What's the difference between aerial photos and topographic maps?

The difference of expression methods: aerial photos are more intuitive and concrete, and topographic maps have high geometric accuracy, but they are not intuitive and concrete.

The difference of projection methods: aerial photography is the central projection of the ground landscape, and it is a ground perspective image formed by the reflection of ground objects through the center of the lens; Topographic map is the orthographic projection of the ground landscape, and each scale is the same.

Difference of scale: the scale of aerial photos is restricted by altitude, focal length and other factors; The most basic point in using topographic map scale is to convert the central projection of aerial photos into orthographic projection, which needs to be corrected and drawn.

Characteristics and technical requirements of aerial photographs

The size of an aerial photograph is called a picture frame. According to different aerial cameras, we can get aerial photos with different frames, such as 18cm× 18cm, 23cm×23cm and 30cm×30cm. For the convenience of use, the edges of aerial photographs are usually printed with various photographic parameters and annotation symbols.

Aerial photography belongs to central projection photography. Because of the tilt error and projection error in the photo, the proportion of each part of the photo is inconsistent. The photo scale usually refers to the average scale. When the focal length of the camera is fixed, the scale of aerial photography mainly depends on the altitude, and the larger the relative altitude, the smaller the scale of aerial photography.

The technical requirements for aerial photos are: the tilt angle of the photos should be less than 3, and the deviation from the predetermined height should not be greater than 5%; the difference between the highest and lowest heights on a route should not exceed 50m; the heading overlap should be 60% ~ 53%, and the lateral overlap should be 30% ~ 18%, not less than 15%. The flight straightness of the route and the flatness of the film have specific requirements.

The quality evaluation of aerial photos requires clear images, moderate gray scale, normal contrast, accurate exposure time and development time, uniform color tone at the center and edge of the photos, proper selection of color filters, and no phenomena such as clouds, shadows, damage and scratches that will affect observation and use.