In the right photo, this first point is now at pp 1 ', and lies left of that photo's principal point pp 2 . In the same way, the height of point b is determined. Then the relief is just the elevation difference between the higher and lower points. Since H and f control the photo scale, these values plus the appropriate parallaxes and the air base B are sufficient to establish the actual distance from the principal point of any location in one of the photos. Similar equations hold for point b .
P is the absolute stereo parallax.
We find the differential parallax by measuring the distance between the base and the top of the feature, or between two proximate terrain points at different elevations on a slope, that we locate in each of the two photos when the pair is in optimum alignment for stereo viewing. We can optically find the values of P and dP by using special devices, such as a stereometer, a parallax wedge, or, most commonly, a parallax bar. We then read the value of dP from a graduated scale. When we do this for a number of points, then, we can calculate heights and relief values for parts of the map. The differential parallax dP for the tower, as measured by a stereometer, is 0.4 mm.