In this exercise you will learn to join a data from an outside table to a spatial file through what is called a table join
Where the data came from
This data is from ICPSR, titled AFRICAN COUP EVENTS DATA, 1986-1990
before continuing, please navigate to the project folder and open the .pdf document titled cb6869.pdf to get a better understanding of what we will be working with
This data was originally in SPSS format, in order to make it compatible with the GIS software the format was changed:
- Rows were reduced to contain one record per country
- Years were moved to columns
- Added fields containing totals for the three categories were created:
- TCOUP - total coups
- TAC - total attemted coups
- TPLOT - total plotted coups
- Field names were reduced to 8 or less characters, spaced were removed (only alpha characters or the underscore '_' can be used
The finished table was saved as a dbaseIV table
Steps for join
Open the ArcGIS software:
Start > Programs > Mapping Applications > ArcGIS > ArcMap
Select the default Start using ArcMap with a new empty map
Maximize the screen
Add the following layers
- africa_cntry92.shp - Africa country boundaries for 1992
- world_cntry92.shp - World country boundaries for 1992
- coup86-90.dbf
Change the color of the world_cntry92 shapefile to a shade of grey, this is only a background layer and is not essential to the data being presented
Zoom in to Africa using the magnify tool
Open the tables for africa_cntry92 and coup86-90, set them side by side
Identify a common field between the two that can be used to join the two layers
Right click on the africa_cntry92 layer and select Joins and Relates
From the pull-down menu, select the field name you would like to use to join the two tables together
From the second pull-down menu, select the coup86-90.dbf file
From the third pull-down menu, select the field name that has the same data as in the africa_cntry92 table
Click on Ok
Now examine the table for the africa_cntry92 shapefile to see the changes
Now create a choropleth map based on one of the variables.
Important features for making a map are a good title (must tell attribute, time, and place), legend, directional arrow, scale, and somewhere on the map should tell who the author is, when the map was created, and what the source is
Saving the layer as a new shapefile
The join between the two tables is only temporary, it will disappear when you close the project. In order to make it permanent you must export the layer as a shapefile.
Right click on the file name and select:
Data > Data Export
An important thing to take note of after you have exported the data into a new shapefile is everywhere in the table where it read "
