Image characteristics of theme musical instruments

Landsat-4,5 is equipped with thematic plotter, and Landsat-7 is equipped with enhanced thematic plotter. TM images and ETM+ images acquired by it have outstanding spatial and spectral characteristics, and are the most widely used and effective remote sensing information sources of Earth resources satellites in the world.

(A) spatial characteristics

The spatial characteristics of TM images mainly refer to the overlapping rate, projection attributes, latitude and longitude, scale and resolution of images.

1. Ground coverage and image overlap.

(1) Image overlap rate: TM images are similar to ordinary black-and-white aerial photographs, with vertical overlap and horizontal overlap. In the data processing center, the vertical overlap rate is designed in advance, which is 10% of the total area, that is, photo overlap 18. 5 kilometers from top to bottom. The lateral overlap rate is determined by the orbit of Landsat satellite. Near the equator, the projection line of satellite orbit on the ground moves westward by 1.43 (longitude) every day, that is, the distance of 159km, and the width of regional images is 185km, thus forming a lateral overlap of 26km, accounting for 14% of the total image area (Figure 3-30

Figure 3-34 Repeated coverage of ground satellites (a) and horizontal overlap of images on the equator (b)

(2) Ground coverage: The ground coverage of TM observation belt adopts bidirectional scanning. That is to say, both forward scanning and reverse scanning of the scanning mirror are effective scanning, and the oscillation frequency of the scanning mirror is 7 times per second, which increases the residence time of the detector on the ground and improves the radiation accuracy compared with Landsat- 1, 2, 3. TM 1-5 band and TM7 band images have 16 lines each time, with a line width of 30m and a surface area of 480m × 185km. A standard image of 185km × 185km needs to be scanned about 386 times, totaling 665438+. The TM6-band image obtained by thermal infrared remote sensor scans four scanning lines at a time in the same surface area, and the line width is 120m. The standard TM6 image frame of a scene consists of 1542 scanning lines.

2. Projection properties

In scanning imaging, every effective scan has a center. A TM image consists of 386 effective scans, so it has 386 centers, so it is called "multi-center projection". The projection center is dynamic, so the image scale of the first line from the center to the edge is not equal. However, because the satellite photographs the ground at an altitude of 700 ~ 900 kilometers, the influence of this deformation is not obvious.

Figure 3-35 TM detector array diagram

3. Instantaneous visual field and ground resolution

The instantaneous field of view of optical scanning sensor refers to the ground area contained in the solid angle (called instantaneous field of view angle) of the beam reflected on the detector element when the scanning mirror is at a certain position, which is the ground resolution of the image on TM image. TM scanning mirror projects 480m-wide ground information to 100 detector units of the imaging board, which are divided into TM 1-5. Seven bands 16 require 96 detector units for each scan, and the instantaneous field of view is 30m×30m；. In addition, four thermal infrared detector units are needed for scanning four scanning lines at a time in TM6 band, and the instantaneous field of view is 120m × 120m (Figure 3-35).

For optical scanning sensors, the instantaneous field of view angle is fixed, but the instantaneous field of view depends on the platform height and scanning degree. As shown in Figure 3-36, if the longitudinal and transverse directions of D are the lengths of the instantaneous field of view along the heading and scanning directions respectively, then

Remote sensing geology

Where: h is the platform height; β is the instantaneous viewing angle; θ/2 is the half scan angle. It can be seen that the ground resolution on the same scanning line varies with the position of the image point, and it is the highest at the bottom point of the image (θ = 0), and D vertical = D horizontal. At this time, the ground resolution is the highest and the image is not distorted. The ground resolution of other image points decreases symmetrically from the middle to both sides, that is, the vertical and horizontal scales of the same belt image are inconsistent, the horizontal scanning line is inconsistent, and the vertical scale is consistent (Figure 3-37). This is because in scanning coverage, in order to ensure that the scanning lines in the middle part are just connected, the overlapping parts from the middle to the two sides gradually increase. Except that the middle scale is equal to the vertical scale, the horizontal scale will gradually decrease to both sides with the change of scanning angle. The inconsistency of vertical and horizontal scales is the main reason for the distortion of optical-mechanical scanning images.

