Complex sentence: Older methods of generating DEMs often involve interpolating digital contour maps that may have been produced by direct survey of the land surface, this method is still used in mountain areas, where interferometry is not always satisfactory.
Split into simpler sentences: Older methods of generating DEMs often involve interpolating digital contour maps that may have been produced by direct survey of the land surface. This method is still used in mountain areas, where interferometry is not always satisfactory.