ISPRS Commission III, Vol.34, Part 3A ,,Photogrammetric Computer Vision“, Graz, 2002 
  
offset, and the reflectance data for the determination of the 
planimetric offset. 
Both height and reflectance data can be used for the 
determination of the strip offsets, but both have there 
advantages and disadvantages. These will be analysed in the 
following paragraphs. Furthermore, new procedures will be 
suggested to improve the accuracy of the matching and to make 
matching possible in a larger number of areas in order to 
increase the number of offset measurements between the strips. 
2. MATCHING HEIGHT DATA 
For stereo matching it is well-known that texture is required in 
order to obtain a good precision of the estimated disparities. 
Similarly, when matching height data, height variations are 
required in order to be able to estimate planimetric offsets. 
However, there are restrictions to the kind of height variations 
that can be used for matching height data. 
* As already pointed out by Kilian et al. [1996] and Maas 
[2000], areas that are occluded in one of the strips should 
not be used for matching. The usage of heights that are 
derived from interpolation over an occluded area results 
into systematic errors in the determination of the offsets. In 
laser altimetry data sets, such occlusions are mostly caused 
by buildings. 
Due to the characteristics of the laser sensor, one should, 
however, also avoid the usage of height jump edges in areas 
that are not occluded. When taking an image with a CCD- 
camera of a checkerboard, pixels that cover a part of a white 
field and a part of a black field will obtain grey values that 
are somewhere in between white and black. Such mixed 
pixels do not occur in height images acquired by laser 
altimetry sensors. If a laser beam at the edge of a building 
roof hits both a part of the roof and the ground, the recorded 
height will be either the roof height or the ground height, 
depending on whether one selects first or last pulse data. 
Hence, the characteristics of a roof edge in a height image 
are comparable with the characteristics of an edge in a 
binary image. The location precision of such edges, and 
thus the precision of matching height images using these 
edges, depends on the length of the edges and the 
orientation of the edges with respect to the grid [Fôrstner 
1986]. For laser altimetry data, the orientation of the grid 
corresponds to the orientation of the scan lines of the laser 
scanner. In the worst case (edges parallel or perpendicular 
to the scan lines) biases of up to 0.5 times the distance 
between the laser points may occur in both strips. Hence, 
matching such edges in data sets with a point distance of 
e.g. 2 meters, may result into an error of 2 meters. 
Although the maximum bias that may occur varies with the 
orientation of the edges and may average out over a large 
number of edges, the quality of the offset estimation is hard to 
predict. Whenever possible, one would like to avoid the usage 
of height jump edges for the estimation of planimetric offsets, 
even if these edges do not cause occlusions. This is in sharp 
contrast to matching grey value images where strong step edges 
give the best matching results. 
In order to estimate the planimetric offsets from the height data, 
height variations are, however, required. These height 
differences then need to be provided by smooth surfaces with 
surface normals pointing in three independent directions. At 
least two of these surfaces need to be slanted. Parts of sloped 
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terrain or slanted roof faces can provide suitable information. 
Unfortunately, the number of locations in strip overlaps with 
such surfaces will usually be small, in particular in rural areas 
with relatively flat terrain. Under these circumstances it will not 
be possible to find sufficient locations where offsets between 
strips can be measured in three dimensions. Therefore, tie 
points should be completed with other tie features such as 
ridges or planes that only supply information on the offset in 
two or one dimension respectively. These dimensions, of 
course, do not need to be parallel to one of the axes of the 
coordinate system. By combining the different tie features in a 
strip overlap, sufficient information should be acquired to make 
the strip adjustment possible. 
In flat terrain usually provisions are made to drain water. Figure 
1 shows an example of such a terrain with many ditches in 
meadows and along roadsides. Such linear structures can be 
used to determine the offsets between strips in height and in the 
direction perpendicular to the ditch orientation. 
  
Figure 1: Height image with ditches. 
The height data of such structure can, however, not be matched 
with a standard image matching tool. Because of the relatively 
small width of the ditches with respect to the distance between 
the laser points, not every ditch part is represented well by the 
laser points. When computing a DEM from the triangulated 
laser points, interpolation between points on either side of the 
ditch produces incorrect height values (figure 2). Such errors 
would effect the performance of a standard image matching 
algorithm. 
Figure 2: Perspective view of a DEM part with a ditch. 
Viewing the point cloud in the direction of the ditch confirms 
that the ditch of figure 2 is uninterrupted. An estimation of the 
offset of such a ditch between two strips can be made if the