One of the important application of stereo-photographs is the determination of elevations of various points in the overlap region. For this purpose, a parallax bar, also known as a stereo-meter, is used for taking the measurements. The pre-requisite is that the elevation of at least one control point is known in the common area (like ground leveling method) before the computation of elevations of other unknown points. The details of the control points are sometimes supplied along with the aerial photographs, else some permanent point is selected and its elevation is determined either from the contour map or leveling or GPS observations.

The first step in creating a stereo-model is that the stereo-pair must be properly oriented under the stereoscope. The process of orientation is called base lining, which is performed as below: 1. On both the photographs, their respective principal point and conjugate principal point are marked, as shown in Figure 4.18a. Principal point and conjugate principal point are joined by a straight line and the line extended on each photo. This line represents the flight line. 2. Under a mirror stereoscope, two photographs are to be kept apart in the direction of flight line on a flat surface with overlap region inwards. 3. The stereo-pair is aligned in such a way that the line drawn on both the photos lies in a straight line, as shown in Figure 4.18a. 4. The stereo-pair is seen through the stereoscope so that the left lens is over the left photograph and the right lens is over the right photograph. The line joining the centre of the lens should almost be matching with the direction of the flight line. 5. The distance between the photographs may be adjusted inward or outward till the two images are fused in the brain and a 3D model of the overlap region is created. 6. In the beginning, it might appear a bit difficult to see the two images fusing together to create a stereo-vision, but with a little more practice and concentration, it will appear to be easy. 7. Once the perfect 3D model is created and the lines drawn on the photographs fall in a line, the photographs are said to be properly oriented (or base lining is completed). 8. Select the features/points on both the photographs in the overlap region whose heights are to be determined. The visual interpretation elements (size, shape, shadow, tone, texture and surrounding objects) will help identifying the objects; but now with the addition of relief, a more natural view of terrain may be seen. 9. Use parallax bar for taking the measurements of these points. The difference between two parallax bar readings will provide parallax differences between the two points.

The change in position of an image from one photo to the next due to aircraft’s motion is called stereoscopic parallax, x-parallax, or simply parallax. The parallax is directly related to the elevation of the point, and is greater for high points than for low points. The parallax bar is a device used to measure the difference of parallax between any two points on the stereo-photographs, more precisely. The parallax bar consists of a graduated metallic rod (in mm) attached with two bars, as shown in Figure 4.19. Left bar is fixed with the rod and right bar is movable with micrometer drum attached on right end of the graduated rod. A pair of glass graticules, one at each bar can be inserted into the grooves provided in the bar. Each graticule is etched with three small identical index marks (cross, dot or circle), called floating marks. The left hand bar is generally clamped at any required distance so that the entire overlap area is covered by the separation of left and right floating marks.

The absolute parallax of a point on a stereo-pair is determined as the algebraic difference between two distances which are measured from the corresponding principal points, parallel to the direction of flight (air base). It is also called x-parallax. Figure 4.20 shows a stereo-pair where locations of two points a and b are marked with their x-coordinates. Let the distance on left photo be x and x on right photo be x' and x'. The absolute parallax of a point is determined as the algebraic difference between two distances of a point, it is computed as given below. p = x - x'. The distances on the left and right photos are measured with the help of parallax scale, which is shown in Figure 4.21. This scale has a better least count that the normal scale (ruler) available for manual measurements. It has a main scale and a vernier scale and the addition of both the readings is the final distance.

Let A and B be two points whose images are a and b on left photo and a and b on right photo (Figure 4.22). The stereo images are taken from L and L with B as air base distance and H as the flying height. Let h and h be the heights of these points with respect to a datum. The parallax difference is related with the difference in parallax bar readings. The parallax p at point A is related as follows. Since the elevation of a ground point A (h_a) is known from reference map or field observation or contour map, the elevation of unknown point B can be determined (say h_b) using parallax bar measurements.