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.
______________________________
LR Disco3 TDV6 HSE 2005 Cairns Blue
Club BAS-FBHIC+Club BAS-Remap
Hidden Winch Mount ( Warn / Tabor 9K ) with Synthetic Line.
A-Frame Protection Bar with Lightforce 170 Xenon
Mantec Raised Air Intake LR Expo Rack OzTent Batwing,+ Frontrunner
GNVP-Suspension Rods+Rock Slider/Step+Full Underbody Protection
GG AT2s 255/60/18
Ham Radio ( HF-UHF ) Blackvue DR590X-2CH HD Camera
Squishy Cup Holders, Engel MT-35 Fridge, Cubby Fridge
GAP IIDtool BT
6th Dec 2013 1:42 am
petersw
Member Since: 17 Nov 2012
Location: Norfolk
Posts: 1135
Thank for that Jan R interesting
I wonder what the margin of error is ?Peter
6th Dec 2013 7:34 am
Robbie
Member Since: 05 Feb 2006
Location: ¯\_(ツ)_/¯
Posts: 17932
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.
2006, D3 SE Auto - gone but not forgotten.
2014 BMW 530d M Sport Tourer.
1977 Triumph Spitfire 1500
_________________________________________________
Disco Picture Website Here
6th Dec 2013 8:06 am
waterbuoy
Member Since: 26 Oct 2013
Location: Argyll
Posts: 2863
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
Previously:
TD5 Defender 110 CSW (230k miles)
300TDi Disco 1 (289k)
4 RR Classics (300-350k each, 2 manual, 2 auto)
110 V8 CSW (220k)
S3 109 hi cap pickup (ex RN)
S2A 88 Safari SW with lpg conversion (bloody lethal)
6th Dec 2013 10:27 am
Swiss-LR
Member Since: 08 Jan 2013
Location: In the middle
Posts: 55
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 3:58 pm. Edited 1 time in total
6th Dec 2013 2:59 pm
petersw
Member Since: 17 Nov 2012
Location: Norfolk
Posts: 1135
Ah ... not reliable then Peter
6th Dec 2013 3:38 pm
viethson
Member Since: 13 Jan 2009
Location: Bavaria
Posts: 136
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 6:10 pm
Bill Wright
Member Since: 03 Nov 2007
Location: Glorious Devon
Posts: 3254
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%?
Bill WrightNever take life too seriously. Nobody gets out alive anyway.
D4 SDV6 SE Tech Kaikoura Stone MY15.
6th Dec 2013 6:16 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
You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot vote in polls in this forum
