SRTM Version 3, Global 1 arc

[nawagers]

I'm starting work on integrating the new SRTM Global 1 arc sec and 3 arc sec data sets. These are available from the USGS EarthData server and requires (free and open) credentials to access. srtm.py will need to be supplied a U/N and P/W to pull those tiles. I am unable to find an authentication-free copy like the SRTM version 2.1 tiles.

The new data sets have the same coverage areas as the previous datasets, but are now 100% void free and have the 1 arc second sampling size over the entire coverage area. The 1 arc second tiles are around Nyquist frequency near the equator, but oversampled at more extreme latitudes (~4x oversampling)

Changes:

• Currently, saved tiles in cache and memory do not save the source information (like 1 arc vs 3 arc). If a previously cached tile uses the 3 arc second grid, and the current instance wants 1 arc second grid the behavior is to just load the 3 arc second grid tile. The saved tiles will need to have source information saved with them.
• The interface for selecting datasets will need to change. I suggest the srtm1 and srtm3 parameters get deprecated and dataset(string of type) and fallback(boolean) get added for a more extensible and cleaner interface
• The dictionary for URLs has lots of duplicated information. I suggest a format change: the tilename could be the key "N00E000" and it would return a tuple or namedTuple of (Region, in1arcSRTMv21, in3arcSRTMv21, inSRTMv3)
• A new interface for supplying credentials will have to be added
• Travis will need to have secret environment variables set with the UN and PW for CI testing

Anyone that would like to suggest interface changes is welcome to comment.

[nawagers]

I forgot to add, if the new interface is well layed out, adding the old version 1 data access should trivial if anyone ever wanted that.