I do the first as mastone suggested paldav...but low poly and then into Zbrush...otherwise the mesh becomes too dense...then normal map. I have used a method that can extract 'z' values from satelite images but its a real bitch. I can use my Survey software but it doesnt translate well into Maya. I did have a tut somewhere...hmmmm
Cheers bullet
bullet1968
"A Darkness at Sethanon", a book I aspire to model some of the charcters and scenes