Importing NetCDF-CF into ArcGIS
By Curtis Price and Dave Blodgett
- Import the netCDF to raster layer using the Make NetCDF Raster Layer (Multidimension) tool. Note this tool does not honor Snap Raster or Cell size from the environment - according to its help.
- Note: The NetCDF "grid" data type can have variable grid spacing or be defined so the grid is skewed from the reference axes. In this case, where the grid does not fit the regular "raster" grid rules, the grid cell centroids can be imported as points using the Make NetCDF Feature Layer tool. Alternatively, you can do geometric operations using raster tools (see below).
- Export the layer to a raster data set (interactively by right clicking the layer/Data/Export or using the Copy Raster tool. Copy Raster can output several formats. The output format is determined by the output path name ("outgrid" for ESRI GRIDs, "outraster.img" for ERDAS .img format). It's good practice to set the bit depth instead of letting Copy Raster "guess" (for example 32_BIT_SIGNED is a good setting for ESRI GRIDs, 8_BIT_UNSIGNED for TIFF files). If you are doing analysis on the raster GRID is the most efficient format to use. (Arc 10 does native read-write of more raster formats, but Arc 9 has to convert raster to grid format behind the scenes to run most raster tools. Even in Arc 9, GRID is the fastest analysis format.)
- Re-Define the raster dataset's spatial reference with the Define Projection tool.
- The multidimension tool box (as of ArcGIS 10.0 SP 1) automatically assigns the WGS84 datum to any imported NetCDF data. Be aware that NetCDF data tends to have ambiguously defined datum information. This results from a general disregard of datum transformations by most atmospheric scientists (the primary creators of NetCDF data). There do not seem to be any reliable assumptions to make regarding what datum to should be assigned to a NetCDF data set, so finding the correct coordinate system may require some research.
- You can use the Shift, Warp, or Rotate tools tweak the raster's layout and location. (a common issue would be center of cell vs lower left corner of cell – ArcGIS raster XY is treated as the lower left corner of the lower left cell. The Shift tool could be used to move the raster 1/2 cell down and to the left if needed to align with other data sets.)
- It would not be unusual to import many similar netCDF layers. Once you figure out a process flow that works for your datasets, you can use ArcGIS's built-in visual programming environment, ModelBuilder, to automate your processing steps: