Turning Digital Data into Images

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is simply a grid of continuous pixels, known as a raster image (Figure 2-27). The digital

number (DN) of a pixel is the result of the spatial, spectral, and radiometric averaging of

reflected/emitted radiation from a given area of ground cover (see below for information

on spatial, spectral, and radiometric resolution). The DN of a pixel is therefore the aver-

age radiance of the surface area the pixel represents.

Figure 2-27. Figure illustrates the collection of raster data. Black grid (left) shows what

area on the ground is covered by each pixel in the image (right). A sensor measures

the average spectrum from each pixel, recording the photons coming in from that area.

ASTER data of Lake Kissimmee, Florida, acquired 2001-08-18. Image developed for

Prospect (2002 and 2003).

(2) The value given to the DN is based on the brightness value of the radiation (see

explanation above and Figure 2-28). For most radiation, an 8-bit scale is used that corre-

sponds to a value range of 0255 (Table 2-4). This means that 256 levels of brightness

(DN values are sometimes referred to as brightness values--Bv) can be displayed, each

representing the intensity of the reflected/emitted radiation. On the image this translates

to varying shades of grays. A pixel with a brightness value of zero (Bv = 0) will appear

black; a pixel with a Bv of 255 will appear white (Figure 2-29). All brightness values in

the range of Bv = 1 to 254 will appear as increasingly brighter shades of gray. In Figure 2-

30, the dark regions represent water-dominated pixels, which have low reflectance/Bv,

while the bright areas are developed land (agricultural and forested), which has high re-

flectance.

2-32

Index