Geoscience Reference
In-Depth Information
2.2 Generalisation of Polygon Data
2.2.1 Operators for polygon data generalisation
Generalisation of polygon themes is carried out utilizing a number of gen-
eralisation operators that perform semantic and spatial transformations.
The most common of these operators are (
Figure 1
): selection, simplifica-
tion, amalgamation, reclassification, collapse, elimination etc.
Figure 1:
Operators used in polygon generalisation (based on McMaster and Shea 1992;
Galanda 2003).
2.2.2 Map overlay operations as auxiliary tools for consistency testing and
polygon generalisation
Many spatial databases exhibit inconsistency problems between their
themes that should be solved before generalisation. Map overlay opera-
tions such as clip, erase, identity, union etc. which allow association of dif-
ferent themes (Laurini and Thompson 1992), can be used to correct incon-
sistency problems. Erase can be used to assure that one polygon theme
does not overlap with another polygon theme, by deleting the common
area from the first theme. Clip can be used to assure that a polygon dataset
is completely inside the area described by another polygon dataset, by
deleting everything that is outside.
One of the basic requirements of polygon generalisation is the topological
integrity of the generalised data. Polygon borders should not self-intersect
or intersect with other objects. A time and effort consuming task is to detect
and correct topological inconsistencies independently of the generalisation
