+Nia Posted September 29, 2002 Share Posted September 29, 2002 my GPS tells me our house is at: N 50 49.250 W 001 22.007 or N 50 49 15.0 W 001 22 00.4 (in hh mm ss.s) but my map tells me its at: N 50 49 14.0 W 001 21 56.0 Why is there a discrepancy, is it anything to do with the 'datum' also what is a 'datum' (i know we use wgs84) and why are there so many different ones? lastly i can find no mention of a 'datum' on my map, what is generally used on maps? Quote Link to comment
+jeremyp Posted September 30, 2002 Share Posted September 30, 2002 Who printed the map? If it's an OS map, the datum will be OSGB36. Try changing the datum on your GPS to OSGB36 (may be in the list as "ordnance survey") and see if it then agrees with your map. The datum is all to do with the fact that the Earth is not a perfect ellipsoid (squashed football). A datum defines an ellipsoid, an equator and a prime meridian. The OSGB36 ellipsoid is designed to fit the Earth's surface in the area of the UK. The WGS84 ellipsoid is designed to be a good fit anywhere on the globe. Positions can then be defined in terms of a latitude and longitude and ellipsoidal height (height above or below the surface of the ellipsoid). The ellipsoidal height is usually discarded as it is not useful for real world navigation. Positions given in WGS84 and OSGB36 can be different by about a hundred metres (it varies from place to place). ------- jeremyp The second ten million caches were the worst too. http://www.jeremyp.net/geocaching [This message was edited by jeremyp on September 30, 2002 at 01:55 AM.] Quote Link to comment
crr003 Posted September 30, 2002 Share Posted September 30, 2002 If you assume the map is OSGB, then the error in location is 29m after conversion, which still sounds a bit off? (Assuming I'm driving the software correctly) Nil Satis Nisi Optimum Quote Link to comment
crr003 Posted September 30, 2002 Share Posted September 30, 2002 If you assume the map is OSGB, then the error in location is 29m after conversion, which still sounds a bit off? (Assuming I'm driving the software correctly) Nil Satis Nisi Optimum Quote Link to comment
+jeremyp Posted September 30, 2002 Share Posted September 30, 2002 quote:Originally posted by crr003:If you assume the map is OSGB, then the error in location is 29m after conversion, which still sounds a bit off? (Assuming I'm driving the software correctly) Nil Satis Nisi Optimum I agree, but if the map is not particularly large scale e.g. 1:50,000 30metres could be an error in reading off the coordinates. ------- jeremyp The second ten million caches were the worst too. http://www.jeremyp.net/geocaching Quote Link to comment
+jeremyp Posted September 30, 2002 Share Posted September 30, 2002 quote:Originally posted by crr003:If you assume the map is OSGB, then the error in location is 29m after conversion, which still sounds a bit off? (Assuming I'm driving the software correctly) Nil Satis Nisi Optimum I agree, but if the map is not particularly large scale e.g. 1:50,000 30metres could be an error in reading off the coordinates. ------- jeremyp The second ten million caches were the worst too. http://www.jeremyp.net/geocaching Quote Link to comment
crr003 Posted October 1, 2002 Share Posted October 1, 2002 While I should quit while I'm ahead (jeremyP agreed with me ), Nia, is it an OS map and what scale?, or are we on the wrong path with this? Nil Satis Nisi Optimum Quote Link to comment
crr003 Posted October 1, 2002 Share Posted October 1, 2002 While I should quit while I'm ahead (jeremyP agreed with me ), Nia, is it an OS map and what scale?, or are we on the wrong path with this? Nil Satis Nisi Optimum Quote Link to comment
+The Hornet Posted October 1, 2002 Share Posted October 1, 2002 The mention of 30 metre errors set me thinking. Could you be using the Hornet datum - guaranteed to be exactly 35' out anywhere in the world _________________________________________________________ It is better to regret something you did, rather than to regret something you didn't do. Quote Link to comment
+The Hornet Posted October 1, 2002 Share Posted October 1, 2002 The mention of 30 metre errors set me thinking. Could you be using the Hornet datum - guaranteed to be exactly 35' out anywhere in the world _________________________________________________________ It is better to regret something you did, rather than to regret something you didn't do. Quote Link to comment
