Overview

Since most computer software is written in languages that use binary code (bits of 1's and 0's for on-off states) and n numbers of bits per byte, the digitization is conventionally based on powers of 2. Thus, 28 when expanded yields 256 integers, normally assigned to a range of 0 to 255. The conversion of voltages that can vary between, say, 0 and 3, which will include decimal values such as 1.56 can be carried out graphically (voltage as ordinate subdivided into decimal units and abscissa from 0 to 255 plotted only as whole number units) or by using a lookup table. If a voltage number plots between two whole numbers, it is assigned to the binary number it is closer to. For the range of electronic signal values encountered in a given scene, a sequence of binary numbers will emerge from the A/D conversion. If, for the example in which a voltage of 0 (no radiance detected) is set as the lower limit or minimum and 3 is an arbitrary upper limit or maximum (the largest radiance expected from experience but not the largest possible), then the equivalent digitized range if expressed in units of 28 will be pegged at 0 V = DN of 0 and 3 V = DN of 255. Voltages between 0.48 and 1.65 V, representing the range actually produced by the radiances present in a scene, when digitized would fall between DN = 48 and DN = 140. Typically there are varying numbers of pixels containing specific voltages (= DNs). When plotted as histograms (frequency of pixels vs DN values) for a given band record covering a sensed scene, histograms such as shown on page 1-1 (Section 1) are the norm. That figure represents a distorted Gaussian distribution. Bimodal and even trimodal or higher distributions are not uncommon.

Digitization is normally accomplished onboard the spacecraft (or aircraft) on which the sensor(s) is/are operating. The DN values are digitized in multi-bit units or bytes (a bit is a number composed of a sequence of 0's and 1's). For Landsats 1, 2, and 3, the signals from MSS bands 4, 5, and 6 are converted to 7 bits/pixel (27)(DN range 0-127) and band 7 to 6 bits/pixel (26) (DN range 0-63). All are converted to 8 bits/pixel on the ground by rescaling, i.e., doubling or quadrupuling the DN values. Later Landsats (4 and 5) (both TM and MSS) and SPOT (RBV) signals are digitized onboard at 8 bits/pixel (28) that establishes the range from 0-255.

The digitized data stream is recorded at the receiving station on an appropriate storage medium, such as magnetic tape or disc. For years the standard was the large reel Computer Compatible Tape (CCT) which was either 7-track or 9-track. The 9-track CCT contained the 28 levels (8 tracks) of digitized DN data and an extra track devoted to parity check. The bits are distributed on the CCT in several densities; one common packing is 1600 bpi (bits per inch [of tape]). A CCT is mounted in a tape drive to load the data into computer memory for processing. Some computer systems will accept data stored on a magnetic disc. These storage devices are appropriate to mainframe or mini-computer processors.

The number of lines that comprise the down-track (along orbital path) dimension of a space image depends on the resolution of the sensor and on an arbitrary cut-off (in kilometers) of that dimension (e.g., 185 km [115 miles] for both Landsat MSS and TM scenes). The number of pixel subdivisions in a line also depends on spatial resolution and scene width; thus the better the resolution, the larger the number of pixels (samples)/line. For the Landsat MSS, the number of lines is 2953 and the number of samples is 3548 (can vary) for a total of ~10,580,000 pixels per band, and ~41,330,000 pixels for all four bands (source: Avery and Berlin, Fundamentals of Remote Sensing and Airphoto Interpretation, 5th Ed., p. 406). For the Landsat TM, the line number is 5965 and sample number is 6967 for a total of 41,560,000 pixels/band and 290,910,000 for the summed 7 bands.

The data can be packed on the storage medium in several ways. Most commonly used is Band Sequential, in which all the lines for a given band are recorded in succession from the top of the scene to the bottom. Another format is Band Interleaved, in which each band is presented in succession one line at a time (e.g., for MSS: line 1, Band 1 through 4; line 2, Band 1 through 4; line 3, ÖÖ). Depending on the nature of the data, a digital file structure can store the data in binary, ASCII, or packed binary modes and the numbers can be expressed as real, integer, or byte. The data we will use with PIT is in byte binary.