Accuracy considerations in creating a topographic data base

Accuracy constraints are of primary importance in developing a strategy for creating a National/Regional (DMDB) Digital Mapping Data Base. Other major considerations are cost benefit analysis and the current and future status of the data base components. Three sets of input sources were available for comparison within two areas, each one 5×5 km, in size: (a) Aerial photography at 1:40,000; (b) Aerial photography at 1:70,000; (c) The existing 1:50,000 - scale map color separations. The obtained data were compared with separately established control and with one another. In addition, data capture from air-photographs through photogrammetric stereoplotters, were analyzed. This combined with analyses of geodetic control and aerial triangulation accuracies was implemented to define the projected accuracy potential of the various mapping products. The cost benefit analysis included work time invested in various procedures involved. The paper presents the result of these investigations which became a basis for policy formulation. The major conclusion arrived at is that the National Digital Mapping Data Base should be based on newly captured data, organized in pre-specified coding system permitting the derivation of graphical output of known accuracy for different purposes. The newly created NDMDB will serve both future production of map series at the Survey and as a national source of digital mapping data for spatial analyses systems.