STAY ON YOUR SIDE OF THE LINE: SUGGESTIONS FOR MAPPING SURFICIAL CONTACTS WITH ELEVATION-DERIVED HYDROGRAPHY

As the Pennsylvania Geological Survey embraces the U.S. Geological Survey’s Geologic Map Schema (GeMS), we have experienced challenges trying to match hydrography data to preexisting surficial units. The GeMS formatting, as applied to recent geologic mapping in southern Bucks County, Pennsylvania, treats hydrography as a surficial unit. Outlines around water polygons act as surficial contacts that represent shared edges between the water polygons and lithologic surficial units; there can be no overlap or space between a water polygon and an adjacent surficial lithologic polygon. The mapping workflow used caused topologic conflicts, which were further complicated by competing priorities of the authors.

The Bucks County geologic investigation workflow began with field data collection and mapping followed by the production of Pennsylvania Hydrography Dataset (PAHD) geometries. The PAHD polygons were added to the surficial lithologic units, and surficial alluvial boundaries were checked and adjusted where they intersected water polygons or crossed PAHD flowpaths. Based on the lessons learned here, the preferred workflow would be to generate hydrography first, use these generated data as a field aid in the mapping of surficial units, and then resolve issues as the surrounding surficial geology polygons are generated.