DTED driver creates incorrect files on latitudes -80, -75, -70 and -50

[pnicolas]

When creating a dted file, pixel width depends on latitude. Values are briefly described on ​https://en.wikipedia.org/wiki/DTED or in the full specification at ​http://earth-info.nga.mil/publications/specs/printed/89020A/89020A_DTED.pdf

Tiles are referenced by the lower left corner, a tile with lower left corner at 50S latitude covers 50S to 49S. The specification is not explicit on the step to be used on such tiles but it should be the step for latitude 0-50.

For instance, the srtm files are distributed as dted on earthexplorer. Tile ​SRTM3S50W075V2 has dimensions 1201x1201. Executing gdal_translate -of dted s50_w075_3arc_v2.dt1 translated.dt1 produces a 601x1201 file.

The thresholds in dted_create.c should be on the lower left for northern hemisphere and upper left for southern hemisphere on ​https://trac.osgeo.org/gdal/browser/trunk/gdal/frmts/dted/dted_create.c#L147

[Even Rouault]

If you've a patch to fix this, this would be welcome

[Even Rouault]

In 38002:

DTED: correctly create files at latitudes -80, -75, -70 and -50 (patch by pnicolas, fixes #6859)

[Even Rouault]

In 38003:

DTED: correctly create files at latitudes -80, -75, -70 and -50 (patch by pnicolas, fixes #6859)

[Even Rouault]

In 38004:

DTED: correctly create files at latitudes -80, -75, -70 and -50 (patch by pnicolas, fixes #6859)

[Even Rouault]

Thanks !