BUHOM: A PROGRAM FOR ENHANCEMENT OF GEOMETRIC
TOPOLOGIC CONSISTENCY OF BUILDING OBJECTS
[. O. Bildirici* *, D. Heidorn°
? Selcuk University, Faculty of Engineering & Architecture, Department of Geodesy & Photogrammetry, 42075
Kampus, Konya , Turkey — bildirici@selcuk.edu.tr
b Institut für Kartographie und Geoinformatik, Universität Hannover, Appelstr. 9A, 30167 Hannover, Germany —
Dieter.Heidorn@ikg.uni-hannover.de
Commission IV, WG IV/3
KEY WORDS: generalization, building generalization, digitizing errors, vector data
ABSTRACT:
Vector data is captured using various methods, e.g. manual digitizing, automatic vectorization etc. Errors or inconsistencies, e.g.
self-cutting polygons, identical points etc, can not be avoided, ev
en if digitizing is done very carefully. Vector data must be analyzed
and corrected prior to any GIS application. This process is called “line-cleaning”. In this paper we introduce a computer program
that is capable of analyzing of building objects. It can correct digitizing errors and improve geometric quality of data, doing right
angle adjustment, making buildings parallel to streets etc. The program was designed to process building objects before
generalization with CHANGE generalization software package. However, it can also be used independently.
1. INTRODUCTION
Spatial data in vector format is captured with various methods,
e.g. manual digitizing, automatic vectorization etc. Errors or
inconsistencies, e.g. self-cutting polygons, identical points etc,
can not be avoided, even if digitizing is done very carefully.
Building objects are generally captured as polygons (area
objects) in vector format. They cannot be considered as
conventional area objects. The structure of their outer contours
(object boundary) is quite different from land use polygons for
example. Buildings are man-made objects, which have mostly
right angles at corners, and are generally located parallel to
streets.
Many GIS systems have tools to examine and correct the vector
data, which are called line-cleaning tools. These tools are
developed to help creating a correct network and/or polygon
topology, but not suitable for building objects.
The software tool BUHOM has been designed to process
1:1000 scaled building data. It is developed with Fortran 90
programming language. The functionality of BUHOM can be
summarized as follows: The objects are examined if there are
self-cuts on contours. Identical points are sought, if exist they
are eliminated. The angles on the corners are examined within
certain pre-specified limitations; the sides are adjusted to have a
right angle on that corner. The system makes sure, that the
direction of the building (bearing angle of the longest side)
stays unchanged during this process. If street data (center-lines
or right and left sides of streets) is available, and if the building,
within a threshold value, is nearly parallel to the nearest street,
it is rotated so it becomes exactly parallel. The center of mass
and the area of the building stay unchanged after processing.
* Corresponding author.
BUHOM's functionality is actually considered as a pre-
processing for the CHANGE software package, a tool for
automatic building generalization. It can also be used
independent from CHANGE. BUHOM has been tested on
several data sets since 1998 at the IKG (Institute for
Cartography and Geoinformatics, Hannover). It is mainly used
before the generalization with CHANGE. Several tests and
analysis on the results showed that the program delivers good
results.
[n this paper, the methods employed in the development of the
program, and the application possibilities will be discussed in
detail.
2. GEOMETRIC IMPROVEMENTS ON BUILDING
OBJECTS
Our strategy for correction of digitizing errors and for
improvement of geometry consists of following steps:
e Small area test. If the object area is smaller than a
threshold value, it is deleted.
« Elimination of identical points within the object
boundary.
e Test on self-cutting objects. If self-cuts exist, the
object is deleted.
e Alignment or elimination of points that are
approximately on the same straight line with prior and
next points (insignificant points, see figure 4).
e Right angle adjustment. The building edges are
sequentially examined. If the angle between adjacent
edges approximates to a right angle within a threshold
value, the right angle is reconstructed by rotating one
of the edges.
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