Characteristics of aerial photographs

Aerial photography is the whole process of taking airplanes or balloons as remote sensing platforms, using aerial cameras carried in the air to reach the ground, collecting and processing target information through photography, and obtaining various images and data. The sensors used in photography mainly include aerial cameras and multispectral cameras. Photographs obtained by photography have the characteristics of large amount of information and high resolution, and can also obtain spectral radiation of ground objects from visible light to near infrared. However, due to the limitation of emulsion, only near ultraviolet, visible light and near infrared spectral information with wavelength of 0 can be obtained. 3 ~ 1.3 can obtain micron, and imaging can only be carried out in daytime.

(1) Type of photo

According to the relationship between the main optical axis of aerial camera and the vertical line, aerial photography can be divided into vertical photography and oblique photography. Vertical photography means that the main optical axis of the aerial camera is kept in the vertical direction, and the maximum included angle with the vertical line does not exceed 3, and the obtained photos are called horizontal aerial photos; If the included angle exceeds 3, it is oblique photography, and the obtained aerial photograph is called oblique aerial photograph.

According to the task and purpose, aerial photography can be divided into single photo photography, strip aerial photography and regional aerial photography.

According to the height of aerial remote sensing platform, it can be divided into: aerial photography (platform height above 9km); Aerial photography (height 6 ~ 9 km); Low-altitude photography (height less than 6 km). Low-altitude and high-altitude remote sensing, with small imaging scale and large coverage, is suitable for large-scale census; Low-altitude aerial remote sensing can obtain a wide range of images and is the most widely used remote sensing means at present.

According to the different photosensitive materials, it can be divided into full-color black-and-white photography, black-and-white infrared photography, color photography, color infrared photography and multi-spectral photography.

(b) Ground coverage and image overlap

In order to ensure the continuous coverage and stereoscopic observation of aerial photography, some images overlap between adjacent photos (Figure 3-29), and they overlap symmetrically along the route direction, and the overlap 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 fluctuates greatly, and the overlapping rate should be improved accordingly.

Figure 3-29 Ground coverage of aerial photography

(3) Spatial characteristics

1. Projection properties and scale

Aerial photographs are the central projection of the ground. Due to the ups and downs of the ground and the tilt 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). When the focal length is fixed, the higher the altitude, the smaller the image scale and the larger the ground coverage scale (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 altitude provided by the aerial photography technical appraisal book is the altitude of the bottom point of the image recorded by the aerial altimeter, and the scale calculated from this altitude is main scale, usually the scale of main scale's representative photos.

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).

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 a a0 is the image point displacement (Δ h) of the elevation difference between point A and T0 on the image plane. Similarly, b b0 is the image point displacement (-Δ) of point B on the image plane that is lower than T0. According to the proportion of the corresponding edge of similar triangles, the calculation formula of the image point displacement (δh) is derived:

Figure 3-30 Influence of focal length on ground coverage

Figure 3-3 1 Influence of Height on Ground Coverage

Figure 3-32 Influence of Terrain Fluctuation on Photo Scale (According to Zhu, 198 1)

δH =δH r/H

Where: r is the radial distance from the image point to the image bottom (principal point), h is the flying height, Δ h is the height difference from the ground point to T0,+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, and the farther the image point is from the image base (principal point), the greater the image point displacement, and the smaller the image point displacement in the central part of the image frame. At the image base (principal point), r = 0, which is the only point without image point displacement; ② Δ H is proportional to Δ H. The greater the height difference, the greater the displacement of the image point, which occurs on the radiation line centered on the image base (principal point), that is, the connection line between the image point and the image base (principal point). When Δ h is positive, the image point moves outward away from the image base (principal point) (from a0 to A). When Δ h is negative, Δ h ③ Δ h is inversely proportional to H. The higher the flying height, the smaller the displacement of the image point.

Figure 3-33 Image Point Displacement Caused by Terrain Fluctuation

3. Spatial resolution

The resolution of aerial photography refers to the ability to distinguish two adjacent objects in a photo, which is usually divided into image resolution and ground resolution. Image resolution refers to the number of lines that a photo or negative can distinguish within a distance of1mm. Image resolution is restricted by the quality of imaging system (resolution) and the quality of photosensitive materials. Ground resolution refers to the size of the smallest object on the ground that can be observed after the image data obtained in the air at a certain height from the ground is amplified by aerial photographic instruments (lens group) or other electronic instruments. 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).

The resolution of aerial photos is not only affected by the lens and photosensitive materials, but also by the shape of objects, the contrast between objects, lighting conditions, photography and the development technology of photosensitive films.

(4) Spectral characteristics of aerial photographs

All kinds of aerial photos reflect the information of electromagnetic wave reflected by ground objects by hue or color and their combination morphological characteristics, so the hue or color of the image is the display of spectral characteristics of ground objects reflection and an important interpretation sign for identifying ground objects from the spectrum.

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, objects with high reflectivity, objects with light color tone and objects with low reflectivity have deep color tone, that is, the depth of color tone of the ground object image is related to the 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.

In black-and-white panchromatic photos, the achromatic color of the object image is the same as or close to the real color of the object, and the color of the colored object image has a certain correspondence with the original color of the object (Table 3- 18).

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 a strong reflection of near-infrared waves and show bright light color, while water bodies show dark (black) color because of their strong absorption of near-infrared waves.

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, at 0. 6 and 0. 7 microns, chestnut soil or rock mainly reflects 0. Six to zero. 7 microns, the image is light, but 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 and strong in three-dimensional sense, and objects with different colors can be seen at a glance.

Table 3- 18 Correspondence between primary colors of colored ground objects and image tones in black-and-white panchromatic photos

The image color of color infrared photos is symbolic. According to its film structure, it does not record blue light, but records the green light, red light and near infrared wave reflected by the ground object as blue, green and red respectively, so it is a false color photo, and the image color is different from the primary color of the ground object. For example, green plants that reflect green light and strongly reflect near infrared waves have magenta infrared images. See table 3- 19 for the comparison between color infrared images 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- 19 Comparison of color of color infrared image and color of ground objects