[at-l] Handheld GPS accuracy

Ryan Brooks ryan at hack.net
Sun Dec 6 16:29:31 CST 2009


Altitude is not always altitude: :-)

http://bhgc.wikidot.com/tutorials:differences-between-pressure-and-gps-altitude

http://www.navitrack.net/  (scroll down).
> Arthur Gaudet
> Lowell, MA
> Rockdancer97 at comcast.net
> "The vessel with the pestle has the brew that is true."
>
> I showed him the GPS
>   
>> that I was using to gather data with. It was absolutely the best
>>
>> hand-held mapping GPS available at the time. So is my current one, but
>>
>> it is not the same one.
>>
>>
>>
>> Anyway, the GPS had been sitting still for 10 minutes or so, making a
>>
>> fixed waypoint, and the satellite pattern was especially good that
>>
>> day. The expected Horizontal error was around 5 feet. Vertical is
>>
>> always about double the horizontal. The GPS disagreed with the GATC on
>>
>> elevation by about 30 feet at that shelter, as it did most of the way
>>
>> through Georgia. It turned out that when Del Doc had gathered data in
>>
>> the area in '02, there was a problem with the antenna that he was
>>
>> using. When the cartographer went to re-do the maps, he had to use the
>>
>> old 80s-vintage Topos as overlays in his GIS to guess at a lot of the
>>
>> elevations and shelter locations. When they later re-gathered the data
>>
>> with a newer, better GPS, the companion and my GPS started magically
>>
>> agreeing, or at least being very close.
>>     
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