The USGS has an API / Web Service for getting the Elevation of a given GPS point (latitude / longitude ).
Here's an example web service call.
The call in that example was for a point somewhere on Algonquin Mountain (Lon x="-73.9853668212891" Lat y="44.1447687597838"). The API tells us that the elevation at that point is: 4850.30527189961 Feet.
You can find more info about the API at the USGS
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