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gradual slope, therefore the ground points are below a certain elevation. The higher points are either vegetation points 
or building points, or extraneous points such as those on power lines that are not of interest in this research. 
The classification approach used to determine 
which points are vegetation and which are 
building points depends on the spatial 
frequency of the data. In high frequency data, 
the spatial distribution of the data may be 
used in the classification process, however 
with lower resolution data it is more difficult 
to incorporate this information. "Vegetation 
has a random spatial distribution, whereas 
building points have a planar distribution, as 
shown in Figure 2. As the information being 
‘used in this research has a low spatial 
resolution, an assumption is made that 
vegetation will be isolated points or groups of 
isolated points, whereas buildings cover a 
larger area. 
Vegetation 
  
Building 
Figure 2. Laser data showing vegetation, buildings and the 
area used for experimentation. 
3.2.5 Surface Discontinuities. The most important edges to be detected are the discontinuities between the roof and 
the ground. These areas are important to the accurate representation of the visible surface, as they contain dramatic 
changes in elevation. Where a roofline has been detected which has no corresponding ground surface breakline, a new 
breakline is added to accurately describe the visible surface. 
The breakline representing the roof will be projected vertically onto the ground surface to produce a new breakline, thus 
describing the surface at ground level as well as roof height. The location of the new breakline must be slightly offset 
outward from the roofline, as the triangulation process being used does not allow points to exist with the same 
horizontal location, as would be the case for a vertical plane. The elevation of the ground surface at the location of the 
added breakline must be determined. It is proposed that the laser data in the areas surrounding the breakline be 
searched. The new breakline will be generated and assigned the elevation of the ground points in that area. 
3.2.6 TIN Generation. The laser points that occur on the breaklines are eliminated, again due to the triangulation 
process not accepting points with identical horizontal locations. The triangulation process is used to generate a TIN that 
utilizes the laser data points and constrains the triangles to follow the breaklines. The inclusion of the elevation 
information to the triangulation provides the digital surface model produced by combining the laser data and the 
photogrammetric data. 
4 EXPERIMENTATION AND RESULTS 
The initial testing was carried out using a data set over Ocean City, Maryland, USA, which includes digital stereo 
imagery and laser data. The data set covers different types of areas, including residential areas, flat terrain, beach front, 
dunes, canals and high-rise buildings. Only a small portion of a residential area has been used for the initial testing of 
the algorithm. For detailed information regarding the test site, see Csathó er al. (1998). 
The laser data have been transformed to the coordinate system of the photogrammetric data using the parameters 
determined by the process of surface registration. The two data sets are therefore on the same coordinate system and 
represent the same surface. Laser points in the new data set are eliminated if they occur within close proximity to the 
breaklines. The breaklines and the laser data are then merged to create a new data set. 
4.1 Surface Generation and Registration 
The transformation parameters were determined using the developed registration algorithm. Rooflines were measured 
analytically to produce planes, and these planes were used with the laser points in the registration process, thus 
providing a result that was not dependent on automatic DEM generation techniques. The transformation parameters 
between the surfaces were in the order of one to two decimeters. Further details are provided in Schenk er a/. (2000). 
  
International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B3. Amsterdam 2000. 567