What is the cause of parallax?

Aerial photographs are the central projection of the ground. According to the principle of central projection, the following research is to understand the causes of parallax, whether it is undulating terrain or any object above the ground. Let's have a look. (1) The image point displacement caused by the tilt of the photo assumes that two photos are taken from the same camera station, one is the tilt camera P and the other is the horizontal camera P. The coordinate system of the two cameras is established with the isocenter as the origin and the main longitudinal line as the Y axis, and the coordinates of the points A and A in the same place on the two photos are (xc, yc) respectively. The momentum of the image point indicates that there are quadrangles around the image; On the contrary, the square image in the photo corresponds to the scene on the ground, not necessarily a square. Generally speaking, the displacement of image points caused by photo tilt is not calculated point by point, but eliminated by correction. For tilt cameras in flat areas, the tilt displacement a of each image point can be eliminated at one time through correction. (2) The image point displacement caused by topographic relief, because the ground is always undulating, and the ground point is higher or lower than the datum, so the linear transfer generated in the photo is called projection difference, which is expressed by H, and the projection difference H is related to topographic relief and the tilt of the photo. According to the coincidence, all vertical lines on the ground converge back to the base number n of the image, and the direction of projection difference is on the image and the radiation line of the image. The tilt angle of aerial camera lens related to surveying and mapping or resource investigation is generally very small, and it is controlled within 3. Therefore, the difference in projection between horizontal photography and vertical photography is mainly the influence of height difference. The above formula is a formula for calculating the displacement of image points caused by topographic relief in photos. Where r is the image distance of point A centered on the image bottom point n, h is the photographic height, and r is the horizontal distance from the ground starting point to the underground point. The above displacement of image points also leads to the change of photo scale and the deformation of graphics. Moreover, because the bottom point of the photo is not on the isochromatic line, considering the influence of the tilt of the photo and the terrain fluctuation, any point in the photo has the displacement of the image point, and the magnitude of the displacement varies with the position of the image point, which leads to the unequal scale of different points in a photo. (1) Projection difference correction refers to the correction from image point A containing projection difference to a0 correction. Correction direction: h0, correction from image point A to bottom point, H0; Image point bottoming point correction, h0. The correction of projection difference in drawing printing refers to the correction from a0 point to bottom point on the drawing. Correction direction: h0, the direction of a0 point on the book facing away from the bottom point is corrected, H0; The a0 point on the drawing is corrected to the bottom point, the a0 point on the book is corrected to the bottom point, and the a0 point on the drawing is corrected to the bottom point direction.