GIS, SINKS, FILL, and disappearing wetlands: unintended consequences in algorithm development and use

Geographic Information Systems (GIS) software has become an important computational tool in several fields. GIS software ranges from command line processors, with maximal control over internal model decisions, to GUI versions with point-and-click access to pre-set modules. Based on the output from this software, some GIS users make important decisions to plan and manage landscapes (e.g., cities, parks, forests) with real consequences for the managed ecosystems. We discuss a programming decision in a GIS algorithm originally used to discern flow direction in hydrological modeling: a first step in mapping streams and rivers. Topographic depressions ("sinks") are "filled" in the algorithm to map water flow downstream; otherwise, the GIS algorithm cannot solve the flow direction. Unfortunately, sinks are often "isolated" wetlands which provide essential habitat for many species not commonly found elsewhere. Thus the algorithmic filling of sinks can make these wetlands "disappear" in GIS output and land-use decisions based on this output.This algorithmic detail may have potentially devastating real-world consequences for numerous wetlands because land-use plans made in ignorance cannot adequately conserve these unique habitats and the vital ecosystem services that wetlands provide. These consequences were not anticipated by the programmers who originally implemented the flow direction algorithm and may not be known to GIS users. We offer several strategies to reduce the impact of these consequences for GIS programmers, users, and policy makers who depend on GIS data when making decisions.

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