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Imagery measurements are collected and stored in raster bands. For example, a panchromatic image raster includes only a single band of measurements, shown as a single layer in figure 2.3, and is typically shown in grayscale. Multispectral imagery contains several bands of measurements, as shown in figure 2.7. Figure 2.8 shows a portion of Landsat imagery over Sonoma County, California. The numerical values of the cells of three bands of the seven-band image are displayed. When the red, green, and blue bands are displayed in the red, green, and blue colors of a computer screen they create the natural-color image of figure 2.8.

Figure 2.7. Multispectral data. If more than one type of measurement is collected for each cell, the data is called multispectral, and each type of measurement is represented by a separate band.

Figure 2.8. The numerical values of three bands of Landsat imagery over a portion of Sonoma County, California.

Hyperspectral data contains 50 to more than 200 bands of measurements and is usually represented as a cube of spectral values over space (figure 2.9). Image cubes are also used to bring the temporal dimension into a set of images, as when multiple Landsat images are analyzed of the same area over time.

Figure 2.9. A hyperspectral data cube captured over NASA’s Ames Research Center in California. Hyperspectral data includes 50 to more than 200 bands of measurements. Source: NASA

Figure 2.10. The impact of raster cell size on the level of detail depicted. The larger the cell, the less discernible detail. In this example a car is represented by three different image cell sizes but displayed at the same scale. The smaller the cell, the more information available to identify the rectangle of eight large reddish pixels on the right as a red sedan on the left.