This package downloads, converts and combines the Global
Multi-resolution Terrain Elevation Data 2010 (GMTED2010) and the
Shuttle Radar Topography Mission (SRTM) Version 2.1 [0] digital
terrain elevation data (DTED) at 3 arc-seconds resolution to one
geotiff [1] file. You really should read [2] for GMTED2010 details
(unfortunately it seems like this report is not freely redistributable
and can't be included in this package).

Important notes:

- GMTED2010: at the moment only the 7.5 arc-seconds mean and for the
  the area between 56d 0' 0.50"S and 90d 0' 0.50"S and for Greenland
  the 30 arc-seconds mean layers are used

ATTENTION: modified data (Antarctic and Greenland)!

- GMTED2010: at the moment the area between 56d 0' 0.50"S and 90d 0'
  0.50"S (Antarctic) and Greenland are interpolated ("upsampled") from
  30 to 7.5 arc-seconds resolution during the conversion process using
  gdalwarp (for details see the Makefile).  This is done to have a
  simple global DEM in one geotiff file. I don't really like that
  solution. In the future I would like to use at least an
  interpolation which if downsampled again (using mean average /
  gdaladdo -r average) produces the original 30 arc-seconds data.
  Though a quick look suggests that the difference between the
  upsampled and then averaged and the original data is very small (<=
  1m)

- unfortunately GRASS GIS doesn't allow an extent > 90S or 180W
  (sub-pixel overlap) => GRASS users might want to use
  freedem_grass.vrt to cut off two columns and one row on the fly
  (s.a. http://trac.osgeo.org/grass/ticket/1734). You can use
  gdal_translate -srcwin ... -of vrt to produce such files (for
  details see the Makefile)

Misc notes:

- the produced geotiff uses internal tiles, lzw-compression and
  includes overviews

- no data for 83d59'59.50"N to 90N
  (arctic ice / water => maybe fill with 0?)

Quick test using gdal:

If you have a good internet connection and gdal you can use the
freedem_remote.vrt available at [3] to take a look at the the result
downloading data on the fly.

(something like:
wget 'http://karme.de/freedem/data/freedem_remote.vrt'
qgis freedem_remote.vrt
should work)

References:

[0] http://dds.cr.usgs.gov/srtm/version2_1/Documentation/

[1] http://en.wikipedia.org/wiki/GeoTIFF

[2] Danielson, J.J., and Gesch, D.B., 2011, Global multi-resolution
    terrain elevation data 2010 (GMTED2010): U.S. Geological Survey
    Open-File Report 2011–1073, 26p.
    http://pubs.usgs.gov/of/2011/1073/pdf/of2011-1073.pdf

[3] http://karme.de/freedem/data/freedem_remote.vrt
