I suspect - which is fine. As long as it matches whatever geocaching.com uses. ===== There's no such thing as a free lunchbox! Quote Link to comment
+Bill D (wwh) Posted December 13, 2002 Share Posted December 13, 2002 Teasel: Many thanks for the explanation, and for the link to the full gory details! Quote Link to comment
+jeremyp Posted December 13, 2002 Share Posted December 13, 2002 quote:Originally posted by Jeremy Irish:Since I'm not too familiar with who actually uses this format (other than the UK), let me know the other countries and I'll add them as well. Jeremy Irish Groundspeak - The Language of Location Nobody else uses British grid format. The OSGB36 datum it uses only fits the Earth in the area of the UK and the grid itself is only big enough to fit the UK into it. ------- jeremyp The second ten million caches were the worst too. http://www.jeremyp.net/geocaching Quote Link to comment
+jeb and co Posted December 13, 2002 Share Posted December 13, 2002 I've written a small application to run on my Palm (actually a handspring) to convert wgs84 locations to osgb grid refs so that I can pick up locations of car parks etc. to locate on OS maps. I know it can be done on the GPS but as the data is already on the palm from downloads, it saves time and typos. The only snag is that the package I used to write it is time limited and will only run for 30 dayswithout shelling out some hard cash. I am trying to rewrite it in c using Cygwin and PilRC. [This message was edited by jeb and co on December 14, 2002 at 02:22 AM.] Quote Link to comment
+paul.blitz Posted December 13, 2002 Share Posted December 13, 2002 quote:Just one small point - using Ordnance Survey's GridInQuest to convert co-ordinates of caches from the Lat/Long on the cache page to OSGB, I'm finding that there are small differences in the last digits of the eastings and northings on the cache pages and in GIQ's results. Typically the easting is one figure different and the northing is around four or five different. Could this be something simple like: the Ordnance Survey will probably be using the OSGB datum, whilst those on the site here use WGS84? See my thread "pick a datum"..... for a chat about this! Paul Team Blitz No, I gave YOU the spare batteries.... Quote Link to comment
+Chris n Maria Posted December 14, 2002 Share Posted December 14, 2002 quote:Originally posted by Team Blitz:[Could this be something simple like: the Ordnance Survey will probably be using the OSGB datum, whilst those on the site here use WGS84? Nope - it is just inacuracies caused by translating from one system to the other. The differences talked about are within the 5 meter range you would expect. OSGB to wgs84 is a more significant difference. That's as far as I understand it. Chris Bear rescues a speciality! London & UK Geocaching Resources: http://www.sheps.clara.net Quote Link to comment
+jeb and co Posted December 14, 2002 Share Posted December 14, 2002 Would it be possible for Jeremy to pack the eastings and northings of the OSGB grid refs with leading zeroes to make them both five figures? There could be some confusion as, for eample, SW 992 993 could be misconstrued as being about 140 km from SW 00992 00993. [This message was edited by jeb and co on December 14, 2002 at 02:18 AM.] Quote Link to comment
+washboy Posted December 14, 2002 Share Posted December 14, 2002 quote: OSGB to wgs84 is a more significant difference. No, really, it's like Teasel suggested. The transormation used by most folks to get from WGS84 to OSGB is a simple Helmert transformation. It will be totally accurate for all points in mainland Britain if there weren't subtle distortions inherent in OSGB. As a result, the transformation of some points could be out by as much as 5m. The Ordnance Survey has developed far more accurate methods of transformation which take into account the distortions (using a lookup table for each square km of the country). Bill D was comparing the results of the Helmert transformation against the results of the OS's "GridInQuest" utility, which uses the more precise techniques of transformation. Hence, the difference in the figures. The Helmert transformation, used by GC.com, is totally adequate for geocaching purposes. After all, how accurate are the initial GPSr readings when the caches are placed? ===== There's no such thing as a free lunchbox! Quote Link to comment
+paul.blitz Posted December 14, 2002 Share Posted December 14, 2002 quote:Just one small point - using Ordnance Survey's GridInQuest to convert co-ordinates of caches from the Lat/Long on the cache page to OSGB, I'm finding that there are small differences in the last digits of the eastings and northings on the cache pages and in GIQ's results. Typically the easting is one figure different and the northing is around four or five different. I just ran a check using my OziExplorer: The datum dufference between OSGB & WGS84 is about 90m in the e/w direction, 70m n/s: if you use 5 digit + 5 digit British Grid figures, then the last digits are 1 metre, so the dufferences you are seeing is 1 metre e/w, and 5 metres n/s: I would put that down 100% to the inaccuracies of the (many) ways of converting between the grids. There is a very interesting, in depth article on the OS site (www.ordnancesurvey.com) desctibing the various grids & datums, and explains the issues in converting between them. 5 metres is within the accuracy they said would exist. Paul Team Blitz No, I gave YOU the spare batteries.... Quote Link to comment
+paul.blitz Posted December 14, 2002 Share Posted December 14, 2002 quote:I added British Grid conversion to all UK Cache pages. Just to point out that the British Grid figures given are, as you might expect, based on the OSGB Datum, and NOT the WGS84 datum. This does mean you do need to remember to set both the grid AND the datum right on your GPS!). Otherwise you'll be looking about 90 metres away from the right place! Team Blitz No, I gave YOU the spare batteries.... Quote Link to comment
+jeremyp Posted December 14, 2002 Share Posted December 14, 2002 quote:Originally posted by Team Blitz: Just to point out that the British Grid figures given are, as you might expect, based on the OSGB Datum, and NOT the WGS84 datum. This does mean you do need to remember to set both the grid AND the datum right on your GPS!). This will only be a problem if you enter the waypoint manually (on Garmins at least). Data transfers (on Garmins) are always in WGS84 format no matter how the GPS is set. One feature I noticed on the Vista is that if you change the format to British grid, the datum changes to OSGB36 automatically. ------- jeremyp The second ten million caches were the worst too. http://www.jeremyp.net/geocaching Quote Link to comment
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