reference for simplifying terrains:
1. http://www.disi.unige.it/research/Geometric_modeling/research.html
2. http://www.cs.cmu.edu/afs/cs/user/garland/www/scape/
3. http://graphics.cs.uiuc.edu/~garland/software/scape.html -
software
4. http://www.ri.cmu.edu/people/heckbert_paul_text.html
5. http://gts.sourceforge.net/reference/gts-surface-simplification-and-refinement.html
6. http://vcg.iei.pi.cnr.it/metro.html
7. pca 2D, dft, dcosin trasform, karhumen-loeve approximation
8. http://www.cs.iupui.edu/~dliao/research/dtmvox.htm
9.
http://www.computer.org/proceedings/hpc/0589/volume%202/05890613abs.htm
10.