What is forestry remote sensing?

(Fang Youqing)

A set of technology that uses optical and photoelectric remote sensing instruments to receive electromagnetic wave information and other information reflected or emitted by forest objects from the air or far away, and then extract information such as forest resources, forest environment and forest natural disasters through transmission and processing. Remote sensing can be divided into space remote sensing, aerial remote sensing and ground remote sensing according to its means of transport. Remote sensors for space remote sensing are installed on satellites, spaceships and space shuttles. Remote sensors for aerial remote sensing are installed on airplanes or balloons. Remote sensors for ground remote sensing are installed on cars or ships. Forestry remote sensing has been involved in all aspects of forestry production. Forest aerial photographs are used to observe and understand the forest environment and the actual situation in forestry work, so as to take targeted measures to solve many problems that were originally thought to be unsolvable by manpower. Forest resource information is the basic data of forestry decision-making, which needs to be investigated. Only by applying remote sensing to modern forest resources investigation can accurate and reliable forest resources data be obtained quickly.

General situation of development

Since the adoption of aerial photogrammetry in World War I, R.Hugershoff and others have tried to apply aerial photographs to forest surveys many times. Later, someone used the precision aerial photogrammetry stereo plotter made by Hugh Hoff to check the area, tree height, canopy density, cross-sectional area and volume, and observed and interpreted various forests in a three-dimensional way with aerial photographs, and calculated and compared them in the same area, which aroused people's interest in the application of aerial photographs. In the 1920s, Sweden began to use aerial photos for forest survey. In 1930s, the Forestry Research Institute of Canada studied the technology of measuring the height of trees through the shadows of trees in aerial photographs. During the same period, some German scholars brought remote sensing technology to Asia, took aerial photos in the exercise forest of the University of Tokyo in Japan at 1932, and tested the forest farm at the forestry experimental field at 1933. During the Second World War and several years after the war, a group of forestry workers were recruited to participate in the war and engaged in military interpretation of aerial photos. After the war, they turned to forest interpretation, which promoted the gradual formation of several forest interpretation techniques and methods. At this time, we can not only interpret forest types (outline and area measurement) on aerial photographs, but also study the interpretation of trees with simple instruments and tools, such as measuring tree height, crown width, canopy density and number of trees per unit area. Using aerial photos to make up for the deficiency of forest visual investigation. In the 1950s, due to the rapid development of aerial photography technology, natural color film and color infrared film appeared, which solved the problem that black-and-white film interpretation could not solve, so that forestry remote sensing technology could be used not only for forest resources inventory, but also for afforestation investigation and design, forest harvesting and transportation engineering investigation and design, soil and water conservation, water sources in arid areas and large-scale forest pest investigation. In the investigation of forest resources, aerial photographs are not only an auxiliary means of investigation, but also can be used for indirect measurement. For example, the height, crown width and number of trees are measured on the super-large-scale aerial photograph (1: 500 ~ 1: 2000), and combined with the ground sampling measurement, the aerial photograph stumpage table is compiled to estimate the volume of aerial photograph sample plots. In addition, according to the theory of mathematical statistics, taking aerial photos as a tool to conduct forest sampling survey has become the main means of modern forest resources inventory. After 1960s, complicated data analysis and calculation were completed by computers, which made the forest sampling survey more perfect.

1952 A forest aerial survey team was established in China. 1953 A forest aerial survey experiment was carried out in Dahailin forest area of Heilongjiang Province, and more than 2,700 square kilometers of forest aerial survey and resource investigation were completed by Dahailin Forestry Bureau. Since 1954, large-scale aerial photography has been carried out in the main forest areas of northeast, southwest and northwest every year. By 1964, more than 400,000 square kilometers have been completed, providing aerial data for forest management inventory and forest resources inventory. 1963, the stratified sampling survey of forest resources was carried out by using medium-scale aerial photographs, and the test results proved that the accuracy was good. 1972 adopts the methods of photo interpretation and measured regression estimation. 1975, the quantitative stand volume scale of three-dimensional forest interpretation samples and aerial photographs was compiled. From 1975 to the early 1980s, a lot of work was done in production and scientific research by directly applying Landsat multispectral scanning images. For example, the inventory of forest resources in Tibet, the planning and investigation of forestry bases in Liaoning Province, and the compilation or revision of the distribution map of forest resources in some provinces1∶ 250,000; Planning and investigation of shelter forest system in North China, Northwest China and Northeast China; Provincial forest resources dynamic monitoring experiment.

applied range

The application of remote sensing in forest resources investigation and management mainly includes the following aspects:

