Wait—One Transformation Isn’t Enough? Moving Plain NAD83 to NAD83 (2011) and beyond
Wait—One Transformation Isn’t Enough? Moving Plain NAD83 to NAD83 (2011) and beyond
Abstract
For decades, many GIS professionals have treated “NAD83 State Plane” as a sufficiently complete coordinate-system description. When data must be projected, we open the Project tool, select the desired output coordinate system, choose a geographic transformation from the list—wait, choose a geographic transformation?—and click Run. Some users never make that choice at all. Others accept the first or default transformation offered, assuming it must be the best one. But geographic transformations are not interchangeable, and the default may not be the most accurate or appropriate option. The issue becomes even more complicated when moving legacy State Plane data from the original NAD83 datum to NAD83 (2011). That process requires a succession of transformations through the intervening NAD83 realizations: NAD83 (1986) → NAD83 (HARN) → NAD83 (FBN) → NAD83 (NSRS2007) → NAD83 (2011). This requirement surprised several GIS professionals with more than three decades of experience. If seasoned practitioners have not encountered it, how many others are projecting data without realizing that one transformation—or the default transformation—may not be enough? This lightning talk will demonstrate where geographic transformations are selected in ArcGIS Pro and which successive transformations are needed to move Illinois State Plane data from “plain NAD83” to NAD83 (2011) and beyond. As the National Spatial Reference System moves toward modernization, now is the time to examine what “NAD83” really means—and whether our legacy data is ready for what comes next.