Thematic mapper image characteristics

The U.S. Land satellites Landsat-4 and 5 are equipped with thematic mappers, and the Landsat-7 satellite is equipped with an enhanced thematic mapper. The TM images and ETM + images obtained by them have spatial characteristics and spectral characteristics. It has outstanding characteristics in all aspects and is by far the most widely used and most effective source of earth resources satellite remote sensing information in the world.

(1) Spatial characteristics

The spatial characteristics of TM images mainly refer to the overlap rate, projection properties, longitude and latitude, scale and resolution of the image.

1. Ground coverage and image overlap

(1) Overlap rate of images: TM images are similar to ordinary black and white aerial photos, with vertical overlap and side overlap. The vertical overlap rate was designed in advance during framing in the data processing center, and is 10% of the total area, that is, the upper and lower parts of the photo overlap 18.5km. The side overlap rate is determined by the Landsat satellite orbit. Near the equator, every day, the projection line of the satellite orbit on the ground moves westward by 1.43° (longitude), which is a distance of 159km. The width of the image frame is 185km, thus forming a lateral overlap of 26km, accounting for 159km of the image. 14% of the total area (Figure 3-34). Since the angle between the Landsat satellite orbit and the Earth's axis is very small, the higher the latitude, the greater the proportion of sideways overlap in the images.

Figure 3-34 Repeated coverage of Landsat (a) and side overlap of images on the equator (b)

(2) Ground coverage: TM’s observation zone ground coverage Bidirectional scanning is used. That is, both forward scanning and retracement of the scanning mirror are effective scanning, and the swing frequency of the scanning mirror is 7 times/s. Compared with Landsat-1, 2, and 3, the dwell time of the detector on the ground is increased and the radiation accuracy is improved. Each scan of the TM1-5 band and TM7 band images has 16 scan lines, each line width is 30m, and each scan covers a surface area of ??480m × 185km. A standard image frame of 185km × 185km requires approximately 386 scans. ***There are 6166 scan lines. The TM6 band image obtained by the thermal infrared remote sensor scans 4 scanning lines each time in the same surface range, each line width is 120m, and a standard TM6 image frame consists of 1542 scanning lines.

2. Projection properties

During scanning imaging, each effective scan has a center. A TM image is composed of 386 effective scans, so it has 386 centers, so it is called "multicenter projection". The projection center is dynamic, so the image scale of the first row from the center to the edge is not equal. However, since the satellite images the ground at an altitude of more than 700 to 900 kilometers, the effect of this deformation is not obvious.

Figure 3-35 TM detector array diagram

3. Instantaneous field of view, ground resolution

The instantaneous field of view of the optical-mechanical scanning sensor refers to When the scanning mirror is at a certain position, the ground area included in the solid angle of the beam of light reflected on the detector element (called the instantaneous field of view) is the ground resolution of the image on the TM image. Each scan of the TM scanning mirror projects 480m wide ground information to 100 detector units of the imaging plate, which are divided into TM1-5, 7 six bands. Each scan of 16 strips requires 96 detector units, and its instantaneous field of view is 30m × 30m; in addition, the TM6 band scans 4 scanning lines each time, requiring 4 thermal infrared detector units, and the instantaneous field of view is 120m × 120m (Figure 3-35).

For optical-mechanical scanning sensors, the instantaneous field of view is fixed, but the size of the instantaneous field of view depends on the height of the platform and the size of the scan. As shown in Figure 3-36, if D vertical and D horizontal are respectively the length of the instantaneous field of view along the heading and scanning direction, then

Remote Sensing Geology

In the formula: H is the platform height; β is the instantaneous field of view angle; θ/2 is the half-scan angle. It can be seen that the ground resolution on the same scanning line changes with the position of the image point. It is the highest at the bottom of the image (θ = 0), and D vertical = D horizontal. The ground resolution is the highest at this point and the image has no distortion. The ground resolution of other image points decreases symmetrically from the center to both sides, that is, for the same strip image, the vertical and horizontal scales are inconsistent, the scales on the horizontal scanning lines are inconsistent, but the vertical scale is consistent (Figure 3-37). This is to ensure that the scan lines in the middle part are exactly connected during scanning coverage, causing the overlapping part to gradually increase from the middle to both sides. The horizontal scale, except that the middle and vertical scales are equal, will gradually shrink toward both sides as the scanning angle changes. The inconsistency between the vertical and horizontal scales is the main cause of image distortion in optical-mechanical scanning images.

