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There has been a
huge growth in interest in mapping in general, and the use of mapping in wildlife
recording and environmental assessment in particular.
Maps are a
uniquely informative way of presenting data, and we all want accurate base maps on which
to plot data.
Most of us have
access to the web, and an odd assortment of maps and mapping services, and to a scanner.
(We should be aware here of copyright issues in terms of internet downloads and in-house
scanning).
Typically we end
up with a bitmap (raster) image, or even a faxed outline from a reserve manager. The
bitmap can be used successfully in a number of situations (specially if we bleed out some
of the colour so that we can actually see the plotted data) – AditSite for instance
will happily treat such an image as a map, if you tell it the location of the top left and
bottom right corners of the bounding rectangle, but what we really want is vector (line
movements) information.
Vector
information improves resizing, reduces storage, increases print quality, and most
importantly allows us to manipulate individual map objects both to improve presentation,
and to allow us to merge maps and map sections.
How do we get
vector information from our raster image?
We can print it out, put it
on a graphics tablet and digitize the picture by hand. We will have minor problems with
accuracy, but the method works. It is also one of the most boring tasks known to modern
man. A PhD in relative paint drying times is probably preferable.
We can use a manual tracing
facility on screen – using the mouse to follow the required lines –
AditMap has
had this facility since version one, and I believe you can find it in
DMAP, and of course
you could have a reasonable go with some of the more advanced drawing packages. Accuracy
is difficult to achieve. The lines produced suffer from every little unintentional mouse
movement, and you tend to forget where you are – in fact you tend to lose the will to
live.
We could consider a computer
assisted or fully automated system.
Its fairly
straightforward really. All we want to do is tell the computer which lines we want it to
follow (maybe choose the colours), and tell it to get on with it – and now you can
– its built in to the latest version of AditMap.
How was this
achieved?
Most map images are made up
of lines with multiple colours, varying thicknesses, gaps and intersections. In addition,
there may be large blocks of colour, and a considerable amount of text. Extracting lines
from such pictures is a simple human activity, but a surprisingly taxing computer task.
The solution was largely a
toolbox approach. You use some tools to get the raster image into a suitable state
(largely through colour reduction and elimination). You involve the user in line selection
where this is appropriate, and in setting a series of parameters that control the tracing
process (the same human skill that allows the user to see what he wants, can be used to
provide clues and hints to the program). Finally, you let the program do the donkey work,
and go and make a cup of coffee.
The results are sometimes
wonderful (see the example trace sequence), you wonder
why someone had not allowed you to use this program before, and you give thanks to those
fine fellows at Adit.
At other times they are
less impressive as so much depends on the original raster image. If its really bad,
you can print it out,
use a felt tip pen to trace the lines you actually want, and scan it back in and get the
program to trace only your felt tip lines – this is a proven technique with faxed or
photo copied maps, and is still much better than the alternatives.
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