+Nia Posted October 1, 2002 Author Share Posted October 1, 2002 Thanks Jeremyp for a nice clear answer. I changed the GPS datum to OS and it matches up a lot better now, still not 100% put closer. The children were shocked to learn that the planet is not a sphere, so to find its not even a perfect elipsoid, means we are thinking of leaving The map i am using is called infomap, its on my Pc and is brilliant for locating postcodes or Long/Lat. The scale can zoom in to to about 1:1000 (on my screen) It seems to be very accurate but it does not list a datum! Tech-no notice Quote Link to comment
+Nia Posted October 1, 2002 Author Share Posted October 1, 2002 Thanks Jeremyp for a nice clear answer. I changed the GPS datum to OS and it matches up a lot better now, still not 100% put closer. The children were shocked to learn that the planet is not a sphere, so to find its not even a perfect elipsoid, means we are thinking of leaving:) The map i am using is called infomap, its on my Pc and is brilliant for locating postcodes or Long/Lat. The scale can zoom in to to about 1:1000 (on my screen) It seems to be very accurate but it does not list a datum! Tech-no notice Quote Link to comment
+jeremyp Posted October 1, 2002 Share Posted October 1, 2002 quote:Originally posted by Nia:I changed the GPS datum to OS and it matches up a lot better now, still not 100% put closer. How far out is it? If it's less than 5 metres, that could be due to inaccuracies in the conversion method used by either the GPS or the map. quote:The children were shocked to learn that the planet is not a sphere, so to find its not even a perfect elipsoid, means we are thinking of leaving Just to get you packing your bags I didn't mention (for reasons of brevity) that with OSGB36 the prime meridian is defined by a mark on the ground in Greenwich. With WGS84 the meridian is defined so that the average movement of the Earth's crust with respect to it is zero. This means that the whole UK is moving North East in WGS84 but is stationary in OSGB36. Then there is how to measure height which is a whole other story. ------- jeremyp The second ten million caches were the worst too. http://www.jeremyp.net/geocaching Quote Link to comment
+jeremyp Posted October 1, 2002 Share Posted October 1, 2002 quote:Originally posted by Nia:I changed the GPS datum to OS and it matches up a lot better now, still not 100% put closer. How far out is it? If it's less than 5 metres, that could be due to inaccuracies in the conversion method used by either the GPS or the map. quote:The children were shocked to learn that the planet is not a sphere, so to find its not even a perfect elipsoid, means we are thinking of leaving:) Just to get you packing your bags I didn't mention (for reasons of brevity) that with OSGB36 the prime meridian is defined by a mark on the ground in Greenwich. With WGS84 the meridian is defined so that the average movement of the Earth's crust with respect to it is zero. This means that the whole UK is moving North East in WGS84 but is stationary in OSGB36. Then there is how to measure height which is a whole other story. ------- jeremyp The second ten million caches were the worst too. http://www.jeremyp.net/geocaching Quote Link to comment
+Nia Posted October 2, 2002 Author Share Posted October 2, 2002 AHA !!! in February when we did one of our early caches - Obelisk on the Prime meridian Line I could not get the GPS and the marker on the ground to agree, so after 7 months i now know why. Thanks again. The remaining discrepancy is just the rounding up or down, the map only does seconds and the GPS does tenths of seconds. How do you know so much (Jeremyp)? Tech-no notice Quote Link to comment
+John Stead Posted October 2, 2002 Share Posted October 2, 2002 So the Earth moved! Try going to Greenwich Hill and putting your GPS on the line marked on the ground to the north of the observatory. Just to cloud the issue more I have found other meridians (meridia?) in Prague and Paris. The next one will be better! Quote Link to comment
+jeremyp Posted October 2, 2002 Share Posted October 2, 2002 quote:Originally posted by Nia: How do you know so much (Jeremyp)? When I first got my GPS receiver I took it down to my brother's house and we compared the reading it gave with an OS map. We couldn't understand at the time why the grid reference was wrong on the GPS receiver (the Garmin manual was less useful than a chocolate teapot in respect of explaining what the datums were for). I noticed I could get a better reading by changing the datum and later decided to do some research on what datums (data?) were. All the info I have given you is available at A Guide to Coordinate Systems in Great Britain ------- jeremyp The second ten million caches were the worst too. http://www.jeremyp.net/geocaching Quote Link to comment
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