Measurement method of aerial photogrammetry

Since 1930s, there are three main mapping methods in aerial photogrammetry, namely, synthesis method, all-around method and block method (or difference method).

Aerial photogrammetry synthesis method is a mapping method combining photogrammetry with flat panel measurement. On the topographic map, the plane position of ground objects and landforms is obtained through photo correction, and the elevation and contour lines of topographic points are measured on the spot by ordinary measurement methods. Suitable for large-scale mapping in flat areas.

The universal method of aerial photogrammetry is based on the principle of geometric inversion in the process of photography, establishing the geometric model of the ground in the stereo plotter and drawing the topographic map (Figure 4). When placing photos on the stereo mapper, the purpose is to make the restored projection beam similar to the photographic beam (the projection beam can also be changed under certain conditions). Because no control points are added in the process of relative orientation of image pairs, only the inherent geometric characteristics of image pairs are used. The orientation of geometric model is arbitrary and the scale of model is approximate, so the map can only be measured by absolute orientation.

Universal mapping instrument is a three-dimensional mapping instrument. There are many forms of such instruments, which can be divided into three types according to the structure of the projection system: ① establishing the optical projection type of the actual projection beam; (2) The actual projected light is from the projection center to the image point, and the optical mechanical projection is replaced by the directional guide rod from the projection center to the model point; (3) Instead of the mechanical projection type of the floodlight, a directional rod penetrating through three universal joints (which respectively represent the image point, the projection center and the model point) is used. The first two types of instruments have been basically eliminated now. The structure of stereo plotter must have several main parts, such as projection system, observation (observation and measurement) system and drawing system. Relative orientation and absolute orientation are realized by the angular motion of two projectors (a few instruments also move linearly) and the placement of marks on the mark frame. After orientation, we can use the measuring points on the instrument to draw the ground features and landforms on the three-dimensional model of the ground through three-dimensional observation. Some instruments can also process photos taken on the ground, while others can triangulate in the air. Since the advent of 1930, the stereo mapping instrument reached its peak in the 1960s. After that, the peripheral equipment of the instrument was mainly developed, such as electronic drawing table, orthographic projection device (see orthographic technology) and coordinate recording device. The electronic drawing table has many functions and can automatically draw some contents.

The division method of aerial photogrammetry (difference method) is a drawing method based on the principle of plane and elevation division. The main instrument used is stereo measuring instrument. It is an instrument to measure the left and right parallax and draw contour lines on the right photo according to the vertical photographic image pair. The difference of abscissa x of a ground point in the left and right photos is called left and right parallax p, while the difference of left and right parallax of two ground points is called left and right parallax comparison Δ p, which is caused by the height difference between two points. In the process of measuring the left-right parallax ratio Δ p, with the help of the correction components on the instrument, the influence brought by the external orientation elements of photography is automatically corrected to make it equal to the left-right parallax or the left-right parallax ratio of the ideal image pair; The elevation difference is calculated by the elevation difference formula; Then, the contour lines outlined in the photo and the ground objects drawn are projected by a projection plotter and drawn into topographic maps in different areas. The X-2 parallax mapping instrument designed and manufactured by our country is based on the stereo measuring instrument, and the plane correction part is added. The improved instrument can solve two steps in division of labor drawing at one time, which improves the work efficiency. Italy and the Federal Republic of Germany have similar instruments.

The mapping methods and instruments of aerial photogrammetry are developing towards semi-automation and automation, and the analytical mapping instruments have made considerable achievements in this respect.