I am relatively new to using ArcMap, so I apologize for what I guess is a relatively easy question. I working with a gridded PRISM precipitation raster (www.prism.oregonstate.edu), which has a standard NAD83 coordinate system. It's spatial reference is listed as:
GCS_North_American_1983 WKID: 4269 Authority: EPSG Angular Unit: Degree (0.0174532925199433) Prime Meridian: Greenwich (0.0) Datum: D_North_American_1983 Spheroid: GRS_1980 Semimajor Axis: 6378137.0 Semiminor Axis: 6356752.314140356 Inverse Flattening: 298.257222101
I am trying to determine if I need to reproject this raster before I use it with a shapefile of watershed polygons that also has a NAD83 coordinate system, but is projected using:
NAD_1983_StatePlane_Virginia_North_FIPS_4501_Feet WKID: 2283 Authority: EPSG Projection: Lambert_Conformal_Conic False_Easting: 11482916.66666666 False_Northing: 6561666.666666666 Central_Meridian: -78.5 Standard_Parallel_1: 38.03333333333333 Standard_Parallel_2: 39.2 Latitude_Of_Origin: 37.66666666666666 Linear Unit: Foot_US (0.3048006096012192) Geographic Coordinate System: GCS_North_American_1983 Angular Unit: Degree (0.0174532925199433) Prime Meridian: Greenwich (0.0) Datum: D_North_American_1983 Spheroid: GRS_1980 Semimajor Axis: 6378137.0 Semiminor Axis: 6356752.314140356 Inverse Flattening: 298.257222101
Part of the issue is that when I reproject the raster, using the Project Raster tool, the output raster has shifted cells. I imagine only one of these rasters (original or reprojected) is appropriate to use in our analysis. A few things have gotten me confused here:
- I thought ArcMap automatically reprojected on the fly to whatever system was specified for the data frame, at least within the same coordinate system. When I use the Project Raster tool, though, and convert the raster from GCS_North_American_1983 to NAD_1983_StatePlane_Virginia_North_FIPS_4501_Feet, it rotates the raster slightly. I have also tried to using the Define Projection tool first on the original raster to NAD83, and the shift still occurs.
- When I change the default view of the data frame from NAD_1983_StatePlane_Virginia_North_FIPS_4501_Feet to GCS_North_American_1983 (View -> Data Frame Properties -> Coordinate System) it rotates both the watersheds polygons and raster slightly. The raster then looks the same as if I had used the Project Raster tool above.
- Finally when I use the Project tool on the watershed shapefile and change its projection from NAD_1983_StatePlane_Virginia_North_FIPS_4501_Feet to GCS_North_American_1983, nothing visually changes (ArcMap apparently reprojects the shapefile on the fly).
Here is a clip of the original raster (grayscale) and reprojected raster (semi-transparent green) on the same map. The cell size is 4 km^2, so the difference is fairly large.