Why do remote sensing images produce parallax and its solutions?

Aerial photos are center projections of the ground. According to the principle of central projection, whether it is an undulating terrain or any object above the ground, the image point reflected in an aerial photo will generally move in position compared to its plane position. This image point position The movement is called image point displacement. It is mainly caused by the tilt of the image, the height difference of the ground point relative to the datum, and physical factors (such as photographic material deformation, flattening error, photographic objective lens distortion, atmospheric refraction, earth curvature, etc.).

(1) Image point displacement caused by image tilt

Suppose two photos are taken from the same camera station, one is the tilt photo P and the other is the horizontal photo For the photo P°, the two establish a coordinate system with the isocenter as the origin and the main longitudinal line as the y-axis. The coordinates of the point a and a° of the same ground point on the two photos are (xc°, yc) respectively. °). The displacement of the image point shows that the imaging sides of any square on the horizontal ground plane on the photo are quadrilaterals; conversely, the square image on the photo does not necessarily correspond to the scenery on the ground.

Square. Generally speaking, the image point shift caused by the tilt of the image is not calculated point by point, but eliminated through correction. For oblique images in flat areas, the oblique displacement δa of each image point can be eliminated at once through correction.

(2) Image point displacement caused by terrain undulations

Since the ground always has undulations, and the ground points are higher or lower than the datum, the image points projected on the photo will The linear shift is called the projection difference, represented by δh. The projection difference δh is related to the terrain relief and image tilt.

According to the nature of the conjunction, the images of all plumb lines on the ground on the image converge at the nadir n, and the direction of the projection difference is located on the radial line between the nadir and the image point.

The inclination angle of aerial photography images for mapping or resource survey is always very small, generally controlled within 3°. Therefore, the projection difference on vertical photography images is mainly the height difference. Influence. The above formula is the calculation formula for the image point displacement caused by terrain fluctuations on the photo. In the formula, r is the image distance of point a centered on the image bottom point n, H is the photographic flight height, and R is the level from the ground point to the bottom point. distance.

The above-mentioned image point displacement also causes changes in the scale of the image and deformation of the graphics. Moreover, since the bottom point of the image is not on the contour line, therefore, taking into account the influence of the inclination of the image and the relief of the terrain, the image on the image There is image point displacement at any point, and the magnitude of the displacement varies with the position of the image point, resulting in unequal scales at different points on a photo.

Correction method of projection inspection

(1) Correction on the image

Correction of the projection difference on the image refers to the correction from the image point a containing the projection difference to a0 correction. Correction direction: h>0, the image point a is corrected toward the bottom point, δh>0; h<0; the image point is corrected away from the bottom point, δh<0.

(2) Correction on the plate

Correction of the projection difference on the plate refers to the correction from point A0 to A’ on the graph without projection difference. Correction direction: h>0, point A0 on the graph is corrected away from the bottom point, δh>0; h < 0, point A0 on the graph is corrected toward the bottom point, δh<0.

Correction of projection differences only needs to be corrected on the photo or on the plate, not both at the same time.