International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B4. Istanbul 2004
insignificant. A threshold value is needed to decide. In other
words, if da is smaller than a threshold value determined by
user, the examined point (point 2 in figure 4) is regarded as
insignificant. It can be aligned or eliminated according to the
choice of the user.
1 uices
= da
Figure 4. Insignificant points
2.3 Building-Street Relationships
Buildings are often located parallel to the next street. If street
data is available, the existence of such relationships can be
examined. If possible, parallelism can exactly be reconstructed.
For this purpose street boundaries or centerlines can be used.
center of mass
search zone
Figure 5. Building-street relationship
The nearest street boundary (or centerline) is searched by using
a search zone (figure 5), which is defined by user (A parameter).
After finding the nearest straight line segment of the street
boundary, the nearest edge, which would be parallel to street, is
sought. If such an edge exists, the difference of the bearings (a,
and a) is examined. If it is smaller than a threshold value, the
objects are considered parallel and the building is rotated
around the center of mass.
If a building is rotated because of parallelism to street, the
direction of the building, at the end of processing, is not
reconstructed, but area and center of mass.
2.4 Topologic Inconsistencies Among Neighbor Buildings
After right angle adjustment topologic inconsistencies can
occur, because the objects are rotated, scaled and moved. These
inconsistencies can be overlaps and gaps between neighbor
buildings (figure 6).
236
In order to analyze and correct the topologic inconsistencies, a
hierarchical approach can be employed.
* All objects are sorted descending according to area.
* Beginning with the smallest one, small objects are
compared with larger ones. Box test technique can
speed up the comparison at this step.
e If any gap or overlap is found, small object is
changed. If there is a gap, small object is extended
towards the large object (object 1 in figure 6 - top). If
any overlap is the case, small object is trimmed
(object 2 in figure 6 - bottom). In both cases small
object can be moved, but this can cause another
inconsistencies with other objects around.
1 1
2 2
gap
1 DL 1
2 2
overlap
Figure 6. Topologic inconsistencies
After these corrections some objects can have small, edges,
which is not desired in the sense of generalization. However,
the enhancements here should be considered as a preprocessing
prior to generalization. In other words, these small edges are
eliminated in the building generalization, which is supposed to
be undertaken thereafter. Another point is the changes in size
and orientation of the objects. Since the gaps and overlaps are
supposed to be small in size, the changes in size can be
considered to be small amounts. The width of gaps is entered by
user, which should be determined within minimal dimensions
on the map (e.g. 0.2 mm on the map). The overlaps normally
occur due to digitizing errors. They must be removed even if
the overlapping area is not a small amount. In this case object
area may considerably be changed.
3. SOFTWARE DEVELOPMENT
The program BUHOM is developed with the programming
language FORTRAN 90. It uses the DBB database in the
background. The input files (building file and street file) and
output file (processed buildings) are in the FDP file format.
DBB format and FDP format are in-house file formats of
Institute for Cartography and Geoinformatics of University of
Hannover. For more information about file structures see IfK
(1998), Bildirici (2000).
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