A graphical representation of the vertical errors in the aerotriangulation of a strip of photographs. The x-coordinates of the vertical control points, referred to the initial nadir point as origin, are plotted along the axis of abscissas and the differences between the known elevations of the control points and their elevations as determined by aerotriangulation are plotted along the axis of ordinates. The Bz curve is a smooth curve drawn through the plotted points. The elevation of any pass point in the strip is adjusted by the amount of the ordinate of the Bz curve for the corresponding x-coordinate of the point.