Page 104 - Digital Analysis of Remotely Sensed Imagery

Overview of Remotely Sensed Data 75

accomplished with excellent location accuracy. Data scanned using
this type of scanner has an accuracy level comparable to that obtained
with analog and analytical photogrammetric devices. Positional
accuracy for photographs recorded on film can be as high as 5 μm or
less RMS error (Bethel, 1995). They are essential in applications that
require high geometric accuracy. By comparison, desktop scanners
are much less reliable geometrically, and hence less expensive (Fig. 2.13).
They find applications in which accuracy is not a paramount concern.
Regardless of their accuracy, all scanners must have an active area of
2
9 9 inches , the physical size of standard aerial photographs, in order
to scan one aerial photograph wholly.
Scanning of photographs is carried out in either the gray or color
mode, depending upon the nature of the photograph being scanned.
If the original photograph is black and white, the gray mode should
be adopted to minimize file size. The color mode is reserved for true
color or color infrared photographs. Unlike scanning the ground
surface from space during initial data acquisition, scanning aerial
photographs allows the analyst to control the process of data

FIGURE 2.13 The HP ScanJet 8200 produces consistently enhanced colors
and razor-sharp images. It is able to scan photographs at 4800 DPI in 48-bit
colors.