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SirSwampThing
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Joined: Jan 22, 2006
Posts: 55
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 Posted: Sat May 06, 2006 8:43 pm Post subject: accuracy of altitude? |
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I have been trialing various apps to choose the right one for me when hiking or backpacking. Vito Navigator 2, GPS Tuner 4.2, MemoryMap and a few freebies I have found while scouring the WWW.
All seem to do what I want them to do, with a few limitations, but the one over-riding problem I have is that ALL of them give innacurate altitude readings....this is even more suprising seeing that MM on my PC gives my house altitude correctly (I have checked this with Ordnance Survey themselves over the `phone) but still gives a way off measurement when on my Mio 168. All give readings that suggest I am approx. 40-60m higher than I actually am.
Is this a fault with my Mio or just one of those things that affects us all due to limitations/restrictions/faults whatever they may be within the satellite data I am receiving?
Any ideas??
cheers |
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oddsock
Pocket GPS Verifier
Joined: May 01, 2004
Posts: 706
Location: Monmouthshire
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| Posted: Sat May 06, 2006 9:40 pm Post subject: |
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Read... This..
Dave
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TomTom one v1,tomtom one v 3,TT Start20,Garmin Oregon 300,Nuvi 300
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SirSwampThing
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Joined: Jan 22, 2006
Posts: 55
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| Posted: Sat May 06, 2006 9:47 pm Post subject: |
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wow!!!
Cheers Dave , explains everything ... |
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daktari
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Joined: Dec 12, 2005
Posts: 17
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Skippy
Pocket GPS Verifier
Joined: 24/06/2003 00:22:12
Posts: 2946
Location: Escaped to the Antipodies! 36.83°S 174.75°E
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| Posted: Mon May 08, 2006 6:58 pm Post subject: Re: accuracy of altitude? |
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| SirSwampThing wrote: | | All give readings that suggest I am approx. 40-60m higher than I actually am. |
The earth is not perfectly round, and the height of mean sea level varies from place to place.
The GPS unit is supposed to figure out what area of the earth you are on and apply a correction factor to give the correct height above sea level.
The problem you are having is probably because you are using a GPS which is based on the SiRF chipset which doesn't do this so in the UK/Europe it will report a height which is about 50 meters too high.
Some GPS programs have a tick box for "SiRF chipset" which will compensate for the incorrect height reported by these units.
Also be aware that the height is not as accurate as the lat/lon positioning. If your lat/lon accuracy is quoted as 10 meters then your height accuracy is typically 1.5 times this - ie 15 meters.
Furthermore, if you don't have 4 sats locked on then the height may be completely wrong because it needs 3 for a 2D (lat/lon) fix and 4 sats for a 3D fix (including height)
I hope that clarifies things. 
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