The purpose of this tutorial is two-fold: to give you practice working with a fairly complex relational database, and to introduce you to an important GIS data set. The SSURGO Soil  Data from the National Resources Conservation Service is an important data set for land use planning, environmental planning, and water resources planning (among other applications). Note for Fall GIS class - this soils data set is a required layer for your project, so make sure you are using the data for your project area.

This part of the assignment has five sections:

1. Familiarize yourself with NRCS soil survey data and the SSURGO data structure and look at the basic SSURGO data in ArcView
2. Understand the SSURGO and related attribute tables  and their use for land use planning
3. Relate (join/link) the SSURGO and MUIR tables in ArcView
4. Create queries in Access that show soil interpretation ratings for key qualities (e.g., septic field construction limitations, road construction limitations, building limitations)
5. Create soil interpretation maps in ArcView using the SSURGO data, plus DOQQs and/or DEMs

1. Familiarize yourself with the SSURGO (Soil Survey Geographic) Soils Data
Read the following selections from the SSURGO Data Users Guide (available in .pdf format from the USDA NCRS SSURGO Data Access Site or on our local Taniguchi GIS Data server under the readings folder; a printed version is also available in the lab in the GIS Readings brown file box):

• pages 20-21 for an introduction to soil surveys
• pages 1-14 for more information about the SSURGO data set - pay very close attention to pages 6-11 which describe the data structure and relationships

Note that the rest of this long document is mostly made up of three appendixes documenting the various tables in the database. You do not need to print these out - they are very useful but you can refer to them onscreen.

Other soil survey and SSURGO web links:

• Land use planning: it all comes down to soil (A short brochure from the USDA that explains the importance of soil data for land use planning)
• USDA Natural Resources Conservation Service, Soil Survey Division
• National Soil Survey Handbook (from the USDA NRCS) - see especially the Soil Interpretations sections, 618-625, to see how the interpretation ratings are assigned.
• The 1993 National Soil Survey Handbook Interpretive Guide is very useful for planners wanting to understand how soils are rated for various land uses. This Guide is available in MS-Word format on our local Taniguchi GIS Data server under Readings the folder or online from the National Soil Survey Handbook, Section 620.01(a)
• USDA NCRS SSURGO Data Access Site - see especially the SSURGO Data Users Guide available in .pdf format on this site.
  Note: I had trouble accessing the data via the web-based data access site above, but the data is also available from the NRCS' FTP site at ftp.ftw.nrcs.usda.gov - follow the directory path pub/ssurgo/online98/data/tx453 for Travis County, and tx491 for Williamson County
• USDA Natural Resources Conservation Service (NRCS)
• NRCS National Soils Information Service

Look at the basic SSURGO data in ArcView

We have downloaded the SSURGO data from the NRCS' anonymous FTP site and placed it on our Taniguchi-GIS Data server under the /local_data/soils/ folder. There you will see folders for Travis and Williamson Counties. Choose data from either county for your assignment.  In each county folder you will find a cov.zip file and another soils folder. The cov.zip file is the compressed file with all the Arc/Info soil coverages for that county. The soils folder contains the unzipped coverages.  (Note the data is also available from the NRCS' FTP site at ftp.ftw.nrcs.usda.gov - follow the directory path pub/ssurgo/online98/data/tx453 for Travis County, and tx491 for Williamson County.)

Copy the cov.zip file  for one of the counties to your personal directory and unzip it. Also copy the metadata file. Start ArcView and  look at the Arc/Info coverages. Note that this data is not in ArcView shape file format, but Arc/Info coverage format. When you go to add a theme in ArcView, you will see numerous folders listed on the left side, and one INFO folder on the right.  Each of the folders on the left is a separate Arc coverage, each one corresponding  to one quad map boundary for the county.  The INFO folder contains additional attribute tables for the entire county (see graphic below). The NRCS calls these additional tables Map Unit Interpretation Records, or MUIR data.

On the left side, click once (and only once!) on the greenish part of a folder ending with the letter a and you will see it open up to show a choice of features, e.g., polygon, labelpoint, and sometimes arc (line).  A coverage can contain more than one feature type. You want to see the polygons, so double click to add the polygon theme.

Open a few of the polygon coverages from different folders and examine them in ArcView. Also look at the attribute tables for these polygon coverages. Use the SSURGO Locator Map to find the SSURGO code for a particular quadmap zone of the county.  Remember, all you want to see are the soils polygons - these are contained in the folders ending in the letter a.

To properly set the map units, you need to know the coordinate system information. The data comes with metadata for both Travis and Williamson counties. In the metadata, you will find that the coordinate system is UTM Zone 14, NAD 83, and map units are meters. Make sure you choose View-Properties and set your map units to meters. Note: you won't be able to show this data with GIS data from the cities of Austin or Round Rock until you all the data sources into a common projection using the Projector  extension - I advise that you wait until the end of the assignment if you want to do this).

Open up the attribute table for one of the themes - you will see it doesn't contain much data.  The musym (map unit symbol) is going to be the link into the rest of the database.

Go to the Tables icon in the Project Menu and add the MAPUNIT, COMP, CODES, and INTERP tables (note that to see these tables, you will have to make sure the data is of the type: INFO, and you will need to open up the INFO directory, as in the graphic below.

Examine these tables.  Look at the relationship diagram to see how they are related. To understand the relationships between these tables, you will work with them in Microsoft Access. (Note it is not necessary to go into Access, but I find it easier to work with  complex tables in Access rather than in ArcView.)

But  first, to get a quick taste of how to use these in ArcView, create a map of prime farmland, which requires only a simple join. To do this:

1. Join the MAPUNIT table to the attribute table of one of your soil polygon coverages, using the musym field as the join field (make sure that the coverage attribute table is the active receiving table).
2. Check that the table joined properly - you should see a field close to the end of the table called primfml (prime farmland)
3. Create a unique values map based on the primfml field - you should see codes ranging from 0-9 (there may be only one or two of these, e.g., 0 and 1)
4. Appendix B (Definition of Soil Codes) of the SSURGO Data Users Guide, lists all fields on all the tables and all the possible values for each field. Look up the primfml field to see how to translate and label your legend. (The SSURGO Data Users Guide is available in .pdf format from the USDA NCRS SSURGO Data Access Site or on our local Taniguchi GIS Data server under the readings folder; a printed version is also available in the lab in the GIS Readings brown file box).
5. Save your project file, naming it Soils1, in your personal directory and close ArcView

Note: you can merge two or more themes together into one theme using the Geoprocessing extension.  After loading the extension, choose View-Geoprocessing, then select the merge function and follow the directions.  The themes you want to merge must all be present in the view.

Proceed to next section

20 September 2001
The School of Architecture at UT Austin
Comments to: Barbara Parmenter