Forest resources investigation and forest distribution map compilation

Using the images and data obtained by remote sensing technology, combined with mathematical statistics sampling technology, computer image processing and mathematical analysis and calculation, the results of large-scale resource inventory can be quickly obtained, which can be used for site condition investigation and land resource evaluation. Because of the superior performance of color infrared film and multi-spectral color composite film, most field investigations can be changed to indoor work. In the wild, we only do some more accurate sampling surveys, observe some typical forest ecological conditions, and make accurate interpretation and accurate calculation indoors. In the past, it was time-consuming and laborious to compile large-scale forest distribution maps, especially small-scale maps of tens of thousands of square kilometers, using tens of thousands of Zhang Dazhong-scale aerial photographs. At present, the ultra-small-scale color infrared photos of1∶ 80000 ~1∶120000, which were taken by high-altitude (20 ~ 35km) flight, or the forest distribution of1∶

Forest management inventory

The second-class investigation of forest resources includes the investigation of forestry production conditions, small-class investigation and professional investigation, which can all rely on large and medium-scale aerial photos. Using aerial photographs, soil types and soil moisture can be investigated, which provides a basis for rational arrangement of afforestation tree species and soil improvement. At the same time, we can also understand the ecological environment of various vegetation types, as well as the hydrological conditions of surface water and shallow groundwater. Aerial photos can be used to explain the factors of forest management inventory, such as forest origin, forest layer structure, tree species composition, forest age, density, stand volume and site type grade, as well as to measure tree height and crown diameter, estimate crown density and number of standing trees.

Forest disaster detection

When the forest suffers from fire, flood, diseases and insect pests, frost and other disasters, using remote sensing technology to detect the disaster situation can quickly grasp the disaster situation and put forward effective prevention measures. For example, identifying the location and distribution of thunderstorms and forest fires from satellite images provides a reliable basis for studying artificial rainfall to extinguish forest fires. Another example is the use of high-resolution infrared scanners and multispectral scanners to detect pests and diseases on satellites, which can be found a few days to ten days earlier than naked eye observation.

Forestry production management

The application of remote sensing technology can timely investigate forest growth, study forest vegetation phenology, collect forestry economic information and play a guiding role in forestry production. For example, using satellite images to study the seasonal phenological changes of vegetation cover, if it is found that the leaves of plants are withered and yellow, it is the season when forest seeds mature; If it is found that the ground is covered with ice and snow, it is the season to use ice and snow for wood cutting and transportation. For example, the boundary between dry and wet in remote sensing images is very obvious. If it shows the lack of soil hydrology, irrigation should be carried out in time. For those areas with known soil water content and drainage conditions, remote sensing information can provide the distribution of surface water resources and the flow direction of shallow groundwater, which is of guiding significance for nursery management, young forest tending and timber river flow direction. Another example is using aerial photographs to survey forest roads, investigate timber circulation channels and calculate the quantity of timber received by water storage fields, which can reduce a lot of field work and improve economic benefits.

Study on forest ecological environment

By means of remote sensing, the objective law of forest ecological formation is discussed, aiming at studying the necessary conditions in the process of forest growth and development and tapping the potential of forestry production. Its main contents are: observing solar radiation flux and estimating photosynthetic potential of plants; Observe the spectral characteristics of ground objects, and measure the spectral characteristic curves of main geographical environment elements such as vegetation, soil, desert, rivers and lakes; Observe and explain meteorological changes, investigate surface water and shallow groundwater, monitor water quality, soil types, soil moisture and environmental pollution in forest areas, and provide basic data for studying forest ecology.

Forestry remote sensing information processing

By means of remote sensing, a large number of forest resources data and ecological environment information are systematically collected, images, charts and statistical databases of forest resources are established, analyzed and processed, the relationship between natural environment and forestry production management is studied, and the potential of forest resources development and utilization and the dynamic development of resources are discussed. In addition, remote sensing tapes are also used to record data, process forestry remote sensing digital images and draw various forestry maps. By integrating the above-mentioned forest resources and environment information database and remote sensing image information, a regional forestry information system can be established to analyze and process regional forestry information and serve forestry decision-making.

prospect

Because of the high cost and long photography cycle, aerial photography can not be applied in time, while satellite remote sensing image has a broader development prospect than the former because of its large photography range, fast dynamic change reflection and unrestricted data acquisition. The fourth and fifth batches of Landsat thematic mappers (TM) in the United States have an image resolution of 30m× 30m, and the French SPOT satellite has improved to 20m and 10m. Using Landsat TM and SPOT satellite images, topographic maps with the scale of1:100000 and thematic maps with the scale of1:50000 can now be drawn. SPOT satellite images can also form stereoscopic images for stereoscopic observation, and a certain area can be photographed repeatedly within 1 ~ 5 days for local and short-period dynamic monitoring. These characteristics are very beneficial to the dynamic monitoring of forest resources and its application in forestry production. The new generation of satellite images can show the texture characteristics of forest images, interpret tree species, forest ages and determine management methods, and its actual resolution is close to that of small and medium-scale aerial photographs. It can be predicted that with the improvement of satellite resolution, the progress of images and the development of database technology, space remote sensing data will play an important role in forest investigation and dynamic monitoring of forest resources.

philology

American society for photogrammetry, handbook of remote sensing, second edition. Sheridan Press, USA, 1983.