EVALUATING GENERALIZATIONS OF HYDROGRAPHY IN DIFFERING TERRAINS FOR THE NATIONAL MAP OF THE UNITED STATES

Introduction: This collaborative effort between the U.S. Geological Survey's (USGS) Center of Excellence for Geospatial Information Science (CEGIS) and two universities addresses research problems in generalization and electronic topographic map design. The work supports the USGS web-based data delivery service called The National Map, and most of the geospatial data available in the service is intended for display scales ranging from 1:20,000 to 1:1,000,000 (20K – 1M). Objectives: Our work is based on two premises. First, a single generalization sequence with uniform tolerance parameters will not produce generalized hydrography data of consistent quality across the country. Second, automating as much as possible of the generalization processing will reduce workloads for the national mapping agency and improve consistency and quality of the results. Methodology: We have generalized four hydrographic subbasins of the USGS National Hydrography Dataset (NHD) to demonstrate that knowledge about terrain and climate can inform decisions about generalization processing. Work on four additional subbasins is in progress. We work with symbol redesign and elimination decisions for feature classes (display changes) throughout the scale range of 20K to 1M and produce generalized Level of Detail database versions (LoDs) only at scales where display changes fail to produce successful topographic map displays. Results are evaluated numerically and visually in the context of full-featured digital topographic maps. Results: We find that the subset of algorithms selected and parameters which guide them vary by landscape regime. Likewise, the sequence of generalization processing is different for different regions. An upstream drainage area attribute plays an important