WMS:GSSHA Join SSURGO Data

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SSURGO (Soil Survey Geographic database) are digital soils data produced and distributed by the Natural Resources Conservation Service (NRCS) of the National Cartography and Geospatial Center (NCGC). The database contains digital soil type and distribution information for 95% of the counties in the United States. Information covered includes soil characteristics, soil properties, and addresses limits, risks, and suitability for various uses.

SSURGO Files

SSURGO files can be downloaded using the USDA Web Soil Survey site.[1]

The DBF files associated with SSURGO shapefiles contain the following attributes: AREASYMBOL, SPATIALVER, MUSYM, and MUKEY. Use the Join NRCS Data command to join more attributes which can then be mapped to the soil coverage. The DBF file is not changed by this process. This allows the DBF and associated shapefiles to be joined to multiple projects.

The Join NRCS Data command opens three TXT files located in the SSURGO tabular folder containing the downloaded SSURGO data:

  • "comp.txt" – Contains HYDGRP data.
  • "chtextur.txt" – Contains TEXTURE data.
  • "chorizon.txt" – Contains KSAT, MOISTURE, FIELDCAP, and WILTINGPT data.

Join NRCS Data Dialog

Join NRCS Data dialog.

The Join NRCS Data dialog is accessed by right-clicking on a shapefile in the Project Explorer and selecting Join NRCS Data.... The dialog has the following options and sections:

  • Join Soil Texture – On by default, this option joins the soil texture data.
  • Join HYDGRP – On by default, this options joins the HYDGRP data.
  • Fill blank values – Turn on this option to enable editing of the spreadsheet.
  • Compute hydraulic conductivity using equivalent conductance equation

When Fill blank values is turned on, the spreadsheet will have two rows visible:

  • Soil Group – A drop-down listing all available soil groups in the SSURGO data.
  • Soil texture – A drop-down list of all available SSURGO soil textures in the SSURGO data.

Because there are often missing fields in the data, this allows WMS to provide a default value to be assigned to missing fields to avoid blanks. The defaults for each of the drop-downs are the most often occurring in the SSURGO data.

After cleaning and filling the data, the attributes are then matched up with their corresponding MUKEY and joined to the GIS layer. If more than one entry exists for an MUKEY, the corresponding parameters that occur most frequently are joined to the layer.

HYDGRP Data

Some SSURGO data may contain hybrid HYDGRP classes, such as "A/B." Because WMS does not support hybrid HYDGRP fields, the entry will be changed to first group listed. For example, if the entry is "B/D," WMS will change the entry to "B."

Using Soil Type Data with GSSHA

GSDA soil type data can be used with GSSHA. See the GSSHA Wiki for more information.[2] A SSURGO GSSHA soil data preparation spreadsheet is also available.[3]

References