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

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

The ground resolution of TM image is also the pixel size. Pixel is the basic unit of remote sensing image, which is formed by the instantaneous field of view of the scanner moving on the scanning line. For example, TM images move continuously on the scanning line of 30m ×30m instantaneous field of view. The radiation reflected by objects in the instantaneous field of view changes continuously with scanning. This continuously changing radiation is received by the detector unit and converted into a continuously changing electrical signal, which is an analog signal. After sampling and quantizing it at regular intervals, the basic unit of the image-pixel is formed, and each value (DN value) of the digital data of each pixel is equivalent to a brightness or gray level. Each pixel contains the comprehensive electromagnetic radiation information of ground objects. If there is only one kind of electromagnetic radiation information in a pixel, 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. Latitude and longitude of satellite images

According to the precise time of imaging, the direction of the satellite and the attitude data of the satellite, the latitude and longitude of the satellite photos are determined by the computer in the data processing center, and recorded on magnetic tape or directly on 700 mm film.

(2) Spectral characteristics of TM and ETM+ images

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

1. Gray scale

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

2. Spectral effect

The spectral characteristics of various objects on the ground are different because of their material composition, surface structure and surface temperature. In multi-spectral remote sensing images, not only the image tones of different objects are different, but also the same object will be different in different bands. Due to the difference of spectral effect, different bands of TM and ETM+ have corresponding recognition ability for different ground objects, as shown in Table 3-20.

Table 3-20 TM image characteristics of each band

Tm 1 (0。 45 ~ 0.52 micron) belongs to the blue-green band, which has strong penetration to water and is sensitive to the concentration of chlorophyll and leaf pigment. The reflectance of vegetation, water and soil in this band is obviously different, which is helpful to distinguish water quality, water depth, chlorophyll distribution in water, coastal water flow, sediment situation and offshore water mapping, and can be used for soil and plant classification. In the image tone, vegetation is the darkest, followed by water, and new snow is the shallowest.

Tm2 (0。 52 ~ 0.60μ m) belongs to the green and yellow light bands, and has strong water transmission ability and light water color. It can reflect the underwater topography with a certain depth (> > 10m), which is beneficial to identify water turbidity, coastal currents, beaches, etc. Chlorophyll has a reflection peak in this band, which is called green peak. Healthy plants have a certain reflection on green light, and the image color is lighter, so it can reflect the distribution range and growth density of vegetation. Detecting the green reflectance of healthy plants and evaluating the viability of plants according to the green peak reflectance can be used to distinguish forest types and tree species. Images of blue, green and yellow ground objects are generally light-colored and darken with the increase of red component. Oil stains and metal compounds floating on the water surface are also shown because they hinder the transmission of green light. On land, light-colored rocks and Quaternary loose sediments, towns and quarries are light-colored. Affected by scattered light, the contrast of the image in this band is small, and the boundary contour of the ground object is somewhat blurred.

Tm3 (0。 63-0.69 micron) belongs to the orange-red light band, and has certain transmission ability to the water body (about 2m), which can reflect the sediment content in the water, underwater landform and sediment flow. This band is also the main absorption band of chlorophyll. The image of healthy plants is dark green, while the dead trees disguised by diseased plants are light in color, so it can reflect the chlorophyll absorption and health status of different plants and be used to distinguish plant species and coverage. Images of orange-red ground objects are generally light-colored, while images of green ground objects are dark. The images of exposed surface, vegetation, soil, water system, rocks, strata and landforms are clear, with many tones and rich information. They are usually used to explain lithology and geological structure according to macroscopic and microscopic geomorphological characteristics and tonal differences. For example, rocks containing more Fe3+ are obviously different from rocks containing more carbon or moderately acidic rocks in hue and shape. The boundary between faults, folds, bedrock and Quaternary loose sediments can be identified from the characteristics, colors and forms of water system. It also has a certain influence on the distribution and classification of coarse and fine particles of Quaternary loose deposits. It has a good effect in the study of geomorphological characteristics.

Tm4 (0。 76 ~ 0.9 micron) belongs to the photographic infrared band, which is a strong absorption band of water and a strong reflection band of plants. The image is clear, the contrast is large, and the three-dimensional sense is strong, which can show various terrain details, such as micro-water system, micro-landforms and some artificial buildings. The water body in the image is black, and the areas and towns with rich shallow groundwater or high soil moisture are darker. It is beneficial to study the distribution of water bodies, divide the land-water boundary, judge whether there is running water in rivers and gullies, find shallow groundwater and identify geological structures and concealed structures related to water. Water-filled faults and new depressions in plain areas are darker in color, uplift areas are lighter in color and water-rich strata are darker in color. The types and formation sequence of Quaternary sediments, such as sediments in different periods, alluvial fans, alluvial plains and coastal plains, are also clearly reflected. It can also be used to study seawater, seawater temperature distribution and geothermal energy.

