the surface discontinuities. The breaklines and the laser data are 
then merged to create a new data set. 
The most significant surface discontinuities occur due to 
buildings, thus the rooflines of buildings are collected and used 
for the data fusion. As only the rooflines are measured, the 
surface at the ground level will not be adequately defined. 
Therefore, to better define the visible surface, it is necessary to 
incorporate measurements at ground level surrounding the 
buildings. This is accomplished by vertically projecting the 
horizontal position of the roofline to ground level. The elevation 
of the ground at that point is estimated using the surrounding 
laser data points. The breakline from these new points is slightly 
offset from the position of the roofline, as the triangulation 
procedure does not accommodate points with exactly the same 
horizontal position. 
4.1 Surface Generation and Registration 
Initial surfaces were generated to enable the registration of the 
two data sets. A digital elevation model (DEM) was generated 
using the softcopy photogrammetry program OrthoMax, under 
the Erdas Imagine environment. The laser surface used for the 
registration process is shown in Figure 2. 
  
  
  
  
  
  
  
  
Figure 2. Laser surface used for registration. 
The transformation parameters were determined using the 
developed registration algorithm. As a check on these results, 
roof breaklines were measured analytically to produce planes, and 
the laser points which occurred in the vicinity of these planes 
were used in the registration process, therefore providing a result 
which was not dependent on automatic DEM generation 
techniques. The results were of similar magnitude in each case, 
with the vertical shift between the surfaces being approximately 
one meter. Further details are provided in Postolov et al. (1999). 
4.2 Results of Data Fusion 
For the current experimentation, the breaklines have been 
measured manually using an photogrammetric workstation. Roof 
breaklines were collected over a residential area in the stereo 
model. The laser points are filtered to delete points which fall on 
breaklines or within a certain distance of the breaklines. A small 
area is shown in Figure 3 to more clearly present the information. 
The same area is presented in the following figures. The surface 
generated from the laser data is shown in Figure 4. 
  
    
International Archives of Photogrammetry and Hemote Sensing, Vol. 32, Part 3W14, La Jolla, CA, 9-11 Nov. 1999 
Figure 3. Breaklines and laser points. 
A new surface is generated using the merged data. Constrained 
triangulation is used to enforce the use of the breaklines. The 
surface using the merged data is shown in Figure 5. Comparing 
these surfaces, the merged surface provides a better 
representation of the buildings than the surface generated using 
only the laser data. 
The breaklines used up to this point only define the rooflines. To 
more accurately define the buildings, the roofline is projected on 
to the ground surface and used as a breakline, thus defining the 
walls of the buildings in addition to the roofs. The result of this 
process is shown in Figure 6. 
Comparing the surfaces, the surface utilizing both the breaklines 
and the laser data better defines the buildings than either of the 
surfaces generated using a single data acquisition method, and 
that the projection of the roofline to the ground surface is 
necessary to accurately define the buildings. 
5 CONCLUSIONS 
This paper describes the development of a general scheme for the 
integration of laser and photogrammetric data. This scheme 
includes the development of a surface registration algorithm and a 
data fusion algorithm. Initial testing of the integration algorithm 
has been carried out using data over Ocean City, MD, and has 
shown that the surface is more accurately represented than when 
using either data set separately. 
The approach presented in this paper is applicable to the 
determination of accurate visible surface models for the 
generation of true orthophotos in urban areas. The use of the 
rooflines to determine surface discontinuities will allow the 
production of an orthophoto that does not have the distortions 
associated with the inaccuracies in the surface models which are 
inherent in automatically generated DEMs. Continued research is 
being undertaken into the automation of the approach, to extract 
the edges of the rooflines and to determine the ground height in 
these regions. 
ACKNOWLEDGEMENT 
This research project is partially supported by the United States- 
Israel Binational Science Foundation, grant no. 97-00433. 
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Internation 
  
  
  
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