Downloading, importing, and projecting PRISM climate data

This was a email I put together for a USU QCNR graduate student. Maybe someone else will find the information useful. I suggest using caution since this information is subject to change as Esri/EPSG may change transformations and PRISM may change data formats/etc. Please leave a note if you find errors and I will attempt to clarify and/or fix. It is was a copy/paste job email to blog - so user be warned.

1. Download the desired data from PRISM (prism.oregonstate.edu)
2. Download (or view) the associated FGDC Metadata. Note that the various details are NOT the same for each PRISM layer. Projections and cell sizes vary product to product.
3. Use the ASCII to Raster tool in ArcToolbox
1. Input the downloaded ASCII file
2. Assign the output file a .tif extension. We generally try to work with tiff files since they are universally read by GIS and RS software. You can also use .img files since they have similar properties if you wish. Avoid leaving it blank (native ArcGIS format) or assigning a .jpg format.
4. Use the Define Projection tool to assign the proper projection to the newly imported file
1. ArcGIS will make a best guess about the initial projection and use that projection in your ArcMap view. Do not rely on the project-on-the-fly functions for anything beyond looking at the data; never for analysis.
5. After defining the projection you need to reproject the data to match the best option for your field data or analysis region. This is done using the Project Raster (for raster data) utility in ArcToolbox. Note that the PRSIM data is generally developed using WGS72, WGS84 or NAD83/GRS80. This is not adequate or acceptable for inter-continental spatial analysis.
1. Depending on the scale of your study, you will want to select the correct projection. If you conducting a multi-state or regional study that would stretch across UTM zones, I suggest using Albers Equal Area (USGS version); if you are working in a small area, such as the state of Utah, and all of your data is within one UTM zone, I suggest stepping from unprojected (geographic) to Albers (or similar) and then to UTM. You can go from unprojected straight to UTM if you wish, but realize that you need to subset the data (extract by mask) to your study area once you complete the process and that any data outside of your UTM zone is invalid as it is spatially inaccurate. 
2. In the Project Raster tool, add your Input Raster; assuming you have defined the original projection, it will populate under Input Coordinate System.
3. Select an output location. I suggest that you do not allow it to go to Default.gdb. You will also need to give it a file extension, similar to above, I suggest that you use .tif.
4. Select the Output Coordinate System. This is where you select the best system depending on the coordinate and system and scale of your study. Don't mix coordinate system if at all possible. Actually, just don't mix coordinate systems!
5. Selection of the correct Geographic Transformation is the next step. Geographic transformations can be thought of as the equations used to convert from one projection to another. Some conversions require these transformations, others do not. Without going in to a long-winded discussion about what/why/how, you can turn here (http://wiki.gis.com/wiki/index.php/Geographic_Transformation) for a basic discussion; and here (http://blogs.esri.com/esri/arcgis/2009/05/06/about-geographic-transformations-and-how-to-choose-the-right-one/) for an in-depth discussion of which one is the correct one to choose.
1. Generally speaking, when your focus area is the 48 contiguous US states and you are moving from WGS84 to NAD83, you can opt for either NAD_1983_To_WGS_1984_5 (don't let the from and to confuse the issue here, it will work correctly even though the logic seems wrong), or according to Esri, the more accurate transformation is now WGS_1984_(ITRF00)_To_NAD_1983. You can also see here (http://support.esri.com/en/knowledgebase/techarticles/detail/24159) and here (http://forums.arcgis.com/threads/40988-WGS_1984_(ITRF00)_To_NAD_1983-when-to-use) for further details. 
2. If you are moving from GCS_WGS_1972 to NAD83, you will need to add to selections to the Geographic Transformations box; that is, the first one will be WGS_1972_to_WGS_1984_2 (which is slightly better than _1) and the second transformation will be WGS_1984_(ITRF00)_To_NAD_1983. Another good discussion about "what to use" is here (http://forums.arcgis.com/threads/48328-Datum-NAD83-NA2011).
6. Leave the Resampling Technique as NEAREST
7. Output Cell Size, set this based on the metadata. Generally PRISM has 1KM and 4 or 5KM data; you can also download 800m Normals data, just make sure you opt for the correct cell size according to the product you download. I suggest not leaving the default as you will end up with oddly-sized cells.
8. Click OK
6. Once it is properly projected to the continental (Projected) projection, you can step down to UTM or use the data as is. You can also subset the data (extract by mask) at this point.

- 19 August 2013