Thursday 15 August 2013

Better GPS track elevation data

Few weeks ago we made a nice little hiking trip near Pazin. I forgot my Garmin and recorded the trip with my Android smartphone. The resulting elevation graph was pretty strange. Look at the black line:

The blue line is the elevation computed from SRTM (Shuttle Radar Topography Mission) data.

Is it obvious why the Android elevations were strange? The first thing is that the graph was cca 40 meters above the actual elevation lines. But this isn't something that bothers me too much. Because if it were only this -- the actual elevation gain should be the same (but the maximum/minimum elevations would be wrong).

But the second thing is what was really strange. Android cooked up a hill in the first 700 meters of our trip. A big hill, almost 420m high. But in reality there was no hill there.

This is why I added an option "Add GPS track elevation" on Trackprofiler. It is online almost two months now. But the actual data used is raw-SRTM data. And that's why the resulting graph was sometimes not a smooth line but a succession of horizontal lines (Look at the blue line in the graph).

I've been experimenting with few different algorithms to make a better elevation graphs from NASA's elevation data. And I think I finally found a good approximation. The result is the red line:

This new (and better) algorithm for elevation detection will be deployed is on Trackprofiler soon now.

PS. I'm not sure but the fact that my Android elevations are cca 40 meters higher may be because of the deviation between the ideal geoid and the actual Earth surface. 

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