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Water flow processes operating on the earth's surface play a fundamental role in shaping various landform elements such
as stream networks, valleys and watershed boundaries. In the digital environment, surface flow paths have been
conventionally derived from a grid-based DEM, where flow directions are assigned based on the direction of steepest
descent determined from grid-cell elevations. However, the assigned flow direction pattern often includes problems such
as unrealistic direction of flow paths especially in nearly flat areas and depressions. A new method for tracing flow paths is
proposed to avoid these problems by two means: 1) Instead of grid-based DEMs, raster contour lines are used as input data;
and 2) Not elevation but information derived from the distance transform performed on a contour map is used to assign
flow directions. The proposed method has been implemented using the C++ programming language. The results obtained
by the new method were compared with those from the existing grid-based methods including the most popular D8 method
and its improved algorithms such as Rho8, FD8, FRho8 and DEMON. The new method resulted in better agreement with
the original stream networks shown in topographic maps, especially in gentle areas. The use of contour lines makes the
new method free from digitizing and interpolation errors, which are often caused in generating a grid DEM from a
topographic map. The high availability of contour maps is another advantage of the new method. The method will benefit
various hydrological analyses and applications.
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