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"Digital terrain model" is the term for the representation 
of a terrain section for planning and design purposes by 
means of storing measured coordinates x,y,z of characteristic 
terrain points in a computer in sufficient quantity and 
significance so that by automatic interpolation the elevation 
of any point, determined by its coordinates x,y can be 
obtained with necessary accuracy and good adaption to the 
natural terrain surface. 
During the past years, digital terrain models have been 
developed for road design purposes. This development 
originates in the aim for optimal designs, especially in 
view of the relation between construction costs and operating 
safety. In order to record and store terrain in digital 
manner, lines such as road centre lines and cross sections 
are no longer recorded, instead strips of terrain wide 
enough to incorporate all possible centre line variations 
are measured point by point, recorded and stored. For 
automatic alignment in the digital terrain model new points 
are interpolated on the initial lines of the project - 
usually road centre line and perpendicular cross sections - 
the positions of which are known from preliminary graphical 
designs. In digital terrain models, which are no longer 
based on the design principle of determining the project 
by means of cross sections, new points are located along 
characteristic lines, like edge of road surface, shoulders 
of earth body, intersection lines of slope surfaces and 
terrain. The accuracy of the earth work quantity determination 
depends on the accuracy of the elevation of new points, 
obtained by automatic interpolation. Since for reasons of 
economy, there has been the tendency of utilizing the 
quantities calculated for the design also as control for 
compilation of the final costs, the aim has been to obtain 
them as accurately as possible. Therefore, the elevation 
of the terrain points used in quantity calculation should 
be determined in a most accurate way. Süll4 this is true 
only up to a certain degree, since it is not the points 
themselves that are of prior importance, but the terrain 
surface determined by those points and its good adaption