The GPS receiver has a " Theoretical Spheric Geoid " that resembles the earth.
The earth is not absolutely round as a ball, it is indeed a slightly squashed sphere..
(the reference ellipsoid used in the geodetic system is itself just an approximation of the true geoid)
So in fact when You check your altitude , it does'nt show your height above the sea..
It shows Your altitude compared to the " Theoretical Spheric Geoid )
The only systems that can give the Correct data is the sort that " Surveyors use "
Because their system has the correct Geographical Data in the Mapping software.
( So Called " GIS Software ( Geographic Information Software ) " )
Hope this bit of info will shed some light over this Matter..
Regards
Jan R.Jan R.
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6th Dec 2013 12:42 am
petersw
Member Since: 17 Nov 2012
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Thank for that Jan R interesting
I wonder what the margin of error is ?Peter
6th Dec 2013 6:34 am
Robbie
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It can be quite a bit when you throw a GPS guided bomb at a coordinate without doing the hard sums.Land Rover - Turning Drivers into Mechanics Since 1948
In the Pyrenees a couple of years ago, we were at about 8000' ISTR and the 4x4 info was about 300-400' out if I recall correctly - or at least out compared to a hand held GPS unit. Whatever that may mean Mark.
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6th Dec 2013 7:06 am
waterbuoy
Member Since: 26 Oct 2013
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When at sea we are (usually) pretty close to mean sea level, yet different GOS units can put us up to 200m+ either side of zero.
As an aside, whilst car based sat navs will usually 'snap' the car onto the most likely feature (ie road) the same does not apply with recreational chart plotters. Consequently, it is often possible for one to sail over 'dry land' from time to time as the charts are often based on cartography that originate from surveys carried out by leadline in the 1700s Currently 2009 Disco 3 SE, 2013 MY D4 HSE and 2016 D4 SE
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6th Dec 2013 9:27 am
Swiss-LR
Member Since: 08 Jan 2013
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Two weeks ago we did some driving in the snow on a pass at an altitude of approximately 1500 m. The difference at the highest point, where the altitude was written on a sign was 105 m.
My watch, which has a barometric altimeter, showed only a difference of 5 m.
Last edited by Swiss-LR on 6th Dec 2013 2:58 pm. Edited 1 time in total
6th Dec 2013 1:59 pm
petersw
Member Since: 17 Nov 2012
Location: Norfolk
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Ah ... not reliable then Peter
6th Dec 2013 2:38 pm
viethson
Member Since: 13 Jan 2009
Location: Bavaria
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I think it really depends on the quality of the receiver. Calculating altitude by GPS signel is quite complicated much more than calculating position. However with a good receiver it can be quite accurate. GPS altitude in an airliner at 39.000' is only maybe 200' off the barometic altitude indicated on the altimeters. I think that is a good value. Also as mentioned GPS altitude is calculated above the reference ellipsioid and not above mean sea level so there always is a difference. But in real life it doesn't really matter.
In Munich (about 500m above MSL) the altitude given by the Disco is accurate to about 10m. Good enough I think.Life in the slow lane
6th Dec 2013 5:10 pm
Bill Wright
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Had a trial today at a known height on Woodbury Common parachute DZ which is registered as 1806ft. My alt was reading 1768ft so out by about 2%?
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6th Dec 2013 5:16 pm
petersw
Member Since: 17 Nov 2012
Location: Norfolk
Posts: 1135
Thats a good idea Bill, Will have to get the O/S map out and have a recce Peter
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