This is a specific technique designed to facilitate conversion of vector surface data, in the form of contours, to hydrologically consistent DEM format. It is provided within ArcGIS and based directly on the work of Hutchinson (1988, 1989) and his ANUDEM (Australian National University DEM) program implementation. Surface interpolation uses a form of thin plate spline that is discretised to allow for discontinuities in the modeled surfaces (ridge lines, cliffs etc.). In order to ensure consistency in the drainage model it pre-fills any pits that it considers spurious (see further, Section 6.2.6, Pit filling and Section 6.4, Watersheds and Drainage). Hutchinson describes ANUDEM as follows:
"ANUDEM ensures good shape and drainage structure in the calculated DEMs in five main ways by:
|•||Imposing a drainage enforcement condition on the fitted grid values that automatically removes spurious sinks or pits. This eliminates one of the main weaknesses of elevation grids produced by general purpose interpolation techniques. It greatly improves the utility of the DEM for hydrological applications. It can also aid in the efficient detection of data errors|
|•||Incorporating surface drainage constraints directly from input streamline data|
|•||Delineating ridges and streams automatically from input contour line data. This is achieved by inserting curvilinear ridge and streamlines associated with corners of contour lines that indicate where these lines cross the elevation contours|
|•||Breaking the continuity of the DEM over data cliff lines|
|•||Ensuring compatibility of lake boundaries with the elevations of connecting streamlines and neighboring DEM points|
The drainage enforcement algorithm is one of the principal innovations of ANUDEM. It has been found in practice to be a powerful condition that can significantly increase the accuracy, especially in terms of drainage properties, of digital elevation models interpolated from both sparse and dense elevation data… The essence of the drainage enforcement algorithm is to find for each sink point the lowest adjacent saddle point that leads to a lower data point, sink or edge and enforcing a descending chain condition from the sink, via the intervening saddle, to the lower data point, sink or edge (Hutchinson 1989). This action is not executed if a conflicting elevation data point has been allocated to the saddle. The action of the drainage enforcement algorithm is modified by the systematic application of two user supplied elevation tolerances."
For further details refer to the ANUDEM website.