NASS CDL grids are generally the preferred data source for quick assessments of agricultural damage. The NASS CDL agricultural data is available online for the continental United States at the following website: https://www.nass.usda.gov/Research_and_Science/Cropland/SARS1a.php.
The NASS agricultural grids are in .tif file format. The projection information is contained within a world file (.tfw). Follow these steps to prepare the agricultural data for use in HEC-FIA:
- Open ArcMap, add the maximum inundation shapefile (i.e., max_extent.shp) obtained in Section 0 by clicking
Add Data. - Next, add the NASS agricultural grid file.
Not all data in the NASS CDL layer are required; only those within the study area should be selected. Right-click on the shapefile in the table of contents and click Zoom To Layer. This zooms the map window to the shapefile extent.
- Next, right-click on the NASS CDL grid in the table of contents and point to Data, click Export Data.
- The Export Raster Data dialog box appears.
- Choose Data Frame (Current) for the Extent. This exports data to the current frame extent, which is based on the maximum inundation shapefile set in Step 3.
- Since the shapefile was added first, the data frame should be using the required spatial reference. Choose Data Frame (Current) for the spatial reference. This selection projects the NASS CDL data into the same projection as the maximum inundation shapefile, which should be the projection system used for all FIA GIS data.
- Leave the middle portion of the dialog with the default values. The Cell Size(cx, cy) is automatically adjusted to meet the selected spatial reference.
- Browse to the location where the shapefile is to be stored by clicking next to the Location box.
- Next select TIFF from the Format list. In the Name box, enter the name of the grid that is to be exported.
- Click Save. The Export Data Dialog box closes and the user is prompted to add the new data to the ArcMap project. Select No, the prompt closes, and the exported data is not added to the ArcMap project.
Finally, create a world file for the exported data.
From the ArcToolbox Tree, double-click Data Management Tools, double-click Raster, then Raster Properties, and then double-click Export Raster World File. The Export Raster World File dialog box opens.
- Browse to the location of the masked digital terrain grid file by clicking next to the Input raster box.
Once the exported .tif file is selected (i.e., westfork_NASS_ag.tif), click OK. The associated world file (.tfw) is created in the same directory as the .tif file. The Agricultural grid is now ready to be imported in HEC-FIA.
