[pct-l] Question about Halfdmile App

[gary_schenk at verizon.net]

 On 12/23/13, Dan Welch wrote:

So my guess is that the GPS function is doing both coordinates and
elevation. Can anyone confirm or deny that?
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If you have to ask... :-)

GPS location is determined by your receiver analyzing data broadcast from the GPS satellites. Basically, your receiver triangulates its position in relation to the center of the Earth in x,y,z coordinates.

The elevation you read on your receiver is based on the equipotential surface. That's determined by mapping gravity. Mountainous wilderness areas are not high on the priority list when it comes to gravity mapping. Your unit should be a bit more accurate in town.

The difference in what your map shows and what your GPS shows can have several causes. One is a simple matter of datum. A lot of maps are based on the old 1929 vertical datum, which uses a different Earth model than modern datums. That can account for a few feet. There are other sources of error, atmospheric conditions, trees, signals bouncing off canyon walls, etc.

The main difference is timing. The GPS satellites broadcast their exact position at an exact time. Your receiver uses this to determine its location. The time is very important. The satellite has an atomic clock. Your phone has a Timex.

Gary
 

On 12/23/13, Dan Welch wrote:

So my guess is that the GPS function is doing both coordinates and
elevation. Can anyone confirm or deny that?