3-3 Scanners for Raster Data Input

Scanner are used to convert from analog maps or photographs to digital image data in raster format. Digital image data are usually integer-based with one byte gray scale (256 gray tones from 0 to 255) for black and white image and a set of three gray scales of red (R), green (G) and blue(B) for color image.

The following four types of scanner are commonly used in GIS and remote sensing.

a. Mechanical Scanner
It is called drum scanner since a map or an image placed on a drum is digitized mechanically with rotation of the drum and shift of the sensor as shown in Figure 3.4(a). It is accurate but slow.

b. Vedeo Camera
Video camera with CRT (cathode ray tube) is often used to digitize a small part of map of firm. This is not very accurate but cheap. (see Figure 3.4(b))

c. CCD Camera
Area CCD camera (called digital still camera) instead of video camera will be also convenient to acquire digital image data (see Figure 3.4 (c)). It is more stable and accurate than video camera.

d. CCD Scanner
Flat bed type or roll feed type scanner with linear CCD (charge coupled device) is now commonly used to digitize analog maps in raster format, either in mono-tone or color mode. It is accurate but expensive.

Table 3.2 shows the performance of major scanners.