Figure 3-36 Ground resolution of optical-mechanical scanning image (according to Pan Shixiang, 1990)

Figure 3-37 Schematic diagram of scale of optical-mechanical scanning image (according to Pan Shixiang, 1990)

TM The ground resolution of the image is the size of the pixel. The pixel is the basic unit that constitutes the image on the remote sensing image. It is formed by the instantaneous field of view of the scanner moving on the scanning line. For example, the TM image continuously moves on the scanning line with an instantaneous field of view of 30m × 30m. The amount of radiation reflected by ground objects in the instantaneous field of view changes continuously with scanning. This continuously changing amount of radiation is received by the detector unit (unit) and converted into a continuously changing electrical signal. The electrical signal is an analog signal, and press Sampling and quantization at certain regular intervals form the basic unit of the image - pixel. Each value (DN value) of the digital data of each pixel is equivalent to a brightness or gray level. Each pixel includes comprehensive electromagnetic radiation information of ground objects within the ground range. If a pixel contains electromagnetic radiation information of only one kind of ground object, it is called a positive pixel; if a pixel contains electromagnetic radiation information of two or more ground objects, it is called a mixed pixel.

4. Longitude and latitude of satellite images

Based on factors such as the precise time of imaging, satellite heading and satellite attitude data, electronic computers are used in the data processing center to determine the latitude and longitude of satellite images. , and recorded on tape or directly on 700mm film.

(2) Spectral characteristics of TM, ETM + images

The spectral characteristics of Landsat images mainly include gray scale, spectral effects, etc.

1. Gray scale

The gray scale of the TM image is divided into 15 levels. Level 1 is the maximum radiation energy level of each channel, which is white on the image. Level 15 is the lowest radiation energy level of each channel (zero radiation energy level), which is black on the image.

2. Spectral effect

Various objects on the ground reflect different spectral characteristics due to their different material composition, surface structure, and surface temperature. In multispectral remote sensing images, not only the image tones of different ground objects are different, but even the tones of the same ground object in different band images will be different. Due to differences in spectral effects, different bands of TM and ETM + have corresponding recognition capabilities for different ground objects. See Table 3-20 for details.

Table 3-20 Image characteristics of each TM band

TM1 (0. 45 ~ 0. 52μm) belongs to the blue-green light band, has strong penetration into water, and is sensitive to chlorophyll and Leaf pigment concentration is sensitive. The reflectivity of vegetation, water, soil, etc. is significantly different in this band, which is helpful for determining water quality, water depth, chlorophyll distribution in water, coastal currents, sediment conditions and offshore water mapping, and can be used for soil and plant classification. In terms of image tone, vegetation is the darkest, followed by water bodies, and fresh snow is the lightest.

TM2 (0. 52 ~ 0. 60μm) belongs to the green-yellow light band, has strong transmission ability to water, and the water color is lighter, which can reflect the underwater terrain of a certain depth (>10m), and has It is helpful for identifying water turbidity, coastal currents, sand bars, etc. Chlorophyll has a reflection peak in this band, called the green peak. Healthy plants have a certain reflection of green light. The image tone is lighter, and the distribution range and growth density of vegetation can be reflected. Detect the green reflectance of healthy plants and evaluate plant vitality based on green peak reflection, which can be used to distinguish forest types and tree species. Images of blue, green, and yellow ground objects generally appear in light tones, and become darker as the red component increases. Oil dirt and metal compounds floating on the water are also shown because they block the transmission of green light. Lighter-colored rock formations and Quaternary loose sediments on land, towns, quarries, etc., appear in lighter tones. Affected by scattered light, the image contrast in this band is small, and the boundary contours of ground objects are somewhat blurred.

TM3 (0. 63 ~ 0. 69μm) belongs to the orange-red light band and has a certain transmission ability to the water body (about 2m), which can reflect the sediment content in the water, underwater landforms and sediment flow. This band is also the main absorption band of chlorophyll. Images of healthy plants have a darker green tone, while dead trees camouflaged by diseased plants have a lighter tone. Therefore, it can reflect the chlorophyll absorption and health status of different plants, and can be used to distinguish plant types and coverage. Spend. Images of orange-red features generally have light tones, while green features have dark tones. The images of exposed surface, vegetation, soil, water systems, rocks, strata, landform features, etc. are clear, with many tonal levels and rich information. They are often used to interpret lithology and geological structures based on macro and micro landform characteristics and tonal differences. For example, rock formations containing more Fe3+ and rock formations containing more carbonaceous materials or medium-acidic rocks have obvious differences in color tone and shape. The boundaries between fractures, folds, bedrock, and Quaternary loose sediments can be identified from the water system characteristics, color tone, and morphology. It also has a certain effect on the distribution rules and classification of coarse and fine particles of Quaternary loose accumulations. It is effective for studying landform features.

TM4 (0. 76 ~ 0. 9μm) belongs to the photographic infrared band, which is a band with strong absorption by water and strong reflection by plants. The image is clear, with high contrast and strong three-dimensional effect, and can display various terrain details, such as micro water systems, micro landforms and some artificial buildings.

