The Community Rating System (CRS) only gives discounts for specific types of open space. These areas must meet the following criteria:
- The parcel is located in the FEMA-designated 100-year floodplain.
- The parcel is open space, meaning it has no buildings, significant areas of pavement or other impervious surfaces, or dumps.
- The parcel is preserved as open space by a long term contract from the private owner or public agency.
Because of these constraints, it's easiest to consider public lands, such as parks, and non-profit lands, such as a church or Boy Scout camp. In many cases, it's difficult to obtain the detailed large-scale data needed for floodplain analysis. This data is more readily available at a national scale. In this lesson, you'll use a Living Atlas raster image of the United States and raster functions to prepare it for parcel-scale analysis. First, you'll filter out impervious surfaces. Then you'll clip the raster so you only have your county of interest. Finally, you'll save the Living Atlas raster locally and create an attribute table for it.
Download and open the project
First, you'll download and familiarize yourself with the data for your community, Georgetown County, South Carolina.
- Download the GeorgetownCo_CRS zipped folder.
- Locate the downloaded folder on your computer and unzip the file to a location you can easily find, such as the Documents folder. Don't unzip the file to your Downloads folder.
The folder contains an ArcGIS Pro project file (.aprx) and a file geodatabase with the community data (.gdb).
- Double-click the CRS_Georgetown_Co.aprx file to open the project in ArcGIS Pro. If prompted, sign in using your licensed ArcGIS account.
If you don't have ArcGIS Pro or an ArcGIS account, you can sign up for an ArcGIS free trial.
The map contains a basemap, feature classes for Georgetown County parcels, the county boundary, the 100-year regulatory floodplain, and a table. The 100-year Floodplain layer has three zone types: A, AE, and VE. These are FEMA codes for subtypes of floodplains.
- In the Contents pane, under Standalone Tables, right-click PADUS_CRS_attrib and choose Open.
Each unique OBJECTID contains all the attributes for the Protected Areas Database of the United States (PADUS) that you'll eventually join to your parcel data. Due to the amount of records, you'll filter the national scale dataset before you join this information back to the data. Lookup tables like this provide an efficient way to store a lot of information.
- Close the attribute table.
Modify a raster image service processing template
Now that you've seen the community data available, you'll add the OSP Activity 420 for FEMA's CRS raster image service to your map. Then, you'll use a raster processing template to filter out raster cells below a certain value. This process will remove impervious surfaces that are ineligible for the program. Then, you'll use on-the-fly processing to clip the data to your community boundary and regulatory floodplain. On the fly means that the clip is performed every time you need to access the data, rather than only once so that you must save the results. Because you're using portal data instead of locally saved data, you'll need to use raster functions instead of the similar geoprocessing tools. Once you finish clipping and modifying the raster, you'll save it to your project.
- On the ribbon, click the View tab. In the Windows group, click Catalog Pane.
The Catalog pane opens.
- At the top of the Catalog pane, click Portal and click the Living Atlas button.
The Living Atlas portal opens. From this portal, you can access the data in the Living Atlas repository, including basemaps, boundary layers, demographic maps, and so on.
- In the search box, type OSP Activity 420 for FEMA's CRS. Press Enter.
- Right-click the OSP Activity 420 for FEMA's CRS imagery service layer and choose Add To Current Map.
The raster layer draws on your map. This layer represents areas that are likely eligible for open space preservation (OSP). The layer is derived from both the PADUS and the National Hydrology Dataset (NHD), and contains the attribute max_imperv with information about the impervious surfaces within each raster cell.
There is a scale dependency set on the image service, so if you are at too small of a scale, it will not be visible.
- If necessary, in the Contents pane, right-click Community_Boundary and choose Zoom To Layer.
Next, you'll use the raster processing template to filter this image service to show only cells that are within a chosen threshold of impervious surface percentage. Areas that impede natural water filtration and movement are not eligible for OSP credit. This includes parking lots, buildings, roads, driveways, and so on. You'll filter the national scale data based on the estimated percentage of impervious surface per grid cell. It has been shown that water quality and ecosystem health are often degraded when impervious surfaces exceed 10 percent of an area. Using this as the threshold, you'll filter the OSP Activity 420 for FEMA's CRS raster service to select only areas that likely have less than or equal to 10 percent impervious surface.
- In the Contents pane, right-click the OSP Activity 420 for FEMA's CRS raster service and choose Properties.
- In the Layer Properties window, click the Processing Templates tab.
- If necessary, for Processing Template, choose OpenSpacePotential.
