[time-nuts] Weird T-Bolt elevation readings...

Bruce Griffiths bruce.griffiths at xtra.co.nz
Thu Jan 14 21:47:24 UTC 2010


Björn

In the case of the Thunderbolt with the default WGS84 datum the height 
is calculated with respect to the MSL geoid.
If another datum is selected the height is calculated with respect to 
the ellipsoid.
The attached snapshot from the Thunderbolt book makes this clear.

The MSL goeid doesnt coincide exactly with MSL at every point on the 
Earth's surface.

Bruce
bg at lysator.liu.se wrote:
> Hi Bruce,
>
>    
>> The GPS altitude is calculated with respect to the geoid.
>>      
> No, the GPS calculates altitude wrt the ellipsoid. Then its a conversion
> with lots of coefficients or a lookup table to add the geoid separation.
> Altitude wrt to the geoid is the same as Mean Sea level, which is what
> normal maps etc gived height in.
>
> --
>
>     Björn
>
>    
>> The geoid surface may have a vertical separation from the actual surface
>> by several meters.
>> This correction varies with location on the Earth's surface.
>>
>> Bruce
>>
>> Michael Baker wrote:
>>      
>>> Hello, TimeNutters--
>>>
>>> While we are on a thread about Trimble T-bolts,
>>> perhaps someone might expand on why my T-bolt
>>> does not ever come up with altitude readings that
>>> are even close.  After a long fix, the Lat-Lon
>>> coordinates are pretty close, but the altitude
>>> is always given as around 2 meters.  We are pretty
>>> low here in Flori-DUH, but not THAT low.  My
>>> ground elevation here is 28M ABMSL and my
>>> GPS antenna is another 8M above that on top
>>> of my fireplace chimney.  I have roughly 50 ft
>>> of RG-59 cable on the antenna, but altering the
>>> cable length value does not seem to have any
>>> effect.  I get these same very low altitude readings
>>> with TBOLTMON and Lady Heather v3 beta.
>>>
>>> Since I know my altitude well withing one meter,
>>> should I enter that manually?  What is the
>>> procedure for doing that?
>>>
>>> Suggestions...??
>>>
>>> Thanks!!
>>>
>>> Mike Baker
>>> -------------------
>>>
>>>
>>>
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>>>        
>>
>>
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>
>
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