Elevation data recorded by GPS devices is notoriously inaccurate, especially in hilly terrain like the Russian River Valley example from our earlier post.
The final elevation track plotted from the Russian River Valley GPX file is suspicious for several reasons. First, the graph shows we descended almost 50 feet below sea level. That’s hard to believe, since we were travelling along the bank of the river, only about 10 miles from the Pacific Ocean.
Secondly, we were headed mostly west, following the river downstream, but the track shows a predominantly uphill trend.
We can evaluate the recorded GPS elevation data by comparing it to information available from the United States Geological Survey (USGS). The USGS operates a Web service that accepts latitude and longitude coordinates, and returns elevation data measured by NASA and assessed for accuracy based on over 13,000 control points in the continental United States.
Using the elevation Web service in an Altova MapForce mapping will let us extract each point from the GPX file, send the coordinates to the USGS Web service, and build a new GPX file with corrected elevation data.