Basic knowledge of 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. Let me introduce some basic knowledge of aerial photogrammetry to you, hoping to help you!

First, aerial photography

Definition: Aerial photography is the use of aircraft or other aircraft (such as balloons, satellites, spacecraft, etc.), on which special cameras are loaded to shoot the ground, among which aerial photography by aircraft is called aerial photography. Aerial photography has the following advantages:

(1) can be observed from the commanding heights;

(2) Aerial photos can objectively record the observed ground objects at the same time;

(3) recording dynamic phenomena;

(4) Aerial photography is a permanent record of the current situation, and there is enough time to study it carefully, so you can move the field to indoor discussion;

(5) Improve the spatial resolution.

1, photography mode

According to the orientation of the main optical axis of the camera lens, the photography methods are divided into vertical photography and oblique photography. Photography in which the main optical axis of the lens is in a vertical position is called vertical photography. In fact, the perpendicularity of the camera's main optical axis is difficult to control, and it often includes a tiny tilt angle. As long as the inclination angle is less than 2 degrees, it is called vertical photography. When the main optical axis of the lens deviates from the vertical position by more than 2 degrees, it is called oblique photography.

2. Requirements for aerial photographs

(1) The image is clear, with even tone and moderate contrast.

(2) Two adjacent photos on a route should have certain overlapping images, which generally require 55%-65% overlapping. Image overlap between adjacent routes is called lateral overlap, which requires about 30% overlap.

(3) The tilt angle of aerial photos should be as small as possible, generally not more than 2 degrees, and the maximum tilt angle of individuals should not exceed 3 degrees.

(4) The ratio of the maximum deviation value of route bending to the total length of the route is not more than 3%.

3, the proportion of photos

The ratio of the distance between two points in a photo to the horizontal distance between two corresponding points on the ground is called the photo scale. Usually expressed as:

-the focal length of the photographic lens; * —— The height of the lens center relative to the ground is called relative height.

Due to the comprehensive influence of various factors, it is impossible to keep the same height all the time when the plane is snake-shaped, and the ground is always fluctuating and the height is inconsistent, so the proportion of each part in the photo is also inconsistent.

Second, the image point displacement caused by ground fluctuation

Vertical objects above the ground, such as chimneys, water towers, telephone poles, etc., are located at a point on the topographic map, but the image in aerial photography is often not a point, but a short line segment. Similarly, when the ground point is higher or lower than the datum, the image in the photo is a point, but compared with the image projected vertically on the datum, it produces a linear displacement, which is called projection error. Generally speaking, the average height of the ground in the survey area is called the starting surface of altitude, that is, the datum plane. Projection error is divided into image point displacement caused by terrain fluctuation, which is called photo projection difference. Verification should be divided into the verification of ground projection difference on the ground.

The law of image point displacement caused by topographic relief;

(1) The projection error caused by the ground fluctuation is on the connecting line between the image point and the image bottom point;

(2) The projection error is proportional to the distance from the image point to the image bottom point;

(3) The bottom point of the image does not produce projection error;

(4) The greater the ground undulation, the greater the projection error;

(5) The higher the flying height, the smaller the projection error.