Lars Arge, University of Aarhus Herman Haverkort, TU Eindhoven Andrew Danner, Swathmore College

1. Henrik Blunck • University of Aarhus 8 72 9 7 7 75 4 71 5 74 73 3 6 25 1 24 21 2 2 27 26 22 23 Watershed decomposition Digital elevation map Pfafstetter labels USGS (manually) Overlay Lars Arge, University of Aarhus Herman Haverkort, TU Eindhoven Andrew Danner, Swathmore College Norbert Zeh,, Dalhousie University • TerraSTREAM: Pfafstetter Labeling • Watershed decomposition • Watershed decomposition is a fundamental task in terrain analysis • Watershed of a river = area which drains to the river • Terrain dividable into (hierarchy) of watersheds • A river’s watershed can be subdivided into the watersheds of the river‘s tributaries • Pfafstetter labels • Identify main river • Find the 4 largest tributaries (orange) • Label tributaries (even) and interbasins (odd) • Recursively label all tributaries and interbasins • “Sanity check”: Pfafstetter decomposition “matches” manually • constructed watershed decompositions I/O-efficient algorithm • Modern terrain data is massive  I/O-efficient Pfafstetter labeling algorithms needed • We develop I/O-efficient algorithm by exploiting • Terrain decomposable into rivers, i.e. into tributaries • Tributaries form a tree • Rivers can be labeled individually • Label prefixes can be “forwarded” to tributaries I/O-efficient algorithm Watershed decomposition Pfafstetter labels Current and future work • Current and future work • Watershed queries: Retrieval (and decomposition) of watershed for individual user-specified location • Meaningful labeling along coast lines, e.g., label the coastal watersheds independently or as tributaries of the “coastal river” + = Subdivision of a watershed Independent costal watersheds Connected coastal watersheds References [1] L. Arge, A. Danner, H. Haverkort, and N. Zeh. I/O-Efficient Hierarchical Watershed Decomposition of Grid Terrain Models. In Proc. 12th Intl. Symp. on Spatial Data Handling, SDH, 2007. [2] K. L. Verdin and J. P. Verdin. A topological system for delineation and codification of the Earth's river basins. Journal of Hydrology, 218, 1-12, 1999. Integration into TerraSTREAM MADALGO – Center for Massive Data Algorithmics, a Center of the Danish National Research Foundation