ISPRS Commission III, Vol.34, Part 3A „Photogrammetric Computer Vision", Graz, 2002 
  
vertical (building) surfaces, windows, road surfaces and other 
surfaces, while surrounding surfaces are generated to enclose 
volume-structured objects, i.e. trees. 
In the followings, we discuss each procedure in details. An 
experiment and discussion is followed, where a real urban out- 
door environment is reconstructed, and the efficiency of the 
method is proved. 
3. CLASSIFICATION OF RANGE POINTS 
As the vehicle moves ahead, LD-As take the cross-sections of 
urban objects by scan lines (1200 range points per scan line). 
Since LD-A has a circular scanning resolution of 0.25  , spatial 
resolution of range points in one scan line (vertical spatial 
resolution) depends on the distance from LD-A to target object. 
If the target object is 20m (r) far from LD-A, range points are 
sampled at a vertical spatial resolution of about 0.087m 
(r*tan0.25). On the other hand, when the vehicle moves straight 
at a speed of 10-20km/h, scanning planes are almost parallel 
with an interval of about 0.1m. Subsequently, horizontal spatial 
resolution of range points at the same sequential number is 
about 0.1m. However, it alters as the vehicle's moving direction 
changes. Classification is conducted by examining the local 
connectivity between the range points of the same and 
neighbouring scan lines. 
3.1 Segmentation of scan lines 
Scan lines are first segmented into linear patches, where 
successive range points are extracted, which are linearly 
distributed with a variance lower than a given threshold. Linear 
patches are then compared with the extraction of neighbouring 
scan lines. Isolated linear patches are discarded, which cannot 
find a linear patch of nearby sequential number and of similar 
direction in the extraction of neighbouring scan lines. It means 
that the range points have only vertical but no horizontal linear 
continuity, so that they are not the measurement of surface 
object. Finally, range points in one scan line are divided into 
four groups as follows. 
1) Range points belonging to vertical linear patches are 
the measurement of vertical building surface. 
2) Range points belonging to horizontal linear patches 
and at ground elevation are the measurement of road 
surface. Relative elevation from the origin of LD-A 
to the nearest ground surface is almost constant. It 
can be measured previously in calibration stage. 
3) Range points belonging to other linear patches are 
the measurement of other surface. 
4) Range points do not belong to any of the above 
groups are scatter points. 
3.2 Correction of window area 
In this research, we assume that the building surface between 
windows of different floor is vertical as a whole, is range 
sampled enough that can be extracted, and its material is neither 
penetrative nor mirror reflective to laser beam. In each scan line, 
penetrative measurement on window area has the following 
characteristics. 
1) They are behind the building surface from the 
viewpoints of LD-A. 
A - 414 
2) Their sequential numbers are between two pieces of 
vertical linear patches, which are on a common 
vertical line. 
Discrimination of window area is conducted using all other 
range points besides those belong to vertical building and road 
surface. To the range points that satisfying both conditions, 
their range values are corrected to fit the common vertical line. 
To the false points (no range value) that satisfying the second 
condition, their range values are interpolated. 
  
i | | Vertical 
| Erroneous line | building surface 
| extraction | 
| No range data | Tree 
: i £n Ground surface 
2, ~~ +} 
eat 4 4 
T a uade t 
  
  
  
  
  
(a) Linear patches that extracted from a scan line, (b) erroneous 
extractions are discarded by comparing the horizontal linear 
continuity with the extractions of neighbouring scan lines 
  
suas Windows m Corrected 
x^ | window data 
T of 
| ~ ^ 
Indoor + Y i 2 
objects 
  
  
  
  
  
(c) Some window glasses are penetrative to laser beam, so that 
indoor objects are measured, (d) Range points at window area 
are corrected using the data of surrounding vertical building 
surface. 
Figure 2. Classification and correction of range points 
3.3 Trees and others 
Range measurement of volume-structured objects, e.g. trees, has 
the following characteristics. 
1) They are in front of the building surface from the 
viewpoint of LD-A. 
2) They have higher elevation values than the ground 
  
surface. 
3) The cloud of range points implies not a surface but a 
volume. 
NS Y 
Vertical building 3 / 
surface on * 
d + 
7 
» 
Cloud of 
Tree points wa 
™N vf 7 
^ UU Road Pd 
9 i AS 
P 4j e o Vertical building 
= 
surface 
Figure 3. Extraction of tree and other volume-structured objects 
  
  
  
Volume-structured objects are discriminated as followed. All 
scatter points that satisfying the first two conditions are 
projected onto a regularly tessellated horizontal plane. For each 
grid pixel i, three values are recorded. They are the maximal 
elevation value 2 the minimal elevation value _ 2 the