Form lines are used where more information must be given about the shape of the ground. Form lines are added only where representation would be incomplete with ordinary contours. They shall not be used as intermediate contours.
Only one form line should be used between neighbouring contours. It is very important that a form line fits logically into the contour system, so the start and end of a form line should be parallel to the neighbouring contours. The gaps between the form line dashes must be placed on reasonably straight sections of the form line. Form lines can be used to differentiate flat knolls and depressions from more distinct ones (minimum height / depth should be 1 m). Excessive use of form lines must be avoided as this disturbs the three-dimensional picture of the ground shape and will complicate map reading.
Minimum length (non-closed): two dashes.
Minimum length of a form line, knoll or depression: 1.1 mm (footprint 16.5 m) outside measure.