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Ichibod
Member Since: 26 Aug 2008
Location: Muscat, Oman
Posts: 44
  
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Can anyone tell me how to calibrate the altitude on the Off-Road Navigation?
Apparently I'm 20m below sea level. (i'm not, beleive me).
Rich
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 16th Sep 2008 10:41 am |
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Jimbob Squarepants
Member Since: 27 Oct 2007
Location: Miles Away from Home
Posts: 1063
  
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The height given you by the navigation system will be derived from the GPS. This is the height above the geoid (a mathematical approximation to the shape of the Earth). The Geoid does not match sea level exactly and varies by different amounts, depending upon where you are in the world. 20 metres does seem a little excessive though. Too much intellectual firepower can damage your common sense.
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| 16th Sep 2008 10:45 am |
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Ichibod
Member Since: 26 Aug 2008
Location: Muscat, Oman
Posts: 44
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Hmmm - so how does the GPS know your height relative to the geoid?
Is there any way of changing the geodetic datum to one more relevent to the actual location. Round these parts, when we're trying to find that expensive black stuff, we use Clarke 1880.
Rich
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| 16th Sep 2008 11:01 am |
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Jimbob Squarepants
Member Since: 27 Oct 2007
Location: Miles Away from Home
Posts: 1063
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I doubt very much that you willbe able to change the datum - it is almost certainly WGS 84.
GPS is basically a 3 dimensional radio ranging solution. The satelites are in a known orbit. They continuously transmit their position and a very acurate time stamp. There's an accurate clock in the receiver. By knowing where the sats are and the time taken for the signals arive the receiver can calculate it's position - if it can see 3 or more sats then you get a 3D position. All calculated relative to the Geoid.
The reason that they use Clarke 1880 will be that previous survey data is based upon it. Nearly all surveys now are positioned using GPS and therefore the raw data is WGS 84 spheroid and datum. A datum shift is applied afterwards to convert it to Clarke. Too much intellectual firepower can damage your common sense.
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| 16th Sep 2008 11:11 am |
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Ichibod
Member Since: 26 Aug 2008
Location: Muscat, Oman
Posts: 44
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Thanks James, I've learned something there.
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| 16th Sep 2008 11:21 am |
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ACB
Member Since: 21 May 2007
Location: BIRMINGHAM
Posts: 20
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Best way to calibrate is manually which is what I do with mine. Check reading at a point that you know the height of probably off an OS sheet and calculate the difference. My set up is about 63 feet different from actual reading to AOD ( above ordanace datum)
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| 16th Sep 2008 1:42 pm |
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Popelka
Member Since: 31 May 2008
Location: Praha (Prague)
Posts: 2430
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can you expand on your instruction or is this possibly only for your make/model of GPS? Experience is a difficult teacher, because she gives the test first and then the lesson afterwards!!!!
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| 17th Sep 2008 5:22 am |
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Ichibod
Member Since: 26 Aug 2008
Location: Muscat, Oman
Posts: 44
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I think ACB means 'mentally', rather than 'manually'.
When I was messing about with the nav screen once, pressing it for 5 seconds to get access to the diagnostics and so forth, I saw some more advanced set-up options, possibly only available to the dealer folk, and as I'm sat in my office and not my car, I can't remember what exactly it was. But I wonder if there's another layer of set-up.
Rich
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| 17th Sep 2008 5:29 am |
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