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NOAA provides QPF data on rainfall in the form of shapefiles. The shapefiles include .prj files with information on the coordinate system used:

GEOGCS["GCS_Sphere_EMEP",DATUM["D_Sphere_EMEP",SPHEROID["Sphere_EMEP",6371200.0,0.0]],PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]]

I was under the impression that NOAA data used WGS84, but it looks like this is not the case and it uses GCS_Sphere_EMEP. Am I misinterpreting the .prj file?

I am using this shapefile alongside route data from Google Maps which uses WGS84. I am not familiar with the Sphere_EMEP coordinate projection. Should I simply reproject the shapefile to use a WGS84 projection?

  • If the data displays correctly in that coordinate system, then you should be fine to leave it as is or re-project it. Don't simply change its coordinate system to WGS84 without re-projecting. – csk Dec 9 '19 at 19:42
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Disclosure: I work for Esri as a Sr. Product Engineer specializing in coordinate reference systems and transformations.

I believe the use of this sphere is specific to the QPF data and not in general usage by NOAA. For instance, National Geodetic Survey is an agency within NOAA and it doesn't use the spheres at all.

If you have access to ArcGIS, try adding the data to a basemap or your other data. It may be difficult to determine if it's lining up properly. If it's offset, it will be offset north-south only.

To add a north-south offset, try using the Create Custom Geographic Transformation tool. Set the input/output coordinate reference systems to EMEP and WGS84. For the method, choose, geocentric translations and leave the parameter values set to zeroes. This transformation converts between the sphere and the ellipsoid, but will only affect the latitude values.

  • Thanks @mkennedy! My difficulty right now is that the QPF shapefile just contains polygons representing rainfall, so there aren't specific reference points to compare to a basemap. I'm having some difficulty coming up with a way to validate these shape files would work with WGS84 coordinate data. – kg1313 Dec 9 '19 at 20:40
  • I'm not sure there is a way unless you could do something with the edge of the data. If there's a stated extent, use that rather than the rainfall? I think it's doubtful though. – mkennedy Dec 9 '19 at 23:17

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