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QGIS Introduction: Joining Data

Purpose of Joins

Joining data together is necessary when you have a standalone table with no address or x,y data.  Joining can be performed on under the following scenarios:

  • you have commonly defined geographic indicators (i.e. state fips, census tracts) in the standalone table.
  • the website or data source you downloaded your data from have a join field.  For instance, if you download a table and a shapefile in the same session from the National Historic Geographic Information System there will be a "GIS Join" field in both the shapefiles' attribute table as well as the standalone table.

Join Example

Joining Data

In order to join data together, you need to have the standalone table and a shapefile.  Both files need to have a common geographic identifier so the table and shapefile can be joined.  For instance, the following dataset was downloaded from the NHGIS and there is a field labeled GISJOIN in both the table and the shapefile.   

  1. Open the layer properties (double-click the layer) and click on the Joins tab in the left column.
  2. Click the green Addition symbol at the bottom of the dialog box.
  3. Change the Join layer to the standalone table and change the join and target fields to the fields that have a common geographic identifier.           
  4. Click OK.
  5. Right click the shapefile and click Open Attribute Table.
  6. Scroll all the way to right and view the new table.  The new fields will have the name of the standalone table followed by an underscore and then the field.
Tags: gis, QGIS