Basic knowledge of photogrammetry

1. Definition of ground photogrammetry

The photogrammetry of the photographed object by using the photos taken on the ground refers to the technology of surveying and mapping the photographed object by shooting three-dimensional image pairs to the target with cameras placed at both ends of the ground baseline. It can be used for surveying and mapping in mountainous areas, small mountainous areas and hilly areas, and can also be used for geological, metallurgical, mining, water conservancy and railway surveys.

2. Classification of ground photogrammetry

Ground photogrammetry is divided into field work and indoor work.

Field work includes photography and measurement. Photography is to use phototheodolite or other cameras to shoot both ends of the baseline in a certain way, so as to obtain a stereo image pair of the target. In the survey work, the photographic baseline is selected first, and then the baseline length, the coordinates and elevation of the baseline endpoint and the inspection point are measured by ordinary survey methods, which provides initial data for indoor photographic processing.

Indoor drawing methods are divided into graphic method, simulation method and analytical method. Graphic method is to design a drawing board according to similar triangles relation according to the coordinates of image points and the left and right parallax values measured by stereo coordinate measuring instrument, and to calculate the plane position and elevation of ground points by graphic method. Simulation method is a method of mapping by using ground stereo mapper. Analytic method is to get the spatial coordinates of ground points in its ground auxiliary coordinate system according to a certain mathematical formula, and then convert them into ground coordinates. Analytical method is a common method with strong adaptability and high precision.

(2) Aerial photogrammetry

Aerial photogrammetry refers to the operation of continuously taking photos on the ground with aerial photographic instruments on the plane, and drawing topographic maps in combination with the steps of ground control point measurement, mapping and three-dimensional mapping.

1. The difference between aerial photos and maps

Aerial photograph is the central projection form of the ground scene, while map is the orthographic projection of the ground scene, which is two different projections. Only when the ground is strictly horizontal and the photo is strictly horizontal, the above two projection results are equivalent.

The map is drawn according to a certain proportion according to the orthogonal projection position of the ground, and its plane position is correct. When the aerial photos have an inclined angle or the ground has a height difference, the photos taken will be different from the above ideal situation. This difference is manifested in the image point displacement, including the image point displacement caused by the tilt of the photo and the image point displacement caused by the terrain fluctuation, the latter is also called projection difference. The tilt displacement and projection difference in aerial photos determine that they cannot be directly used as maps.

2. Image point displacement caused by photo tilt.

Generally speaking, the photos obtained by aerial photography are inclined. At this time, even if the ground is strictly horizontal, the target object in aerial photography will be deformed or the image point will be displaced because of the tilt of the photo. The result of this displacement is that the geometric figure in the photo and the geometric figure on the ground are deformed, and the image scale in the photo is different everywhere. Because of this difference, the central projection aerial photograph does not have the map function of orthographic projection. In photogrammetry, the displacement of image points caused by the tilt of photos can be corrected by photo correction.

3. Advantages of aerial photogrammetry

1) The aerial film completely and objectively records the instantaneous state of ground objects and landforms in the process of photography. Therefore, it has the characteristics of large amount of information, realistic shape and uniform precision.

2) A large part of aerial survey will be moved from outdoor to indoor. Therefore, a lot of manpower and material resources are saved, and the influence of weather season is also reduced.

3) Aerial survey has the characteristics of fast mapping speed, good accuracy, low cost and high efficiency.

4. Indoor and outdoor contents of aerial photogrammetry.

Aerial photogrammetry needs both outdoor work and indoor work.

Aerial survey field is to provide aerial survey field with control survey results and difference photos, including the following work: ① Joint survey of photo control points. The photo control points are generally the landmark points laid on the ground before aerial photography, or obvious feature points (such as road intersections). ) can choose, their plane coordinates and elevation can be determined by ordinary measurement methods. (2) photo adjustment. It is a general term for image interpretation, investigation, drawing and annotation. Through the interpretation of photos, draw features, landforms and other elements with prescribed topographic map symbols; Surveying and mapping without images and new important features; Pay attention to the place names obtained through investigation. The main surveying and mapping objects of field surveying and mapping include independent feature surveying and mapping, residential area surveying and mapping, road and its ancillary facilities surveying and mapping, pipeline, fence and boundary surveying and mapping, water system surveying and mapping, landform surveying and mapping, soil and vegetation surveying and mapping, place name survey and annotation.

The indoor work of aerial survey includes: ① Encryption of surveying and mapping control points. In the past, radiation triangulation was generally used in flat areas, and three-dimensional mapping instrument was used in hilly areas to establish single-line simulated aerial triangulation encryption control points. ② Using various optical and mechanical instruments and computers to measure the original topographic map.

(3) Space photogrammetry

Photogrammetry using space photographic data.

1972, the United States successfully launched the first earth resource satellite (later changed to a land satellite), marking the beginning of the era of space photogrammetry. After that, the United States launched the land satellite 1 ~ 5, the French successfully launched the SPOT satellite 1 at 1985, and China also successfully launched the geodesic satellite.

Satellite images (remote sensing images) are mainly used for mapping topographic maps and making orthophoto maps or various thematic maps. This paper briefly lists the relationship between satellite image resolution and mapping scale, as well as several common satellites and their sensors.

1. Relationship between satellite image resolution and mapping scale

See table 1- 10 for the relationship between various satellites and image scale.

Table 1- 10 Satellite resolution and mapping scale

2. Brief introduction of commonly used satellites

(1)Landsat satellite series

Landsat satellite series belongs to sun-synchronous polar-orbiting satellites, with the orbital height and inclination of 750km and 98.2 respectively, and the revisiting period is 16 days. Since 1972 launched the first Landsat satellite, NASA has launched seven Landsat series satellites, which have been continuously observing the earth for 35 years. The last Landsat-7 satellite was successfully launched in April 1999.

(2)SPOT satellite series

The French SPOT satellite series belongs to the sun synchronous quasi-regression orbit, with the orbital height and inclination of 830km and 98.7 respectively, and the revisiting period is 26 days. However, due to oblique observation, it is actually possible to observe the same area every 4-5 days. It is equipped with two high-resolution remote sensors HRV, which has the advantage of stereo observation through side view. From 1986 to 1998, France launched 1 ~ 4 stars successively. The resolution of SPOT-5 launched in May, 2002 has reached 2.5m, which has significantly improved a series of aspects such as data compression, storage and transmission.

(3) New high-resolution remote sensing satellites and sensors

At present, the common new high-resolution remote sensing satellites are: IKONOSⅱII II, Quick Bird, SPOT-5, P5, ALOS, WorldView- 1, GeoEye- 1 and so on. See table 1- 1 1 for the main sensor parameters.

Table 1- 1 1 new high-resolution remote sensing satellites and sensors

(4) domestic satellite system

At present, the main remote sensing satellites in China are CBERS-02 B China-Brazil Earth Resources Satellite, CBERS-2 Satellite, CBERS-2 Satellite, Beijing No.1 Satellite, Environment 1 HJ 1-B Satellite, Remote Sensing No.1 Satellite, Remote Sensing No.3 Satellite and Environment No.1 HJ 1-A Satellite.