Do I need to reproject a NAD83 raster (with no projection) before using it with a shapefile that has a specific NAD83 projection?

Question

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.

https://dl.dropboxusercontent.com/u/4405912/raster.jpg

Answer 1

It's good practice to reproject, but not necessary, as ArcMap does reprojections on the fly. Reprojection does help to cut down on processing and server lag time, but in all cases it is not practical to reproject every time you add the layer to a map (webservices, dynamic datasets you receive from other agencies, etc). If you are in a situation where changing the projection is the best route, then trying to figure out the right way to do it in ArcMap can be a little confusing.

I think this blog post does a good job of noting the differences between the different projection tools in ArcGIS:

Define Projection - This tool does not change a projection. It changes the metadata describing the current projection of the dataset. You should only use this tool if the projection is currently described as unknown or is known to be incorrect. With this tool you are defining or describing the dataset by saying "Hey, those points are in this projection."

Project - This tool operates on features to change the current projection from one defined projection to another. For example, if you have a point dataset in a UTM projection, each point has a pair of numbers describing that point. When you use this tool to change its projection, each number is recalculated to reflect its equivalent value in the new projection.

Project Raster - This tool operates on rasters to change the current projection from one defined projection to another.

As you probably already know, different coordinate systems will have different distortions during projection as the image below from University of Nebraska - Omaha shows:

Though you probably already know about the distortion, it's a good thing to keep in the back of your mind.