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I am playing with srtm files and I dont quite understand my results. I am testing a lat lon of N39 W77 or 39.0,-77.0. I have tested using the python script from this post: How to extract elevation from .HGT file? and using gdallocationinfo. When I plot the point in google earth, it says the elevation should be 73-74m, gdal reports 81m. I'm curious why the difference?

gdallocationinfo N39W077.hgt -wgs84 -77 39
Report:
  Location: (0P,1200L)
  Band 1:
    Value: 81

enter image description here

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From MIL-PRF-89020B (which includes SRTM requirements), Section 3.3.1.1.2, DTED1 absolute vertical accuracy is 30m (90% Linear Error, to MSL). DTED2 absolute vertical accuracy is 18m (Section 3.3.2.1.2).

So 6-7 metre differences are well within spec for SRTM. Even pixel to pixel accuracy can vary by more than that and still be in spec (Section 3.3.2.2).

Given the difference, it appears Google Earth is using something other than SRTM for that location - there is plenty of good height data from other sensors for some parts of the US. You could try USGS Earth Explorer for some freely available ones.

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