ONE-VARIABLE AND TWO-VARIABLE CHOROPLETH MAPS: AN
EXPERIMENTAL RESEARCH
A.M. Leonowicz
Polish Academy of Sciences, Institute of Geography and
Spatial Organization, Warsaw, Poland
aleon@twarda.pan.pl
Looking for the relationship between geographical phenomena
is one of the possible ways to explore spatial data; it is often used in
geography as a part of an investigation process. Visualization considerably
facilitates that process. Choropleth maps are one of the most popular
cartographic visualization tools. However, the most frequently used
one-variable choropleth technique is not a perfect method to visualize spatial
relationships. It allows depicting only single geographic feature on a separate
map. Comparing visually such maps to discover similarities or differences
between the distributions was found in many experiments to be a demanding task
for the map readers. It can be assumed that information about geographical
relationship would be easier to notice if two distributions were combined on a
one single map. Such maps (called two-variable choropleth maps) are well known
in cartographic literature. They are however not often used in practice,
probably because they were thought to be controversial. This opinion was based
on few experiments which results evidenced that this mapping method is too
difficult for the readers to understand. The aim of my research was to verify
this opinion. It was assumed that two-variable choropleth maps can be well
understood by their readers, if only the maps are properly designed (by the use
of proper graphic and reduction of the number of classes). The experimental
test was conducted in order to compare the effectiveness of one-variable and
two-variable choropleth maps. Students participating in the test were asked to
perform different, typical map-reading tasks, like extracting specific and
general information from maps. They were asked questions about the
distributions as well as about variables relationship, and after that about
their opinions and preferences about maps. The
results verify opinion about poor readability of two-variable choropleth maps.
Students were more accurate in reading spatial distribution on one-variable
maps (especially the general pattern) and in reading spatial relationship on
two-variable maps. They found two-variable maps a little bit more difficult to
interpret but this form of presentation seemed more unusual and interesting.
They found also that one-variable solution is more appropriate to read
distribution and two-variable solution – to read variables relationship.