Claus Brenner
e roof elements for which there is no hint in the ground plan (such as dormer windows) cannot be reconstructed.
e each segment of P triggers a new roof surface. It is not possible that one roof surface intersects different building
borders at different heights.
e ground plans P which are too detailed lead to unnecessary complex roof reconstructions.
In order to improve this, the DSM must also be integrated in the process of finding the roof structure, rather than being
used for measurement only.
3.1 DSM segmentation
Figure 8 shows the results of several segmentation algorithms for a laser scanner DSM with a ground resolution of one
meter. Figure 8(a) shows a segmentation into regions which have normal vectors compatible to the ground plan, and figure
8(b) shows a segmentation based on contours. Finally, figure 8(c) shows a RANSAC-based (Fischler and Bolles, 1981)
segmentation into planar regions; in this case, the ground plan is not used except for limiting the segmentation area. In our
opinion, approaches which use the ground plan to guide segmentation needlessly narrow the scope for later interpretation
steps. Thus, we have chosen the RANSAC-based planar segmentation as input for further processing.
(a) (b) ©
Figure 8: Results of several DSM segmentation algorithms. (a) Normal vector compatibility. (b) Contour based. (c) Seg-
mentation into planar regions.
3.2 A rule-based approach to obtain roof structure
Looking at figure 8(c), one sees that all major parts of the roof result in a segmented region. The task is now to filter the
regions and build a final interpretation. Region filtering can be done on the basis of general criteria such as region size,
shape and normal vector orientation. Using only constraints of this kind, however, means that no specific building model
is part of the reconstruction process. Often, this results in odd roof shapes. Therefore, in our approach we try to combine
the segmented regions from figure 8(c) using the ground plan and some rules as follows.
According to their normal vector, regions “along” a ground plan edge can be assigned certain labels (Fig. 9 (left)). For
example, if the normal vector of a region is parallel to the normal vector of the ground plan edge, it is called “compatible”
(c). Other labels are parallel to previos (p) or next (n) edge, perpendicular to ground plan edge (left (1) and right (r)) and
inverse of previous (a) or next (b) edge. Figure 9 (right) shows how some typical building parts would be labeled in the
ideal case. Note that one region can get several labels (for example, {n,r,a}). From the sequence of labeled regions along
one edge, sequence parts are accepted as follows (< denotes the start, > the end, z* one or more occurences of label x,
* stands for any label):
detected pattern accepted pattern
<ptatet + <yptat
ctbtnt> > bfnt>
«p'xn*» + <ptandn* >
[rt Urt
Additionally, all remaining patterns c* are accepted. Regions which have been grouped together during acceptance are
inserted into equivalence classes. Each equivalence class undergoes a new plane estimation process. Figure 10(a) shows
the regions which have been selected using this algorithm. Note the upper part where a cluster of regions could not be
explained by the rules and have been excluded.
International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B3. Amsterdam 2000. 89
