Landsat: The Thematic Mapper

A more sophistical multispectral imaging sensor, named the Thematic Mapper (TM) has been added to Landsats 4 (1982), 5 (1984), and 6 (the latter failed to attain orbit during launch and thus has never returned data). These flew on a redesigned, more advanced platform. Although similar in operational modes to the MSS (which was also part of the 4 and 5 payload, to maintain continuity), the TM consists of 7 bands that have these characteristics:

The six reflectance bands obtain their effective resolution at a nominal orbital altitude of 705 km (437 miles) through an IFOV of 0.043 mrad; the IFOV for the thermal channel is 0.172 mrad.

Band 1 is superior to MSS 4 in detecting some features in water; it also allows quasi- natural color composites to be put together. Band 5 is sensitive to variations in water content, both in leafy vegetation and as soil moisture; it also distinguishes between clouds (appearing dark) and bright snow (light). This band also responds to variations in ferric iron (Fe₂ O₃) content in rocks and soils, with materials containing this substance showing higher reflectances as its percentage increases. Band 7 likewise reacts to moisture contents and is especially suited to detecting hydrous minerals (such as clays or certain alteration products) in geologic settings. Band 6 can distinguish a radiant temperature difference of ~ 0.6 degrees C and is helpful in discriminating rock types whose thermal properties permit varying extents of heating and consequent differences in near surface temperatures; it often can pick out changes in ground temperatures due to moisture variation and can single out vegetation due to its evaporative cooling effect. The higher resolution achieved in the reflective bands is a significant aid in picking out features and classes whose minimum dimension is usually on the order of 30 m (100 ft) . Thus, houses and smaller buildings, which were unresolvable in MSS images, can often be discerned.

The size and shape of TM images from Landsats 4 and 5 are identical to the MSS images. At first glance, the quality and characteristics of the full scene TM images from 4 and 5 seem similar to those made by the MSS after optimal computer-based processing but on closer inspection they do appear sharper. This apparent similarity is due to the need to resample the TM images for TV monitor displays (which are not high resolution systems capable of reproducing all TM pixels) by dropping some pixels. The influence of the better TM resolution (when un-resampled) becomes apparent whenever photographs of full scenes are enlarged (pictures more than a meter on a side can be produced with exceptional clarity) or subscenes are extracted and enlarged.

All the Landsats follow a near-polar orbit (inclined ~ 9 degrees to west of longitudinal lines; passing within 8 degrees of the poles) and are sun-synchronous (satellite precesses about Earth at same angular rate as Earth rotates; thus, in descending mode [N to S] it crosses the equator each time between 9:30 and 10:00 A.M. local time). Landsats 1-3 make 14 full orbits (each successive one displaced 2875 km [ 1785 miles] to the west) each day (3 over U.S.) and after 252 orbits repeat their previous ground tracks every 18 days; Landsats 4-5, from lower altitude (705 km [437 miles]), after 233 orbits cover the same area again every 16 days. To fully image the entire Earth's land surface (except for polar regions), ~11000 scenes are required.

For each band detector, the electronic signal from this IFOV results in a single digital value (called its DN or digital number, which for the MSS can range from 0 - 255 [2⁸]). The value is related to the proportionally averaged reflectances from all materials within the each IFOV and, since the mix of objects on the ground will constantly change, will vary in DN magnitude from one to the next IFOV. Each IFOV is represented in any b & w image of which it is a part as a tiny point of uniform gray-level tone known as a pixel (a contraction of "picture element") whose brightness is determined by its actual DN value. In a Landsat MSS band image, owing to a sampling rate effect in which there is some overlap between successive 9 microsecond intervals, a pixel has an effective ground-equivalent dimension of 79 by 57 m (259 x 187 ft) but contains the reflectances of the full 79 m² actually viewed .The average (but variable) number of pixels within a full scan line (representing 185 km) across the orbital track is 3240 (185 / 0.057). In order to image an equi-dimensional scene, which requires 185 km of down track coverage, the average total number of lines to do this is set at 2340 (185 / 0.079). Each band image will therefore consist of approximately (again variable) 7,581,600 pixels - a lot to handle during computer processing.

The continuous stream of pixel values can be used to drive an electronic device that generates a uninterrupted light beam of varying intensity which sweeps systematically over film to produce a b & w photo image in which tone variations are proportional to the DNs in the array. Or, the pixels generated from these sampling intervals can be displayed as an image of each band by feeding their DN values sequentially into an electronic signal array. That is then projected line by line on to a TV monitor (in which the resulting image is an assemblage of light-sensitive spots [also called pixels] of varying brightnesses).

Best MSS Bands for Identifying Surface Features
Item	Category	Best Bands	Salient Characteristics
a.	Clear Water	7	Black tone in black and white and color.
b.	Silty Water	4,7	Dark in 7; bluish in color.
c.	Nonforested Coastal Wetlands	7	Dark gray tone between black water and light gray land; blocky pinks, reds, blues, blacks.
d.	Deciduous Forests	5,7	Very dark tone in 5, light in 7; dark red.
e.	Coniferous Forest	5,7	Mottled medium to dark gray in 7, very dark in 5; brownish-red and subdued tone in color,
f.	Defoliated Forest	5,7	Lighter tone in 5, darker in 7 and grayish to brownish-red in color, relative to normal vegetation.
g.	Mixed Forest	4,7	Combination of blotchy gray tones; mottled pinks, reds, and brownish-red.
h.	Grasslands (in growth)	5,7	Light tone in black and white; pinkish-red.
i.	Croplands and Pasture	5,7	Medium gray in 5, light in 7, pinkish to moderate red in color depending on growth stage.
j.	Moist Ground	7	Irregular darker gray tones (broad);darker colors.
k.	Soils-bare Rock-Fallow Fields	4,5,7	Depends on surface composition and extent of vegetative cover. If barren or exposed, may be brighter in 4 and 5 than in 7, Red soils and red rock in shades of yellow; gray soil and rock dark bluish; rock outcrops associated with large land forms and structure.
1.	Faults and Fractures	5,7	Linear (straight to curved), often discontinuous; interrupts topography; sometimes vegetated.
m.	Sand and Beaches	4,5	Bright in all bands; white, bluish, to light buff.
n.	Stripped Land-Pits and Quarries	4,5	Similar to beaches ­ usually not near large water bodies; often mottled, depending on reclamation.
o.	Urban Areas: Commercial Industrial	5,7	Usually light toned in 5, dark in 7, mottled bluish-gray with whitish and reddish specks.
p.	Urban Areas: Residential	5,7	Mottled gray, with street patterns visible; pinkish to reddish.
q.	Transportation	5,7	Linear patterns, dirt and concrete roads light, in 5; asphalt dark in 7.