Cross sections provided by the surveyor are usually as indicated in figure 4:

Figure 4

Each point is input in terms of its cross-wise abscissa and its elevation. The section may be introduced either from the left bank or the right bank.

Imagine that one begins from the left bank. The first point need not be entered because the canal bank stops at point 2; similarly for point 9. If one enters points 1 and 9, they will be disregarded by the program. In fact the left and right points of the section have to be disregarded if their elevations are less than or equal to the elevation of the canal bank itself.

You must therefore take these point deletions into account if you enter a section that you want to close to the ceiling. So rather than closing "abruptly" horizontally, or even less well when going down, which would give 2 points at the same height, or an extreme point lower, it will be necessary to maintain a slight increasing slope when returning towards the initial point.