A mathematical model of images that make up a stereo pair is presented. The model links the coordinates of points of images showing some scene (usually terrain) from different angles with the distribution of brightness caused by simultaneous photographic recording of all points of the scene. The paper gives a mathematical relationship of three objects, each being a function of two variables (the distribution function of brightness on the left and right stereo pair images and the distribution function of the relief of the terrain), and states that for any pair of objects selected from these three the third object can be constructed. We propose a piecewise linear approximation of the relief in the basal plane, we show how the task of building the relief in the form of a function of two variables is reduced to a set of tasks of constructing a function of one variable and we explain how to find the parameters of the relief. The above procedure is universal and can be used for restoring a nonlinearly defined funct