ON THE ACCURACY OF HEIGHTING FROM AERIAL PHOTOGRAPHS AND MAPS - IMPLICATIONS TO PROCESS MODELERS

A concern to all scientists engaged in modelling landform must be the accuracy with which heights can be obtained from aerial photographs an d their derivatives, conventional topographic maps. Morphological info rmation is of importance for terrain description and analysis and incr easingly as a critical boundary condition for models of geomorphologic al processes. This paper offers some cautionary advice in the use of a erial photographs and maps to provide morphological information. Curre nt mapping specifications are reviewed and the prospects for improved accuracy in heighting, given new equipment such as cameras equipped wi th forward motion compensation, are analysed in the light of recent pu blished material. An analysis is presented of the manner in which rand om errors in photogrammetric observations can be propagated through a block of aerial photographs. A further analysis is made with the addit ion of a small systematic error of a type and magnitude occasionally e ncountered in practice. These analyses show that the use of published map data (digital or not) for producing landform models should be made only after careful assessment of the accuracy of those data and that some of the claims currently being made for height accuracy obtained w ith new photogrammetric equipment are valid only in special cases.