Coordinate Transformations for Using NEXRAD Data in GIS-Based Hydrologic Modeling

Gridded precipitation products created as part of the U.S. National Weather Service's Next Generation Weather Radar (NEXRAD) program are referenced to a national grid called the Hydrologic Rainfall Analysis Project (HRAP) grid, which is used to mosaic precipitation estimates from different radars into a single national precipitation map. The HRAP grid is defined in a polar stereographic map projection that is formed on a plane intersecting a spherical earth datum at 60° N. This paper describes a method for transforming HRAP grid cells into a coordinate system commonly used for mapping geographic information system data sets, quantifies mapping errors associated with using the HRAP coordinate system, and outlines an approach to reduce these mapping errors. The NEXRAD radar rainfall processing software assumes that the earth is spherical rather than using a more accurate ellipsoidal representation. This assumption causes east-west distances to be distorted relative to north-south distances. The magnitude of this shape distortion and the magnitude of the scale factor distortion associated with the polar stereographic projection it self are quantified for different latitudes.