Datums, Projections, and Coordinates: A Spatial Gee-Odyssey
What makes geospatial data “spatial”? Geodesy. Geodesy provides the foundation for defining datums, which are the basis for projections, and both can yield coordinates. But what are these things, where do they come from, and how are they related to one another? And how does the geoid and gravity fit into all of this? Such questions often go unanswered as we busily create, manipulate, analyze, and display geospatial data. Yet one of the most fundamental functions of a GIS is the ability to correctly overlay spatial datasets. That function relies on an understanding of such questions, especially as the resolutions, precision, and (hopefully) accuracy of spatial data increases. This presentation seeks to provide an explanation of this geodetic menagerie within an unrealistically short time. Particular emphasis is on the modernization of the National Spatial Reference System in 2022, which leads to the unsettling idea that coordinates change with time. Safety is not guaranteed.