- In the Inputs section, for Parameter, type max_imperv. For Value, type 10. Click the validate button to verify that your Inputs information is valid.
- Click OK.
The resulting raster shows only those cells that are estimated to have no more than 10 percent impervious surface coverage. To qualify for OSP credit, the protected areas also need to be in the regulatory floodplain. You'll clip the national raster to Georgetown County's 100-year floodplain.
- On the ribbon, click the Imagery tab. In the Analysis group, click the Raster Functions button.
The Raster Functions pane opens.
- If necessary, in the Raster Functions pane, click the System tab.
- Expand the Data Management section and click Clip.
The Clip Properties function opens.
- In the Clip Properties pane, click the General tab.
- For Name, type PADUS_CRS_clip (this will be the name of the output raster). For Output Pixel Type, choose 32 Bit Unsigned.
- Click the Parameters tab. For Raster, choose OSP Activity 420 for FEMA's CRS.
- If necessary, for Clipping Type, choose Outside.
Specifying Outside will remove the raster cells outside the clipping geometry. The other option is Inside, which removes the raster cells inside the clip features.
- For Clipping Geometry/Raster, choose 100-year Floodplain. Check the Use Input Features for Clipping Geometry box.
Using input features for clipping geometry specifies that the raster will be clipped to the floodplain layer you chose. Alternatively, you can set an extent by drawing a rectangle around the area you want clipped.
- Click Create new layer.
The function may take several minutes to run. When it finishes, a temporary layer called PADUS_CRS_clip_OSP Activity 420 for FEMA's CRS is added to the map. The default name is long, so you'll rename it.
- In the Contents pane, click PADUS_CRS_clip_OSP Activity 420 for FEMA's CRS to select it. Click it again to make the layer name editable and change the name to PADUS_CRS_clip.
Save the clipped raster
Now that you've filtered and clipped the national raster dataset down to a useful size, you'll save it locally for future use. Using the Export Raster tool, you'll save the raster for your area of interest as a TIFF, a compact format. Then, you'll create an attribute table for the .tif file that you'll use to join the raster with its attribute table for analysis in the next lesson.
- In the Contents pane, uncheck OSP Activity 420 for FEMA's CRS and 100-year Floodplain to turn the layers off.
Now the new clipped layer is the only one visible on the map. Unlike geoprocessing tools, the raster functions tools apply an operation to a raster image on the fly, meaning that the original data is unchanged and no new dataset is created. If you remove the layer from the map, you'll erase the PADUS_CRS_clip layer you just created. This enables faster processing because it requires less storage and memory on your computer. Because you'll need the raster later, you'll save it.
- In the Contents pane, right-click the PADUS_CRS_clip raster, point to Data, and choose Export Raster.
The Export Raster pane opens.
- On the General tab, for Output Raster Dataset, name the output PADUS_CRS_final.tif.
- For Clipping Geometry, choose Community_Boundary.
- Check the Use Input Features for Clipping Geometry check box. If necessary, for Clipping Type, choose Outside.
- In the lower part of the pane, make sure that Cell Size is 30 for X and Y, andPixel Type is 32 Bit Unsigned
- For NoData value enter 2147483648, which is a value just beyond the range of actual useable values, and check that Output Format is TIFF.
- In the Settings tab, for Snap Raster choose PADUS_CRS_clip. Click Export.
The new layer is added to the map.
- In the Contents pane, right-click the PADUS_CRS_clip layer and choose Remove.
The data is now clipped to the extent required by FEMA. Your output is in TIFF format, which is an efficient file type but doesn't have an attribute table. To use this layer later, you'll create one.
- On the ribbon, click the Analysis tab. In the Geoprocessing group, click Tools.
The Geoprocessing pane opens.
- In the search box, type build raster. In the list of results, click Build Raster Attribute Table.
- In the Build Raster Attribute Table pane, for Input Raster, choose PADUS_CRS_final and click Run.
- Right-click PADUS_CRS_final.tif and choose Attribute Table.
If you can't open the attribute table, try removing and re-adding the PADUS_CRS_final.tif layer. This helps refresh the data.
The Value field in this attribute table represents the most common value from each cell. Later, you'll use this field to join the PADUS attribution to your parcel dataset.
- Close the attribute table and save the project.
In this lesson, you modified a Living Atlas raster layer to fit your community's needs. To start the CRS analysis, you filtered out impervious surfaces and clipped the national raster down to Georgetown County, South Carolina. In the next lesson, you'll perform a parcel-scale analysis on this data to determine the likely acreage of eligible open space per parcel in your community.