IMPLEMENTATION OF OBJECT ORIENTED GIS USING FORMAL DATA
STRUCTURE WITH PLANAR TOPOLOGY - SOME CONSIDERATIONS
Hongguang Yang and Wolfgang Reinhardt, Munich/FRG
Abstract:
This paper is intended to characterize how an object oreinted
model for Geographical Information System (GIS) of new gene-
ration may be designed and realized. Special emphasis will be
laid on the data structure aspect. Design methodes, on the one
side used specially for topological structuring of spatial data, and
on the other side used generally for object oriented software en-
gineering will be considered here. As a bridge combining the ap-
plication requirement and software engineering methode the
Fomal Data Structure (FDS) from the GIS theory of Molenaar is
quite suitable.
It will be assumpted that the reader has been already confronted
with topological structure of spatial data and one of the Mole-
naar’s papers about the FDS.
Key Words: Formal data structure,
Object oriented model, Planar topology.
1. Introduction
Nowadays GIS must possess a planar topological structure over
its spatial data so that spatial analysis or spatial query of Geo-
graphical Information are supported with an acceptable perfor-
mance, or even possible.
A history GIS which existed before this knowledge is well
known shows often the problem that planar topological data
structure can hardly re-implemented because at the begin of the
system design data topology were not taken into consideration
enoughly. Even a re-implementation of planar topological struc-
ture is possible, re-structuring of existing enormous data set and
re-design of processing algorithms are necessary which would
be very work-consuming.
The design and realisation of a new generation GIS is subjected
not only to the essential requirement from topological aspect of
data structuring, but also from software engineering methode of
object oriented design and programming.
There is yet no standard methode to implement planar topology
for a GIS to make data and algorithms portable in a meta-level,
between different dates (evolutional aspect) and systems
(common use of data). In the practice every GIS manage the
spatial and non-spatial data at its own way and in different data-
base model (relational or network). The multiple use for chan-
ging applications within one GIS and common access of data
from differnt GIS are subjected to very strong restrictions.
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The planar topological structure of spatial data has taken its
standard form, originated from the DIME and succeeded by
TIGER (Boudriault 1987). The advantages have been shown in
last years contineously. Nevertheless no practicable and widely
acceptable methode do exist to make it possible to implement a
planar topological structure in an object oriented system envi-
ronment.
The Formal Data Structure of Molenaar can serve as an meta-
language for description of geographical information as a whole,
including topological, spatial and non-spatial aspects and inde-
pendent of used data model. The inherent connection of FDS
with object oriented model makes it suitable to standardize the
process for GIS design and realisation. To do this, the FDS must
be refined and made more understanable.
In this context the following sections will deal with FDS, object
oriented model and the connection between them.
2. Characterizinz Formal Data Struct
After Molenaar difference must be made between data and in-
formation in a GIS. Geographical data is defined in a GIS which
is implemented in a computer. But geographical information is
resulted from the thinking process of human. We capture geo-
graphical data from the reality to a computer by tranforming the
geographical information or we get geographical information
from a computer by tranforming the geographical data. The in-
teractions between computer and human are 'semantic transfor-
mations'. In a wide sense the process such as get a display file
from a database is also a semantic transformation.
We are used to think of geographical objects its data form is
called "terrain features' (point, line and area features). Every
feature is associated with a 'feature identifier' (Fig.1). Every
identified feature can be described by a geometic and a thema-
tic aspect.
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coor-
dinates
Fig.1 FDS for Single Valued Polygon Map (Molenaar 1989)
