Import grid from USGS SRTM (Shuttle Radar Topography Mission) data.
You find data and further information at:
http://dds.cr.usgs.gov/srtm/
http://www.jpl.nasa.gov/srtm/
Farr, T.G., M. Kobrick (2000):
'Shuttle Radar Topography Mission produces a wealth of data',
Amer. Geophys. Union Eos, v. 81, p. 583-585
Rosen, P.A., S. Hensley, I.R. Joughin, F.K. Li, S.N. Madsen, E. Rodriguez, R.M. Goldstein (2000):
'Synthetic aperture radar interferometry'
Proc. IEEE, v. 88, p. 333-382