Characteristics of frame-to-frame photographic photos

Common aerial photos are mostly in frame format, which are image photos obtained by aerial photography to reflect the characteristics of the ground. Aerial photography refers to the whole process of optically imaging the ground with an aerial camera installed on the aerial platform, directly recording the electromagnetic wave of 0.3- 1.3μm band reflected by ground objects with photosensitive film, and obtaining photos. Modern remote sensing technology has entered the space age, and the above concept has been extended to include all kinds of frame photos obtained by optical photography of the earth, the moon and other planets in the solar system from outer space.

A vertical aerial photograph whose angle between the main optical axis of an aerial camera and the vertical line is less than 3 degrees is called a horizontal aerial photograph; An inclined aerial photograph with an included angle greater than 3 degrees is obtained as an inclined aerial photograph. According to the working band and the film used, aerial photos can be divided into full-color black and white, natural color, infrared black and white, infrared color, multi-band aerial photos and so on.

(2) The ground coverage of frame aerial photos overlaps with the images.

Aerial photography mainly provides basic data for topographic mapping and resource and environment investigation, and needs to cover the investigation area, so aerial photography for this purpose is called regional aerial photography. As shown in Figure 32-9, regional aerial photography consists of many parallel linear routes. In order to ensure the continuous coverage and stereoscopic observation of image pairs, some images need to overlap between adjacent photos, and the overlapping rate is required to reach 60% or not less than 53%. Two adjacent photos with this overlapping relationship are called stereo image pairs. The image overlap between two adjacent routes is called lateral overlap, and the overlap rate is usually 20%-30%. The terrain is undulating, and the overlapping rate should be improved accordingly.

(3) Spatial characteristics of framed aerial photographs.

1. Projection properties and scale

Frame aerial photograph is the central projection of the ground. Due to the ups and downs of the ground and the inclination of the photos, the proportions of the images in the photos will be inconsistent. The horizontal aerial photography on the flat land shows that the image scale is consistent everywhere, regardless of the length of the line segment direction, it is1/m = f/h. When the height is constant, the longer the focal length, the larger the image scale and the smaller the ground coverage (Figure 3-30). If the focal length is fixed, the higher the flying height, the smaller the image scale and the larger the ground coverage (Figure 3-3 1). In areas with large topographic relief, because the relative elevation of each image point is inconsistent, the scale of ground object images with different elevations is different (Figure 3-32). The greater the height difference, the greater the relative height difference and the greater the scale difference. Only ground objects with the same elevation have the same image scale. Therefore, the scale of aerial photography in hilly areas can only be roughly expressed. The height provided by the aerial photography technical appraisal is the height of the bottom point of the image recorded by the aerial altimeter, and the scale calculated from this height is called the main scale, which usually represents the photo scale of main scale.

Figure 3-29 Ground coverage of regional aerial photography

Figure 3-30 Influence of focal length on ground coverage

Figure 3-3 1 Influence of Height on Ground Coverage

2. Image point displacement and image distortion caused by topographic relief.

According to the principle of central projection, any image point projected on a horizontal photograph by a ground point higher or lower than the datum plane has a position offset relative to the image point vertically projected on the datum plane due to topographic relief. Because of the central projection, the coordinates of points with the same plane coordinates but different ground elevations are different on the image plane. This movement of image point position is called image point displacement (projection difference) (Figure 3-33).

Figure 3-32 Influence of Terrain Fluctuation on Photo Scale

Figure 3-33 Image Point Displacement Caused by Terrain Fluctuation

As shown in Figure 3-33, T0 is the datum plane (the horizontal plane where the underground point N is located), point A is higher than T0, and the elevation difference is Δ h, a0 is the vertical projection of a on T0, A and A0 are the image points of A and A0 in the photo, and the line segment aa0 is the image point displacement (Δ h) of the elevation difference between point A and T0 on the image plane; Similarly, bb0 is the image point displacement (-Δ h) of point B below T0 on the image plane. According to the ratio of similar triangles to the corresponding edge, the calculation formula of image point displacement (hδ) is derived: δ h = △ h r/h, where r is the distance from the image point to the image base (principal) point, which is called radial diameter; H is altitude; Δ h is the height difference between the grounding point and T0, with "+"above T0 and "-"below T0.

According to the above formula, the law of image point displacement is as follows: ① Δ h is proportional to R. The farther the image point is from the bottom (main) point of the image, the greater the displacement of the image point, and the smaller the displacement of the image point in the central part of the image frame. The bottom (main) point is the only point without image point displacement. ② δ is proportional to H△ H. The greater the height difference, the greater the displacement of the image point. The displacement of the image point occurs on the radiation line centered on the image base (principal point), that is, the connecting line between the image point and the image base (principal point). △h is positive and positive, and the image point moves outward away from the image base (principal point) (a0→a). △h is negative when it is negative, and the image point is negative. ③ Δ h is inversely proportional to h, and the higher the flying height, the smaller the displacement of the image point.

