In the last section, we discussed the creation of 3D breaklines for enforcing situations where the elevation must be a constant along the breakline. The most common example of this application is “water body flattening” such as lakes and ponds. In this final section we will consider the case of varying elevation along the breakline.
Recall that, for our purposes, a 3D breakline is a vector (feature) that has an elevation value (Z) associated with each vertex. Generally, 3D breaklines can be divided into two categories – those with the same elevation for each vertex (used for flat water bodies, for example) and those with the ability to store a different elevation value for each vertex (a down-stream flow polyline, for example).
In this section, let’s look at a varying Z example such as the edges of a road or a downstream flow. As with flat water bodies, a common method of collecting breaklines is to use heads-up digitizing from an orthophoto for the X, Y (planimetric) aspects of the construction and to probe the LIDAR (or, more generally, point cloud data) for the Z value. Unlike the constant Z flat water body breaklines, we must store the Z for varying elevation breaklines at each vertex. This means that these breaklines are always represented by 3D features.