A methodology for aligning raster flow direction data with photogrammetrically mapped hydrology

The requirement to digitally delineate accurate watershed boundaries is a routine task necessary in numerous natural science disciplines. The base topographic data necessary for watershed analysis in many jurisdictions is typically contained within two primary photogrammetrically interpreted base topographic layers: the hydrology (streams, rivers, wetlands, lakes, etc.), and the elevation (contours, spots, digital terrain model points, etc.). From these two data sources, a flow direction corrected, single line, vector hydrology network and a raster digital elevation model can be developed. There are numerous approaches to watershed delineation, but the most accurate implementations employ both these enhanced data sets in extracting watershed divides. A popular method to increase the accuracy of extracted watershed boundaries is to utilize that hydrology network to post-condition the digital elevation model, thus ensuring the local topography honours the stream network. This process is known as stream burning. This paper outlines an alternative approach to accomplish a similar objective. The outlined approach unambiguously translates the topological relationships within the vector hydrology layers directly into a raster flow direction grid. Initial testing of this methodology has shown several accuracy and processing advantages for extracting watershed boundaries.