ArcMap Join Tutorial

Two data files you will use in this tutorial: 72nd Congress Shapefiles; 1930 Election Data.

This tutorial will allow you to begin navigating in ArcGIS (specifically ArcMap) and perform one of the most important functions, Join. It is not intended to take the place of considered and intense study or learning, such as through a course on GIS — just to get basic functionality.

Open ArcMap and create a New Map from a Blank Template. This will create a .mxd file to work with your data.

This is the Data View, the basic ArcMap interface. The left column is the Table of Contents listing your data and sources. The large window shows the map and its presentation when you add the data. You can toggle to the Layout View under the View Menu.

To add data, from the toolbars at the top, click the Add Data button (yellow diamond with a black plus sign) and navigate to your data on your computer. You may need to Add a Connection (yellow folder with a plus sign in the dialog box) to tell ArcMap where to look for your data. The boundary (geographic) files are “shapefiles,” .shp. These are a bundle of 3 to 6 files. In ArcMap, you only see the .shp, not all the files. For this tutorial, I am adding the boundaries of the 72nd U.S. Congress.

Here the shapefile has been added (shown on the Table of Contents). This is shown with a particular projection, which comes with a set of visual distortions. These closed districts are called polygons (the other possible features are lines or points).

To change the projection or coordinate system, you can change the underlying data (not shown here) or you can change how the data frame displays the projection. Under View>>Data Frame Properties>>Coordinate System you can choose a different projection, in this case Albers Equal Area Conic.

Here you can see the difference that changing the projection makes.

By right-clicking on the shape file layer in the Table of Contents and opening the attribute table, you can see the underlying data. This data is for the shapefile; you can also look at the attribute table of straight quantitative data sheets once they are added to ArcMap.

This is the attribute table for an Excel spreadsheet of Congressional elections data for the 72nd Congress. The “ID” column is a necessary creation to enable the matching or “Joining” of the quantitative elections data to the geographic district data. If you download prepared data from a source like NHGIS.org, it will already have this join-friendly data. If you create your own, you will have to think this through and enter it beforehand.

Using the Selection Tool(s) in the Selection menu or on the toolbar, you can isolate the information of any single polygon or feature within a shapefile. This screen shows how you can toggle between the data of the selected polygons and all the polygons (the two turquoise buttons at the bottom of the dialog box).

To Join data, right click on the shape file and, from the menu, choose Joins and Relates, then Join. This dialog window will come up allowing you to tell the program how to put the election and district data together. The matching column in each is called GISJOIN in the shapefile and ID in the spreadsheet.

Once you have joined the data, you need to check if it is successful. This is the successfully joined shapefile attribute table, where the election data is now part of the shapefile data. There are a few problems in this case — the “” in the two right columns means something isn’t quite right with that record.

With the data successfully joined, you have the ability to visually represent how it is displayed. Right-click on the shapefile layer, select Properties, the go to the Symbology tab. Since we are interested in categorical representation (ie what party a winner is in, not how big was the margin of victory), we choose Categories. Then we Add All Values from the Party field in the Value Field drop-down menu.

We can choose colors to represent the categorical data. In this instance, we are drawing on an established Red/Blue convention, changing the lavender associated with “0” (Democrats) to blue, the pale yellow of “1” (Republicans) to red, and the pale blue of “2” (Independents) to yellow. There are other options, depending on what you want the map to convey. We could choose shading for margin of victory or voter turnout, or something representing our other data sources.

Here we see the data displayed and now shown in Layout View, where you have the ability to add a legend, add other explanatory text or labels, add a scale bar or north arrow (if applicable), and make this into a more useful map. Then you can export (File>>Export) as a PNG. PNGs, remember, are better for vector/line drawings like this, vs. JPGs for photos.

Remember to SAVE OFTEN and keep all your files (your .mxd, your shapefiles, and your spreadsheets) in one place so when you open this up again, ArcMap knows where to find them. Joining is a reversible process (right-click the layer in the Table of Contents, then choose Join>>Remove all Joins). You could also make it permanent by right-clicking the layer, choosing Data, and saving the joined layer as its own permanent layer.