Geographic Information Systems: An Introduction, 3rd Edition

7.4 Quantizers

Quantizers are devices that convert analog data, such as data on ordinary maps, into digital data, as used in GIS. The quantizers used most frequently include the following:

• Digitizer: a combination of manual positioning and electromagnetic sensing on a plane surface; historically, the first analog-digital converter for GIS applications
• Scanner: an automatic electromechanical system that converts a picture into a raster of points (pixels)
• Video: the electronic equivalent of motion-picture photography, often followed by rasterization similar to that employed in a scanner
• On-screen digitizing and automatic line tracer: a function performed through software programs on raster data that have been scanned and displayed on screen

7.4.1 Digitizers

A digitizer consists of a flat nonconducting surface in which a grid of wires is embedded (Figure 7.10). Electric currents passing through the wires give rise to electromagnetic fields, which together form a pattern on the surface. A small magnifying glass fitted with crosshairs is used to locate objects on a map placed on the table. The glass is also fitted with a sensor coil, which detects the position of the crosshair center with respect to the underlying wire grid. Thus, positions are entered continuously or at the command of a keystroke. A keypad with up to 25 keys connected to a microprocessor allows identifiers, thematic codes, and so on, for objects that have been located to be entered, and also facilitates interaction ...