Digital terrain models (DTMs) and orthophotos have become essential tools for the planning, construction and management of all major transportation networks. Plans for roads, motorways, railways and airports are based on maps and accurate large scale altimetric measurements produced by laser-scans and digital cameras. During the planning stage DTMs can be used to study the environmental impact of new structures. During construction, GPS systems mounted on earth moving equipment can be used as a guide for accurate levelling. DTMs are also essential for the study of acoustic impact along road- and rail corridors and near the airports. They are used in the planning of effective sound prevention barriers. DTMs are used in airport areas to localise and identify obstacles or the Obstruction Identification Surface.