The water body in the image has a black tone, and areas and towns with rich shallow groundwater or high soil moisture have a darker tone. It is beneficial to study the distribution of water bodies, divide water and land boundaries, determine whether there is flowing water in rivers and gullies, search for shallow groundwater, and identify water-related geological structures and hidden structures. Water-filled faults and new depressions in plain areas have darker tones, while uplift areas have lighter tones, and water-rich strata have darker tones. The types and formation sequence of Quaternary sediments, such as sediments, alluvial fans, flood plains and coastal plains of different periods, are also clearly reflected. It can also be used for research on seawater, seawater temperature distribution and geothermal heat.

Healthy plants have strong reflection of near-infrared waves and have bright light tones, while diseased plants have darker tones. Broadleaf trees are lighter in color, while conifers are darker in color. Through contrastive study of image tones and texture feature analysis with TM2 and 3, it is easy to delineate the distribution range of vegetation, distinguish whether plants are woods, crops, or grasslands, investigate the amount of plants, and measure crop growth. Through the correlation between plants and water, certain rocks, strata or hidden structures covered by vegetation can be studied on images, such as mudstone strata with developed vegetation, limestone strata with poor vegetation growth, water-filled faults, etc. There is a noticeable difference in the images.

TM5 (1. 55 ~ 1. 75μm) belongs to the near-infrared band. This band is within the absorption band of water (1. 4 ~ 1. 9μm). It is sensitive to the moisture content of ground objects and can be used for Soil moisture, plant moisture content investigation, water status research, crop growth analysis, etc. The differences between pastures and broadleaf forests, granite and bare soil are enhanced, and the ability to distinguish between different types of crops is greatly improved. Processed TM5 images distinguish between exposed, grass-covered, and tree-covered epigenetic deposits. In the image, the snow is darker than the clouds, and the aquatic clouds are lighter than the ice crystal clouds. It is easy to distinguish between clouds and snow, clouds and bare ground, and the glacier snow line is easier to identify.

TM6 (10. 4 ~ 12. 6μm) belongs to the thermal infrared band. According to the difference in radiation emitted by ground objects, herbaceous plants and woody plants can be distinguished on images, and large-scale desertification can be identified. Information on wetland freshwater and saltwater mixing, depth of small water bodies, littoral water levels, and heat sources is available. Regional ground humidity changes are also clearly reflected. It can be used to study regional magmatic activity and human-related surface heat flow changes. Nighttime thermal infrared images have been used to distinguish differences in lithology. Since near-surface water is usually concentrated on fault planes and joint planes, its temperature is lower than that of the surroundings, so it can also be used to identify fault structures. It is also used to observe surface temperature changes in lakes, rivers, coasts and snow-covered areas.

TM7 (2. 08 ~ 2. 35μm) belongs to the near-infrared band, which is an additional band for geological research. Located in the strong absorption band of water, the reflection characteristics of the soil are similar to those in the visible light band, the water body appears black, and the images of other ground objects are similar to those in the visible light band. This band is the peak section of the reflection spectrum of most rock-forming minerals. Hydrogen-containing minerals (such as clay) and carbonate minerals (such as calcite) have distinctive characteristic spectral absorption bands, which appear dark in the image. Therefore, the TM7 image is more sensitive to clay and carbonate minerals directly exposed on the surface. The comprehensive use of TM7 and TM2-5 images can detect iron-containing clay minerals that are signs of hydrothermal alteration characteristics, and map lithofacies changes in carbonate formations and hydrothermal alteration distribution maps in arid and semi-arid areas.

ETM + image spectral effects are as follows:

B1 (0. 45 ~ 0. 52μm,) belongs to the blue-green band and is used for water penetration and soil vegetation resolution.

B2 (0. 52 ~ 0. 60μm), which belongs to the green band, is used for vegetation discrimination.

B3 (0. 63 ~ 0. 69μm) belongs to the red band and is in the chlorophyll absorption area. It is very effective for observing roads, bare soil, and vegetation types.

B4 (0. 76 ~ 0. 90μm), which belongs to the near-infrared band, is used to estimate the number of organisms. Although this band can distinguish water bodies from vegetation and distinguish moist soil, it is not as effective as road identification. TM3.

B5 (1. 55 ~ 1. 75μm) belongs to the mid-infrared band. This band is considered to be the best of all bands. It is used to distinguish roads, bare soil, and water. It can also There is good contrast between different vegetation and good ability to penetrate the atmosphere, clouds and fog.

B6 (10. 5 ~ 12. 5μm), which belongs to the thermal infrared band, senses targets that emit thermal radiation, and the resolution is 60m.

B7 (2. 08 ~ 2. 35μm), which belongs to the mid-infrared band, is very useful for distinguishing rocks and minerals, and can also be used to identify vegetation cover and moist soil.

B8 (0. 52 ~ 0. 90μm), which belongs to the panchromatic band, produces black and white images with a resolution of 15m, which is used to enhance the resolution and improve the resolving power. When used, it is blended with other bands to increase its resolution.