Geography Reference
In-Depth Information
Exercise 5-3 (Project)
Digitizing and Transforming
“Digitizing” is a process in which a graphic representation (a drawing, a map) is turned into numbers
(digits) and characters that give the computer some of the information that is “in” the drawing. Generally,
that information makes it possible for the computer to reproduce the drawing on its screen or on paper
and to answer questions about it. We, of course, are most interested in digitizing maps.
You have had experience with one method of digitizing: creating a geodatabase feature class by typing
in the “digits” of the coordinates of TextToFeature project for the Foozit Court for the elementary school.
Doubtless you have had all of that sort of data entry you want. Sometimes, however, it is necessary when
the absolute coordinates of some important feature are known precisely.
Another way of digitizing, requiring much less human effort, at least initially, is scanning , where a map is
placed on a rotating drum, or laid flat on a table, and through a combination of electronics and mechanics,
the map's contents are finely gridded into little squares. The pixels that result are stored in a computer
file. At this point, all the user has is a picture. However, with some direction from the user, sophisticated
computer programs can then interpret the picture according to rules and can create an intelligent GIS
map. For example, the program could recognize a contour line—perhaps even to the extent of reading its
height off the map and placing it as an attribute in a table. High-precision scanners are expensive, and the
work to make programs that interpret map image files is ongoing.
A third technique of digitizing is to mount the paper map 8 on an electronic drawing board, called (surprise)
a digitizer, and trace over the lines of the map with a stylus or puck; this process transmits x-y coordinates
to the computer. The user can add attributes to the lines or areas that are digitized.
A Plan for Digitizing and Transforming
Here we will look at a fourth way of getting graphic information into the computer; it is called “heads-up”
digitizing—so named because you look up at the monitor rather than down at a paper map, as in
traditional digitizing. In heads-up digitizing, an electronic image of a paper map is made by scanning . In
this case the map is the Foozit Court. You will be able to see this image on the screen of your computer.
By tracing around the lines of the map with crosshairs (a cursor or pointer, controlled by the mouse), you
will be able to automatically supply coordinates to ArcMap.
This process is a little like learning to ride a bicycle: not really hard, but difficult to describe in all its detail
(e.g., if the bicycle begins to tip over to the left, turn the handlebars to the left—just a little now—and . . .),
so I will simply provide general directions and let you figure out how to digitize mostly on your own.
A caveat: In the process of scanning and then displaying a map systematic error may creep in—since scanners,
and particularly computer screens do not maintain exactly the same aspect ratio(height to width) as the paper
or Mylar map. It's something to think about, measure, and correct on the final product if necessary.
The product of your digitizing will be an ArcGIS shapefile . As you know, since we have discussed the nature
and characteristics of these elements in the last chapter, a line shapefile is made up of beginning points ,
ending points , and vertices , which define polylines .
8 Or Mylar map—Mylar is used because it is a “stable” medium, unlike paper, which stretches and shrinks with
environmental factors like humidity.
 
 
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