Transformations - Spatial Mathematics: Theory and Practice through Mapping - page 69

line was re-etched through the paint. Often, the best solution was simply to
throw the film away and start over again.
GIS not only permits dynamic response to such changes, but it also offers
hundreds of algorithms that allow the user to have a wide-ranging control of
maps and data with the freedom to customize them creatively at will. Thus,
understanding of what one is doing is critical!
The introduction of digital mapping caused a revolution in mapping: The
process became dynamic. A change in the database produced a correspond-
ing change on the map, and vice versa. Errors on a map could be corrected
almost as easily as errors in a text document. There was no need to start over.
Production improved in quality and speed. What remains critical in both
approaches, however, is to understand the concepts involved and to think
about what we are doing as we create maps.
So, drawing from the classical analogy, what is the “paper” for a digital map?
Clearly it is the computer screen. Having the map there is what makes it pos-
sible for mapping to become dynamic. To be effective, however, we need to
understand the mechanics of the computer screen as well as we understood
paper types and paper finishes.
The computer screen is composed of a set of small squares, called pixels, each
of which holds a small part of the image—like pieces in a jigsaw puzzle of tiny
squares. Generally speaking, the smaller the squares, the sharper the image;
sharper images are images of high resolution. Thus, we can imagine assigning
location on the computer screen in terms of pixel location (the third pixel down
from the top and the seventeenth pixel over from the left). That sort of assign-
ment is like using parallels and meridians. It is relative to the pixel size. Instead,
one might also use mathematical coordinates to assign location independent
of pixels. This approach using standard mathematics is much like introducing
standard circular measure to replicate results on the sphere (see Chapter 1).
When maps or other images are created, they might be composed of a set of
instructions on how to form the shapes with specific numerical values; they
are made up of mathematically-defined paths as line segments and curves
( Figure 3.7a ) . This sort of image is called a vector image; it does not use
pixels and is based on standard mathematics. Or, images can be composed
of a set of pixels with each pixel having a specific value or color. This sort
of image is called a raster image ( Figures 3.7b and c ) . There are merits and
drawbacks to maps based on images formed in either approach. Bear in mind,
though, that the computer screen is itself composed of pixels and therefore all
images, no matter how constructed, will appear as raster-like on the computer
screen. How they print out is a different matter.
For vector images, in the associated vector database, each image feature has
an attribute in the underlying table. Categories of data can be suppressed by
the software producing the map, so that there is capability to show denser
data by zooming in. The image can be rotated and have text remain upright.