Overlays (Spatial Joins) - Relate Two Attribute Tables via  a Shared Location in Space

• How many building permits were granted in each census block group and what is the total value of new construction per census block group?
• How many Toxic Release Inventory sites are there in each county and and what are the total releases per county?
• Who has jurisdiction over roads?
• Where is there an incompatibility between zoning and current land use?

You can visually inspect building permits and block groups by bringing both themes up at once - but what if you want more precise information about total new construction in each block group?  This requires an overlay process, generally referred to as a point-in-polygon overlay (or line-in-polygon, polygon-in-polyon, etc.).  It is also called a "spatial join" or "spatial relate"  because you are joining (relating) two geographic theme tables based on a common location instead of a common attribue field.

Aggregating data between themes requires a spatial overlay process

• For example, to total construction costs and square footage of all building permits by block group, we need to overlay block groups on permits
• The GIS program has tools that allow you to count up the number of building permits in each block group and to summarize other fields (e.g., cost and footage)
• In ArcView this process requires you to go through 3 steps 

3 steps for "overlays" in ArcView

1. perform a spatial join - joins attributes from one theme table (e.g., census block groups) to another theme table (e.g., building permits). In this example, every building permit would then have a census block group id added to it.
2. create a summary table - in the example, this step creates a table that has a record for each block group id and how many permits were found in each block group, plus any other aggregated calculations you set up (e.g., summed construction value per block group, average construction value per block group)
3. perform a regular table join using common field (joins the summary table back to the original block group theme attribute table, so that now the latter has fields containing a count of building permits (and, if you set this up, total construction value and average construction value) for each block group

Step 1: Spatial Joins in ArcView - permits per block group example
(Note: in ArcView 3.1, this first step can be done using the Geoprocessing extension -  make this extension available by choosing File-Extension, then choose View-Geoprocessing Wizard, the use the Assign Data By Location option; continue with steps 2 and 3 as described below)

Instructions (for ArcView 3.0 or not using the Geoprocessing Wizard)

1. Add 90census and permits themes to the view
2. Open the attribute table of each theme
3. Highlight the "Shape" field on 90census - this will be the source table
4. Highlight the "Shape" field on permits - make sure it is the active (destination) table
5. Join the tables - result is block group attributes are joined to permit based on its location within the block group (see graphic example)

The result of spatial join is a permits table with block group ID attached:

Step 2: Create a summary table based on blockgroup ID field

1. In permits table, highlight the name field (this is the block group FIPS code)
2. Choose Field - Summarize (creates a new summary .dbf file)
3. Define a name for the new table
4. Define which fields you want summarized and the method for summarizing (e.g., sum, average, count)
   
5. Result is a new table summarizing permit fields (e.g., construction cost, square footage) by block group

Step 3: Join Summary Table to Original 90census table

1. Highlight name field on summary  table (this is source table)
2. Open 90Census attribute table and highlight Name field (this is destination table)
3. Join the tables so that the new permit information is added to the original 90census attribute table:
   
4. Result is an expanded 90census attribute table with summarized information about permits in each block group

The result is a 90census block group attribute table with aggregated permit information:

Use newly integrated information

• Export new information to another program (SPSS, Excel) for statistical analysis
• Create map based on new information

13  March 1998
The School of Architecture at UT Austin
Comments to: Barbara Parmenter