The difference between carving and filling (1)
People using TopoToolbox asked me quite frequently about the difference between the two preprocessing options when calculating a flow direction object (FLOWobj): ‘carve’ and ‘fill’.
First and most important: These options do not alter the DEM, they only control how FLOWobj internally preprocesses the DEM to optimize how flow is routed through topographic depressions.
In this first post I’ll describe how the option ‘fill’ works.
Option ‘fill’: The constructor function FLOWobj fills all topographic depressions (function fillsinks). It then identifies all flat sections (function identifyflats) and calculates an euclidean distance transform (function bwdist) within each flat section seeded at their outer rim. Subsequently, the complement of the distance transform is calculated in each flat area. The complement grid is subsequently used as weights in a gray-weighted distance transform (function graydist). This generates an auxiliary surface in each flat area. The surface has the nice property that flow will run through…
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