3. Spatial resolution

The resolution of aerial photos is generally 25 100 line pairs /mm, and the ground resolution is related to the image resolution and scale. The relationship between them is as follows:

Remote sensing geology

For example, an aerial photograph of 1:50000 has an image resolution of 40 line pairs /mm and a ground resolution of 50000/40×1000 =1.25 (m).

4. Stereoscopic observation

Stereoscopic observation of remote sensing images is an important means of visual interpretation. When the stereoscopic observation conditions are satisfied, the two-dimensional image can be transformed into a three-dimensional optical model, thus highlighting the spatial characteristics of ground objects and facilitating human eyes to identify ground objects and determine their spatial positions.

People's eyes have natural stereoscopic vision. As shown in Figure 3-34, put a transparent glass sheet P 1 and P2 in front of your eyes, and you will see the stereoscopic scenery behind you through P 1 and P2. Assuming that the observed scene image can be kept on P 1 and P2 (like a photo), and then the scene is removed, according to the reversibility of light, the spatial shape of the scene will still be seen, but this time it is not a stereoscopic image of the actual scene, but an artificial stereoscopic-artificially simulated optical stereoscopic model. This three-dimensional model can be observed by observing the three-dimensional pairs of aerial photos such as P 1 P2 with stereoscope and other instruments.

Figure 3-34 Principle of Artificial Stereo Vision

(4) Spectral characteristics of aerial photographs

Various aerial photographs reflect the electromagnetic wave information of 0.3- 1.3μm band reflected by ground objects with hue or color and their combination morphological characteristics. Therefore, the hue or color of the image is the expression of the spectral characteristics of ground objects, and it is also an important interpretation sign for spectral recognition of ground objects.

Tone refers to the black-and-white depth of the image on the black-and-white photo, which is a record of the photochemical reaction between the electromagnetic wave reflected by the ground object and the photosensitive film. The reflection spectrum characteristics of different ground objects are different, and the colors appearing in the photos are also different. Generally, the objects with high reflectivity in the film photosensitive belt or multi-band corresponding channels have light tones; Objects with low reflectivity have a deep hue, that is, the hue of the ground image is related to the color sensitivity of the film. Tone difference is represented by gray scale (or gray scale). From white to black, it is divided into white, gray, light gray, light gray, gray, dark gray, dark gray, light black, light black and black.

Black-and-white panchromatic photograph, the color tone of achromatic object image is the same as or close to the real color of the object; There is a certain correspondence between the hue of the color object image and the primary color of the object (Table 3-3).

Table 3-3 correspondence between primary colors of color objects and image tones in black-and-white panchromatic photos

(According to Zhu, 198 1)

The hue of black-and-white infrared photos depends on the reflection intensity of ground objects to near-infrared waves, and has nothing to do with the perception of objects by human eyes. Healthy plants, especially broad-leaved trees, have strong reflections on near-infrared waves and bright colors. Due to the strong absorption of near-infrared waves, the water body is dark (black).

The hue of multi-band black-and-white photos mainly depends on the reflection intensity of ground objects on electromagnetic waves in corresponding bands of each channel of multi-band aerial camera. This is especially important for colored objects. For example, in the channel of 0.6-0.7μm, reddish-brown soil or rock mainly reflects orange-red light of 0.6-0.7μm, and the image is light tone; However, the light reflection of plants in this band is very weak and the image is dark.

Natural color photos record the visible light selectively reflected by ground objects, and the image color is basically the same as the primary color of ground objects, so they are also called true color photos. The image is rich in color, strong in three-dimensional sense, intuitive and realistic, and objects of different colors can be seen at a glance.

The image color of color infrared photos is symbolic. According to its film structure, blue light is not recorded, while green light, red light and near-infrared wave reflected by ground objects are recorded as blue, green and red respectively (Figure 3- 10), so it is a false color photo, and the image color is different from the primary color of ground objects (plate 18). For example, green plants that reflect green light and strongly reflect near infrared waves have magenta infrared images. See Table 3-4 for the comparison between color infrared image and primary colors of ground objects. Compared with natural color images, it is less influenced by the atmosphere, with higher color saturation, brighter colors and clearer levels, because the spectrum of ground objects recorded by it moves in the long wave direction.

Table 3-4 Comparison between Color of Color Infrared Image and Geological Color

(According to Li Yongyi 199 1)

(5) Image quality evaluation of aerial photos.

The aerial photos used for geological interpretation should not only evaluate the required heading overlap of aerial photogrammetry not less than 53%, the lateral overlap not less than 15%, the inclination angle of the photos less than 3, the yaw angle less than 6, and the straight route, but also evaluate the image quality. Black-and-white aerial photos should have clear images, moderate blackness, normal contrast, rich tone levels, uniform tone, no black spots and clouds, and no scars. Color aerial photos should have clear color difference, normal color difference, obvious brightness change of various parts of ground objects, rich colors and high saturation. In addition, it should also be able to identify images unrelated to ground objects, such as dendritic patterns generated by electrostatic radiation, fingerprints, bubbles generated by development, white spots generated by dust, and uneven blackness caused by improper fixing. The inspection and evaluation of image quality is usually based on experience or with the help of standard photos.