HRAP (Hydrologic Rainfall Analysis Projection) Transformation

Discussion created by jbarnabe on Jul 29, 2013
Latest reply on May 30, 2018 by jbrengel
I am trying to transform precipitation data I got from NOAA from HRAP into Albers Equal Area Conic (USGS).  It is an older file (2005) data that is in a point file (grid).  Each point represents the total rainfall (inches) for 2005 for that location. Derived using these methods: http://water.weather.gov/precip/about.php.  There are no transformations available for HRAP.  I even checked the extensive list here to make sure: http://downloads2.esri.com/support/TechArticles/Geographic_Transformations_10.1.pdf

I found this about reprojecting it here: http://www.nws.noaa.gov/oh/hrl/distmodel/hrap.htm
But this requires an XMRG file, which was not sent to me. 

I am ultimately trying to take the precipitation data and get an Average Yearly Rainfall of each County that the points fall in using a spatial join and the average summary. (or even using Zonal Statistics with the Counties as the zones)

So the question is....If HRAP doesn't have a Transformation, is there a way to do it without an XMRG?  And if not, do you think its safe to just ignore the Geographic Coordinate System Transformation warning and let ArcGIS do it's best on the fly?  Since this is an average, would it really be that off if I didn't do any transformation?  I just want to make sure I'm understanding all of this correctly since I've never worked with hydrologic data before.