Additional processing of the data can to some extent compensate for vegetation induced surface anomalies. The height of the aircraft and focal length of the camera lens determines the scale of the diapositive. The scale limits both point height accuracy and the grid spacing. The limit of accuracy for point heights on 1 in 25,000 scale diapositives is about 0.8 metres. The minimum valid contour intervals equals the span of accuracy.
Greater accuracy requires photography from a lower height.
This would need to be flown at about 7,500 feet. Such a photo has a 3.45 Km side, which limits the model to 714 ha. Stereo Plotter quality limits what information on the diapositives can be identified and processed. Ground Control provides real information about relative heights and distances upon which to scale the 3D model. Some type of ground control of the area is necessary to produce a contour map from aerial photographs.