What is an aerial photographic forest survey?

（forest in ventory by aerial photographs）

（Dong Naijun）

Forest survey aerial photograph scale and resolution comparison table

Utilize the interpretation and mapping capabilities of aerial photographs to improve and complete the collection, analysis and processing of forest resource information. Aerial photos of forest areas taken from airplanes record instantaneous images of land types and forest vegetation in the forest area, which are valuable records of forest resources. The main functions of aerial photos in forest surveys are: providing forest resource distribution and structure information, which is the basis for formulating forest survey plans; performing photo interpretation, forest area classification, and mapping; and obtaining forest stand factor interpretation values ??as auxiliary variables. Improve the estimation method of target variables; directly provide relevant information on area and accumulation; be used for sampling point location and control field surveys; provide sampling probability data for multi-level forest inventory systems, etc. Due to the application of aerial photos, the workload of ground surveying and tree measurement has been reduced, the accuracy of map area and storage volume has been improved, the plasticity of forest survey technology has increased, and the plan has become more flexible. Some highly efficient sampling estimation methods have emerged, such as aerial photos. Sampling by number, stratified sampling, etc. The main functions of aerial photos in statistical analysis of forest sampling are: ① Reduce the changes between first-order units through interpretation of zoning, stratification, etc.; ② Provide accurate expansion coefficients, such as the number of overall units, the weight of sub-populations, etc., so that the estimated values The variance is reduced; ③In the forest resource information system, the overall estimated value is better allocated to each plot.

Historical evolution

In 1919, Canada used airplanes to take photos of forest areas to compile forest survey maps. In 1923, Toronto Forestry University began to study the determination of forest height and crown width from aerial photographs. In 1929, Canada began to use aerial photography in forest regulation and management planning. In 1933, the United States conducted its first study on tree species identification. In the same year, Sweden tested the interpretation method of accumulation volume. During World War II, aerial photography and interpretation technology developed rapidly. In the 1950s, the preparation of aeronautical accumulation tables, the application of infrared films, and aerial photo sampling technology were applied to forest surveys. In 1962, low-altitude ultra-large-scale sampling photography was carried out. In the 1970s, mid-to-high-altitude ultra-small-scale aerial photos and the development of aerospace remote sensing technology. Push aerial photo forest survey technology to a multi-level application stage.

China's forestry aerial photography began in 1936. The first forest aerial photogrammetry team was established in 1952 to complete the aerial survey of the large forest areas in Northeast China. In 1954, we began to use elephant photos to conduct forest manager surveys, compile forestry pictures, and use aerial photos to conduct aerial surveys. Stratified sampling of test images in 1962. In the 1980s, major forest areas across the country entered the third generation of aerial photography, and aerial photographic application technology has become an important means of forest survey.

Photographic Requirements

The forest survey requirements for aerial photos are as follows: ①Photographic scale: It affects the resolution of the photo and determines the amount of information recorded in the image. The forest survey application image scale and resolution are as shown in the table. Forest surveys that focus on measurement and interpretation require that the image scale be greater than 1:15000 in order to obtain quantitative data on forest stand factors. Aerial photographs larger than 1:25000 should be used for the outline and qualitative interpretation of zoned forest land. The scale of aerial photos generally used in forest surveys is 1:10000~1:25000. ②Photography time: Special aerial photography is required to be completed one year before the survey. Photos are required to reflect the current situation. ③Types of aerial photos: For forest surveys that mainly focus on zoning forest land classification, it is better to use infrared black and white films and color infrared films. For drawings and measurements, full-color black and white films should be used. ④ Coverage: Coverage is generally required to be based on mapping specifications and standards. Only in large-area forest inventory work that does not require mapping, strip or point sampling photography can be organized according to the sampling plan. ⑤ Magnification of elephant films: Forest survey work requires mapping and annotation on elephant films. Small-scale elephant films below 1:25000 should be magnified by 1 time to improve the accuracy of interpretation of zoning. Another function of enlarging the image is to approach the image scale and improve the rendering efficiency and accuracy.

Aerial photo forest survey methods

According to the actual role of aerial photos, they can be divided into four categories: ①Aerial photo interpretation methods of forest resources: mainly based on aerial photo interpretation methods, combined with a small amount of ground Check, correct or quickly observe methods to collect forest resource information. For example, aerial surveys using aerial photos, methods for estimating the sampling area of ??aerial photos, etc. ②Aerial photo forest area division and interpretation combined with ground sampling method: that is, the qualitative interpretation of aerial photo, mainly zoning and classification, combined with the quantitative survey method of ground samples. Such as stratified sampling of aerial photos. ③Quantitative interpretation of aerial photographs combined with ground sampling methods: This method fully utilizes the measurement and forest interpretation potential of aerial photographs. Such as aerial photo double sampling, multi-stage sampling and other methods. ④Aerial photo contour interpretation combined with ground comprehensive survey: Aerial photos are used mainly for subdivision zoning and mapping, and subclass factors are obtained from ground comprehensive surveys. Aerial photos are also used to design survey routes and determine survey points. Such as full and small flight film forest manager survey.

Development Trend

①Aerial forest survey has become the information collection subsystem of modern forest resources information system. Images processed from aerial photos, forestry maps, etc. are recorded into the computer database through graphic digitizers and image scanning input devices, and combined with survey data to form a complete resource database.

It updates data in a timely manner, accumulates and analyzes information, and clarifies the direction of information collection, making periodic aerial forest surveys more effective. ②Aerial forest survey has developed from a single forest stock volume to a resource survey system. Multi-objective optimal interpretation analysis and sampling estimation schemes have become important topics in current research. ③Aerial photo surveys of forest resources have evolved from current situation surveys to resource monitoring, measuring dynamic changes in land use and accumulated resources within a certain period. Aerial photos are objective records of resources and play a huge role in monitoring the dynamics of land species. ④ Forest interpretation from aerial photos is an important means of obtaining information for forest surveys. The digitization of images and the development of human-machine image analysis technology will promote the development of aerial photo forest interpretation technology towards automated analysis. ⑤ Multi-level sampling surveys combining aerial images with satellite images are the future development direction of forest survey technology. For example, the application of satellite images, high-altitude photography color infrared films, combined with larger-scale aerial sample photography, and the use of PPS sampling to estimate large areas. area forest resources, has proven to be a highly efficient method of forest resource inventory.