Measurement differences resulting from analyzing natural resource spatial databases referenced to multiple map coordinate systems

Map projection on-the-fly capability allows natural resource GIS analysts to geoprocess spatial data layers referenced to different map coordinate systems. This study’s primary objective was to provide examples of spatial measurement differences that can result when point, line, and polygon natural resource spatial databases are referenced to different map coordinate systems and are used for a set of typical spatial analysis tasks. The GIS software used for evaluations was part of the ArcGIS product suite. We performed three separate spatial analyses: the intersection of spatial line features to polygons, the identity of polygons with other polygons, and a spatial join analysis that involved several components. Four separate spatial databases representing gauges, streams, watershed boundaries, and forest ownerships were each referenced to four different map coordinate system settings. An initial assessment of distances, areas, and categories of features resulting from geoprocessing tasks involved only spatial databases referenced to the same map coordinate system. The measurement and categorical results of this initial assessment became comparative benchmarks. Subsequent analyses involved the same geoprocessing tasks but mixed databases referenced to different map coordinate systems. We found differences from the benchmark values in many of the comparisons between databases of different map coordinate systems. There were relatively small measurement differences in the amount of line lengths measured within specific polygons resulting from overlay intersections between databases in different map coordinate systems. The identity of polygons with other polygons resulted in large area differences in some cases from initial baseline measurements, particularly when datums were substantially different (NAD 27 and WGS 84). Distance differences were greatest for the spatial joining of points to other point and line features when involving databases referenced to map coordinate systems with substantially different datums. Results indicate that GIS analysts using ArcGIS analysis tools should be cautious when analyzing spatial databases referenced to different map coordinate systems.