AutoCAD Civil 3D – Breaklines – What do Supplementing Factors do?

By Geoff Woodhead

Breaklines are one of the best features of Civil 3D for site design. For those of you with little experience of them they are a line, be it a polyline, feature line, arc or simple line that is applied to a surface. It’s purpose is to tell all the triangles in our surface that they may not pass through the line but instead must break upon it. This results in an increased level of triangulation in our surface, giving a much better defined model.

Those of you that are familiar with breaklines have probably noticed that when they are added to a surface the newly split triangles can only have a point on one of the vertices of the added breakline. This does not always give the desired look to the surface. Users may be happy with it but probably only because they’ve not experienced the added benefits we get from supplementing factors.

Supplementing factors can be found at the bottom of the Add Breaklines dialog box as shown in fig. 1 below. Supplementing factors work by adding extra interpolated vertices to the surface at the specified intervals. The great thing about adding the data in this way is that if we ever want to amend the levels of the line in the future we aren’t going to have a whole load of extra physical vertices to manage as the software is handling all of the extras behind the scenes and not even revealing them to us. The two important values are the distance (on straights) and the Mid-ordinate distance (on curves). I tend to use values of 1m for straights and 0.01m for curves.

Fig. 1:

I’m going to finish off with a comparison of un-supplemented breaklines vs supplemented breaklines but I’m sure the differences will speak for themselves. Fig. 2 shows un-supplemented breaklines. Look at the curves and notice how they are not very curved. The right turn lane is showing as more of an arrowhead. Fig. 3 shows supplemented breaklines with 1m between points on the straights and 0.01m between them on the curves. We can see that the curves are looking a lot more curved and the amount of triangulation on the whole is increased, there are no jagged edges and the surface blends together much better. The two surfaces are almost uncomparable but were created from the exact same objects.

Fig. 2:

Fig. 3:

If you’re new to breaklines or just to supplementing factors I’m sure you will find the above useful.