http://dds.cr.usgs.gov/srtm/version2_1/SRTM3//srtm/version2_1/
http://dds.cr.usgs.gov/srtm/version2_1/SRTM3/Africa/
http://dds.cr.usgs.gov/srtm/version2_1/SRTM3/Australia/
http://dds.cr.usgs.gov/srtm/version2_1/SRTM3/Eurasia/
http://dds.cr.usgs.gov/srtm/version2_1/SRTM3/Islands/
http://dds.cr.usgs.gov/srtm/version2_1/SRTM3/North_America/
http://dds.cr.usgs.gov/srtm/version2_1/SRTM3/South_America/
