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I was at the Royal Observatory at Greenwich this weekend. I noticed that my iQue shows 000.00.000 about 300 feet east of the location for the icon for the Royal Observatory, which BTW is labled "National Maritime Museum." (The National Maritime Museum is on Romney Road at the bottom of the hill and is correctly labled there.) The Royal Observatory is at the end of Blackheath Avenue where the map shows the second National Maritime Museum. But when you look at the location of the "National Maritime Museum" at the end of Blackheath Avenue the longitude is given as W000.00.045.
What was worse was that it appeared that the icon was actually on the prime meridian, but the GPS and the Longitude grid were in agreement showing 000.00.000 about 300 feet east of the prime meridian.
Doesn't this seem a little sloppy when locating the mother of all longitude?
I learned that the prime meridian was set at the Greenwich Observatory because they put a telescope there locked on north and south, and measured the exact time of transit of stars etc directly overhead. The original telescope, from which all else came, is still there. The prime meridian is set at its eyepiece.
Last time I was there they had the prime meridian layed into the ground in brass, so you could stand in two hemispheres at once.. did you see that? It's in the park.. did it line up with the gps?
It's a grand design the prime meridian and the brass line that has an atomic clock at one end so that you can capture your precise time of position is wonderful. Behind the clock and in the building itself lie the beautiful Harrison clocks. Anyone who shares an interest in time, and everyone who delights in GPS should aim for Greenwich.
Last time I was there they had the prime meridian layed into the ground in brass, so you could stand in two hemispheres at once.. did you see that? It's in the park.. did it line up with the gps?
The prime meridian is in the ground lined up, with the eye piece, and it is very near the icon on the map, but the gps read W000.00.045! Anyone with European detail map can check the icon for the National Maritime Museum at N51.28.665 W000.00.057. That is actually the Royal Observatory and it should be at W000.00.000. But it appears that the garmin gps longitude is off by 000.00.057. The Garmin reads 000.00.000 about 300 feet east of the observatory.
The UK stands upon a tectonic plate which has moved a bit since that brass line was laid down.
Just how far is debateable, in that the origional degree of accuracy, whilst the best at the time, is uncertain.
Remember, the meridian was laid down in a time before GPS when all they had was the stars and clocks that suffered heavy gains and losses. A ship's master would navigate by stars and sextant, and they would drop leading lines and bring back on board the material of the sea bed below them. Some masters could predict their position by the composition of the sand. Some could take an urchin or a prawn and by sniffing it's ass could tell if it had eaten 30 miles due south of one cliff or 25 miles due west of another.
The choice of prime meridian was arbitrary. It could have been the spot where a drunken geographer took a wizz. They were defining where prime meridian was. Where they said it was where it should be by definition .
Somewhere along the way to GPS, someone must have dropped the ball.
Originally posted by JMckie The choice of prime meridian was arbitrary. It could have been the spot where a drunken geographer took a wizz. They were defining where prime meridian was. Where they said it was where it should be by definition .
Somewhere along the way to GPS, someone must have dropped the ball.
The Royal Observatory was chosen because it was on a hill and was far away from London at the time that fires and smoke did not affect astronomical sightings. The exact point was chosen I believe by the architect who decided to have a dump and the pile was used as the reference point. Of course they don't teach you that part of it at school.
There is a monument erected at a village a few miles from where we live that supposedly is bang on the line. I must go there one day with my iQue and see if it is.
What map datum were you using? The Ique default is WGS 84 and WGS 84 uses the Bureau International de L'Heure defined zero meridian as it's reference and not the prime meridian, and then only after many other definitions. You can read more here.
What was worse was that it appeared that the icon was actually on the prime meridian, but the GPS and the Longitude grid were in agreement showing 000.00.000 about 300 feet east of the prime meridian.
Doesn't this seem a little sloppy when locating the mother of all longitude?
I learned that the prime meridian was set at the Greenwich Observatory because they put a telescope there locked on north and south, and measured the exact time of transit of stars etc directly overhead. The original telescope, from which all else came, is still there. The prime meridian is set at its eyepiece.
Like everything else in the GPS world, the iQue uses standard mathematical models of the world and depending on which model you have selected, you'll get a different lattitude/longitude reading from the exact same spot.
For example, I have a waypoint (my home) and with the GPS off, when I hit the "I" button (info on the point) it shows I'm at N 40:08:59.8 lattitude and W 105:08:38.7 longitude. That's using the WGS84 geoid model. If I simply change to the WGS72 model, my waypoint suddenly moves (the GPS is not on, it's the saved position being recalculated) to N 40:08:59.6 Lat. Or 0.2 seconds of lattitude. IF I pick the Irish Grid instead (1965) suddenly my location is now: N 40:08:41.8" and W 105:09:00.7
Again...the fixed spot on the map moved by 18 seconds of latitude and 1 second of longitude.
It's all based on the model used, and none of them are 100% accurate.
Assuming that you used WGS84 as the map datum in your iQue, I think this is a map datum issue.
In Britain the OSGB1936 datum is used as map datum, so the prime meridian at Greenwich is based on this datum.
Using a co-ordinate trnasformation program, I transformed the position with latitude 51°28' & longitude 0°00'00.000" (prime meridian at Greenwich) based on OSGB1936 to WGS84. The longitude in WGS84 turns out to be 0°00'05.750" W, which is about 110 meters, about 360 feet, from the prime meridian. This corresponds pretty much with your observation.
Linear Mode
