INTERACTIVE CHOROPLETH MAPPING ON A RASTER-ORIENTED COLOR DISPLAY
Dieter Steiner, Benedetto Flury, Hans Gilgen and Markus Ilg
Department of Geography
Swiss Federal Institute of Technology (ETH)
Zurich
ABSTRACT
This paper discusses some basic considerations for the establishment of
a computer-assisted interactive and raster-based choropleth mapping sy
stem, in particular with respect to a color display unit with a refresh
memory, a video lookup table and a CRT monitor. A tabular relational
model serves as a framework within which tasks and problems can be de
scribed, structured and evaluated. The model is introduced in section 2.
In section 3 it is shown how it can serve as a design tool to derive
and evaluate alternative mapping strategies. Section k discusses some
implementation aspects in the context of a color display system. Section
5 addresses the special problem of composing suitable color scales by
interactive means. Finally, we list some desirable components of an in
teractive legend facility in section 6 and use the relational model
again to demonstrate how the related problems can be solved.
1. INTRODUCTION
We assume to have, for a particular geographical region, a data base
consisting of a digital base map in raster form and a matching attri
bute data file, the latter containing statistical data collected by some
survey or census for some, usually irregularly bounded geographical
areas. The task is to design an interactive system for the production
of choropleth type maps. The term 'choropleth map' is used in a wide
sense to mean simply a statistical mosaic map. The following discussion
is based on experiences gained at the ETH Zurich in the course of deve
loping a general geographical data processing system, using a minicom
puter and a color display unit (STEINER 1980, 198la).
2. A RELATIONAL MODEL FOR RASTER-BASED CHOROPLETH MAPPING
We begin by considering for the mapping task in question a relational
model, following the general idea of TOBLER's (1979) transformational
approach to cartography as well as the more explicit notions of MORRI
SON'S (1978) functional definition of cartography. We make use of the
specific concept of relational modeling by CODD (1970) (however, we are
not concerned with data base organization aspects here). The model
serves the purpose of describing and structuring the mapping process in
a formal way. A graphic representation of the model is given in Fig.l.
The nodes stand for sets of elements associated with four kinds of spa
ces: Map space, geographical space, attribute space and graphic space.
The edges indicate relationships between the element sets concerned.
Each relationship is fully defined in the form of a binary relation li
sted in a table, as indicated by Ti (i = 1,...,8).
Suppose that we follow in Fig.l the path from m to s. From the corre
spondence type labels encountered along the edges (explained in the cap
tion to Fig.l) we observe that each edge represents a functional depen-
