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Another effect of the different measurement modes occurs when measuring at discontinuities in elevation structure, i.e. 
break lines at the border of buildings partly covered by a footprint. By using first pulse mode, the footprint represents 
the higher elevation level at this position; using last pulse mode, the same footprint (at the same position) the ground 
level is acquired. Therefore, the same building may be acquired with larger or smaller dimensions compared to the 
correct measures, depending on the scanning mode (Figure 3). 
       
   
  
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Figure 3: Effects on the size of objects by using laser scanning in different modes, first (left) and last 
pulse (right); circles illustrate footprints of laser rays, filled circles indicate measured 
points on higher level; original contour lines (solid) and acquired contour lines (dashed) 
2.4 Characteristics of laser scanning data 
In first pulse mode, usually the upper canopy of objects is registered (DEM). Therefore, besides solid objects like 
buildings also most vegetation (trees, bushes etc.) is acquired (Figure 4) which has to be excluded during further data 
processing for a robust building recognition and reconstruction. 
  
  
   
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Figure 4: Subset of laser scanning derived DEMs of test area Karlsruhe; intensity corresponds to elevation 
(brighter points are higher); first (left) and last pulse mode (right) 
On the other hand, first pulse mode delivers a good coincidence of building contour lines compared to cadastral maps. 
But some (for this approach) irrelevant details on the roofs, e.g. antennas, dormers, chimneys etc., may disturb the 
determination of building surfaces, especially of roof planes. This can be observed in Figure 5. At the highest building 
in the background some antennas are mounted on the roof. In first pulse derived DEM (left hand side), these antennas 
are acquired, but not in last pulse mode (right hand side). 
In DEM of last pulse mode (Figure 4, right side) most vegetation are nearly eliminated, a big advantage for building 
recognition. A disadvantage of this mode is that buildings appear generally too small, i.e. they have missing borders of 
about 1 to 2 pixels (compared to cadastral maps) (see section 2.3). Another effect is the smoothing of roof top edges and 
the loss of details on the roof if desired. For a better illustration of these effects and a visual comparison of the resulting 
data, some CAD models of buildings (manually measured by means of aerial images) are superimposed with DEM data 
acquired in first pulse and last pulse mode respectively (Figure 6). The brighter silhouettes represent the CAD models, 
  
International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B3. Amsterdam 2000. 861