Geoscience Reference
In-Depth Information
process (Edwardes et al. 1998). Topological errors due to simplification
can be avoided by generalising an auxiliary dataset. The auxiliary dataset
is a combined polygon theme that is created from the application of the
map overlay method “union” to a number of independent polygon themes
with inter-theme constraints. If it is used for the simultaneous simplifica-
tion of the geometry of these themes, inconsistency problems that would
result from more than one independent simplification procedures are
avoided. Most commercial GIS software, provide simplification tools that
can assure topological consistency in a single dataset.
In addition to the above, the auxiliary dataset can be used to address incon-
sistency problems of the original themes. By the application of the “union”
operation, a number of polygons are created. If the original themes are
fully compatible, a “one to one” relationship exists between the polygons
in the original themes and the polygons of the combined dataset. When in-
consistency problems exist, new polygons are created in the combined
dataset that correspond to each one of the problems. The polygons of the
combined dataset carry all the attributes of the themes that are used. The
difference in the values of these attributes makes it possible to identify
each polygon. The identification and the appropriate handling of these
polygons result in a consistent dataset.
Moreover, the auxiliary dataset can be used to better handle other carto-
graphic generalisation issues such as minimum area, imposed by the limits
of visual perception. Minimum area limits are applied to independent
polygons, enclaves and small polygons between different themes.
The auxiliary dataset is introduced as a structure that can assure consis-
tency to original and generalised data. It is created only for processing
issues and does not change the original data schema.
Afterwards the original
themes can be reconstructed.
Example: Establishing consistency between Administrative units, Built-up areas
and Watercourse Areas
The application of the above concepts will now be presented with a con-
crete example. Most medium class spatial databases contain themes such
as Administrative Units, Built-up areas and Watercourse areas that are rep-
resented as polygons and have obvious inter-theme relationships. Each
theme (
Table 1
) has at least one attribute that describes one important
characteristic of this theme.
