Use Copernicus GLO-30 DEM instead of SRTM GL1 for latitudes above 60 or below -56

[mkrupczak3]

The SRTM dataset is of excellent quality, however it only covers latitudes from 60 N to -56 S.

This can cause the OpenAthena software to fail to function at higher latitudes. Notable areas not covered by SRTM include:

• Most of the state of Alaska
• Greenland
• Iceland
• Norway
• Sweden
• Finland
• Northern Canada
• Russian Siberia

In cases where latitude is out of bounds of the SRTM dataset, OpenAthena should use the Copernicus GLO-30 DEM instead.

This dataset was generated by the German TanDEM-X satellite mission using SAR. This is very similar to the SRTM mission, however the shorter wavelength of X band compared to C band radar means it will penetrate less vegetation (making it more of a digital surface model rather than DTM)

See the OpenTopography API Docs for request format:
https://portal.opentopography.org/apidocs/#/Public/getGlobalDem

[mkrupczak3]

Vertical datum of GLO-30 is EGM2008 rather than EGM96. We should consider using Android's new AltitudeConverter class for converting vertical datum to WGS84:

matthiaszimmermann/EGM96#1 (comment)