Evaluation of Automatic Contouring Methods for Drainage Design

ABSTRACT PRODUCTION of a contour map from spot elevation data is often an essential step in the preparation of plans for subsurface drainage systems, land levelling projects and other agricultural developments. Contour drawing in the agricultural industries is usually done by drastsmen. It is a routine activity which is time-consuming and expensive. In other, non-agricultural ac-tivities, such as geography and geophysics, very large quantities of contour maps have been produced by means of processing elevation data by computers and computer-steered plotters. Program packages available from commercial, institu-tional and governmental sources are usually oriented to geographical, geological and geophysical uses. To draw contours by digital plotter in a sufficiently economical manner for agricultural purposes, it was necessary to write a new contouring program package specifically oriented towards small areas and data sets. The calculation of the contour locations is usually a two-step process: gridding and contour drawing. Three gridding and one contour drawing method were im-plemented in the programs. Two of the gridding methods (weighted average and finite difference) were found to be suitable. The gridding methods were adaptations of well-known schemes; the contour-drawing method was of novel design. The contouring package was tested by using it to generate contour maps from spot elevation data of four real and three mathematical surfaces. A novel contour-ing testing method was developed to evaluate the ac-curacy of contour placement of a human or a machine relative to the location of ideal contours. The performance of the package was evaluated according to three criteria: map quality, contour ac-curacy and map cost as compared to human perform-ance. The contouring package was found to produce maps of better quality and equivalent accuracy when compared to four experienced draftsmen. The cost of computer contouring was also found to be lower than human contouring and potentially could be 50 to 75 per-cent cheaper under commercial production conditions.