Healthy plants have a strong reflection on near-infrared waves, which are bright and light, while sick plants are darker. Broad-leaved trees are lighter in color, while conifers are relatively darker in color. By comparing the hue and texture characteristics of TM2 and TM3 images, we can easily delineate the distribution range of vegetation, distinguish whether plants are trees, crops or grasslands, investigate the number of plants and determine the growth of crops. Through the correlation between plants and water, we can study some rocks, strata or hidden structures covered by vegetation, such as muddy strata with poor vegetation growth, limestone strata with poor vegetation growth, water-filled faults and so on. There are obvious differences in image.

Tm5 ( 1。 55 ~ 1.75μ m) belongs to the near infrared band, which is in the water absorption region (1. 4 ~ 1.9μ m), and it is sensitive to soil moisture, so it can be used for soil moisture, plant moisture investigation, water status research and crop growth analysis. The difference between pasture and broad-leaved forest, granite and bare soil is strengthened, and the ability to distinguish different types of crops is greatly improved. The processed TM5 image can distinguish bare, grass-covered and tree-covered supergene minerals. The image color is darker than the cloud, and the water cloud is shallower than the ice crystal cloud, so it is easy to distinguish between the cloud and snow, the cloud and bare land, and the glacier snow line.

Tm6 (10.4 ~ 12.6μ m) belongs to the thermal infrared band. According to the different radiation emitted by ground objects, herbaceous plants and woody plants can be distinguished in the image, and large-scale desertification can be identified. It can provide information such as the mixing of fresh water and salt water in wetland, the depth of small water body, coastal water level, heat source and so on. The change of regional ground humidity is also obviously reflected. It can be used to study regional magmatic activity and human-related surface heat flow changes. Thermal infrared images at night have been used to distinguish lithologic differences. Because the near-surface water body is usually concentrated on fault plane and joint plane, its temperature is lower than that around it, so it can also be used to identify fault structures. In addition, it is also used to observe the surface temperature changes in lakes, rivers, coasts and snow-covered areas.

Tm7 (2。 08 ~ 2.35μ m) belongs to the near infrared band and is an additional band for geological research. Located in the strong absorption zone of water, the reflection characteristics of soil are similar to those of visible light band, the water body is black, and other ground objects are similar to those of visible light band. This area is the peak of reflection spectrum of most rock-forming minerals, while hydroxyl-containing minerals (such as clay) and carbonate minerals (such as calcite) have distinguishable characteristic spectral absorption bands, and the images are dark, so TM7 images are sensitive to clay and carbonate minerals directly exposed on the surface. Comprehensive utilization of TM7 and TM2-5 images can detect iron-bearing clay minerals, draw the lithofacies change map of carbonate strata and the distribution map of hydrothermal alteration in arid and semi-arid areas.

The spectral effects of ETM+ images are as follows:

B 1 (0。 45 ~ 0.52 micron) belongs to the blue-green band, which is used for water infiltration and soil vegetation discrimination.

B2 (0。 52 ~ 0.60μ m) belongs to the green band and is used for vegetation discrimination.

B3 (0。 63 ~ 0.69 micron) belongs to the red band and is located in the chlorophyll absorption area. It is very effective for observing roads, bare soil and vegetation types.

B4 (0。 76 ~ 0.90μ m) belongs to near infrared band and is used to estimate biomass. Although this band can distinguish water from vegetation and wet soil, it is not as effective as TM3 in road identification.

B5 ( 1。 55 ~ 1.75 micron) belongs to the mid-infrared band and is considered as the best of all bands. It is used to distinguish roads, bare soil and water bodies, with good contrast between different vegetation and good penetration ability to the atmosphere and clouds.

B6 (10.5 ~ 12.5μ m) belongs to thermal infrared band, which induces the target to emit thermal radiation with a resolution of 60m.

B7 (2。 08 ~ 2.35μ m) belongs to the mid-infrared band, which is very useful for distinguishing rocks and minerals, and can also be used to identify vegetation cover and wet soil.

B8 (0。 52 ~ 0.90μ m), which belongs to panchromatic band, and the black-and-white image with resolution of 15m is obtained, which is used to enhance and improve the resolution. When used, it is combined with other bands to improve the resolution.