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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.

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

  • When changing the default projection (point two), are you using a reprojection tool to achieve it? ArcMap does reproject on the fly--it's just easier on the processing side to have all data in the same projection. In large datasets, this can cause significant slowdown. – MaryBeth Dec 14 '15 at 14:09
  • Ah, by that I mean when I go into into the View menu, Data Frame Properties, Coordinate System, and changed the default view of my map from NAD_1983_StatePlane_Virginia_North_FIPS_4501_Feet to GCS_North_American_1983. I'll edit the above text for clarity. Thank you! – jww Dec 14 '15 at 14:31
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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: Map Projection Distortion

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

  • Hi MaryBeth, thank you for taking the time to write up this post. To put my question another way, if ArcMap is reprojecting on the fly, then why does the raster visually shift when I apply the Project Raster tool? My thought was that it shouldn't be causing the raster to shift around at all. The polygons don't shift when I apply the Project tool, which would make sense if ArcMap is reprojecting on the fly. I'm worried that either the original or reprojected raster is now off, and I imagine I should only use one or the other in our analysis. Thanks again! – jww Dec 14 '15 at 15:33
  • Project Raster is doing the same thing that Project does, only for the raster. What might be happening is that your raster is defined as one projection when it actually is something else. You may want to try 'Define Projection' for the raster first to see if that helps. – MaryBeth Dec 14 '15 at 15:40
  • I tried using the Define Projection tool first, assigning it to NAD83, and then using the Project Raster tool; the same shift in the cells occurs. I've edited to the main text to include a link that shows a clip of some of the 4 km^2 cells, as well as highlight that I'm worried about the discrepancy between the rasters. Thanks again. – jww Dec 14 '15 at 18:57

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