This is a way i found to make very large terrains from satellite images (Usgs Dem,srtm Data,Globe Tile,Lidar point cloud,Mars Mola,Geo tiff dem,Terrain Matrix bin,gtop030 tile,XYZ POINT CLOUD,Mars polar Mola mgt img format)first download the free version of 3dem Visualization here (http://www.simtel.com/product/view/id/6700)dem tutorials here (http://www2.cs.uh.edu/~somalley/DemTutorial/)and you must download a earth globe tile for 3dm visualization here you can download only the tile from earth you need not all earth (http://www.ngdc.noaa.gov/mgg/topo/gltiles.html) now go to load terrain model in file menu of 3dm program and select the option (globe tile selected from map) select or resizing the image or location you what and save then in file menu Save terragen terrain. At this point go to Terragen (free program to)go to landscape and open ter format,in export options open the raw 8 bites file export, depending from image size in 3dm visualization you can have raw 8 bites whit 256 levels terrain height 512 levels or 1024 l this is already a very high definition terrain level. If you what go to google earth an whit pro version take a shoot satellite map whit high definition and aplay in your favorite program the texture in terrain: they are ter plugin for max in guru ware. And then only must load terrain raw 8 in unity 3d. Thats all people some questions put a replay in mi post.