A raw manuscript directly plotted by stereo plotter contains many mistakes and failures which should be edited and compiled manually. In the case of contour lines, editing process takes long time and needs high skills.
Now analytical plotters and analogue plotters equipped with digital encoders became available and thus digital contour line data can be obtained, and an automatic method to compile digital contour lines has been studied.
Compilation of contour lines is a kind of smoothing of the surface plane expressed by contour lines. Therefore, simple curved surface (named Local DTM, DTM=digital terrain model) is calculated to represent the local terrain within the neighborhood of the concerned point on a contour line. Then the point is to be moved horizontally so that the point is dropped on the computed surface.
Local DTM of the neighboring points are so strongly correlated to each other that we can obtain smooth contour lines consist of the moved points.