Your encounters with GIS to date may be similar to
those of a Martian arriving on Earth and being faced
with a motor car. Imagine a Martian coming to
Earth and stumbling across a motor car showroom.
Very soon he (or she) has heard of a ‘car’ and may
even have seen a few glossy brochures. Perhaps you
are in the same position. You have heard of the term
GIS, maybe even seen one or two demonstrations or
the paper output they produce.
Developing the analogy of the Martian and the
car leads us to a dilemma. There are two approaches
to explaining to the Martian what the car is and how
it works. The first method is a bottom-up approach.
This involves taking the car apart into its component
pieces and explaining what each part does.
Gradually, we put the pieces back together so that
by the time we have reassembled the car we have a
good appreciation of how the car works in theory.
However, we may still have little idea about how to
use it or what to do with it in practice.
The second method, the top-down approach,
starts by providing several examples of what the car
is used for and why. Perhaps we take it for a test run,
and then explore how the different components of
the car work together to produce an end result. If
this approach is adopted, we may never be able to
build a car engine, but we will have a clear appreciation
of how, when, why and where a car could be
used. In addition, if we explore the subject in sufficient
technical detail we will know how to choose
one car in preference to another or when to switch
on the lights rather than the windscreen wipers.
We feel that the same two methods can be used to
inform you about GIS. Since we believe you are reading
this book not because you want to write your
own GIS software, but because you wish to develop a
better appreciation of GIS, the approach adopted is
similar to the top-down method. We focus on the
practical application of GIS technology and where
necessary and appropriate take a more detailed look
at how it works. In a book of this size it is impossible
for us to explain and describe every aspect of GIS.
Therefore, we concentrate on those areas that
enable you to make sense of the application of GIS,
understand the theories upon which it is based and
appreciate how to set up and implement your own