Topographic rendering from SRTM (Shuttle Radar Topography Mission).

I got the data from http://srtm.csi.cgiar.org/ where you have a nice map to select from.
I used the sript http://www.stevematt.f9.co.uk/sl/App1/srtm.html to preview the .hgt data.
Then I used the online data conversion (http://converter.mygeodata.eu/) to convert .hgt file to .asc file which I opened with Notepad. I left some data rows and columns aside to keep the ones I wanted. (You can use any spreadsheet to handle the .asc file, by converting the data separated by spaces).

Then I used the surface() function in openscad to render. Dont forget you have to mirror the Y axis, and dont forget to scale down the altitudes. 1 x or y count represents 1/1200 (depending on the data file of course) of a 1° angle.
So 1° angle represents 60 time a mile = 1.852*60 = 111.12 km
So each step on X or Y axis represents 111.12km/1200 = 92.6m.
You should scale down Z axis by 1/92.6. (0.0108)

Inspired from http://www.thingiverse.com/thing:16851