Creating a Basic Choropleth Map in ArcMap
A choropleth is a thematic map where areas are colored according to some characteristic of those areas.
An example of a choropleth commonly seen in the media is a red/blue state map that shows the dominant political parties in the different US states. In this map of the 2012 presidential election, the red states sent Republican electors to the electoral college to vote for Governor Romney, and the blue states sent Democratic electors to vote for President Obama.
This tutorial will cover the creation of a simple choropleth image that can be used as a graphic or figure in a document.
Preparing a Project Directory
When starting a project, you should first create a project directory on your desktop to store the files you will use in that project. Maps created with ArcMap commonly require a number of different files, and it will be helpful to keep them together in a single directory.
The election data for this tutorial is available here as a zipped shapefile. A shapefile common format for distributing geospatial data that was originally developed by ESRI (the company that also created ArcMap) in the 1990s. A shapefile is actually a collection of files that have a common name, but different suffixes like .shp and .dbf.
Shapefiles are commonly distributed in a .zip archive to keep all the different files together.
You can then unzip the shapefile by opening the WINDOZE EXPLORER, browsing the desktop, and double-clicking on the .zip file. Select all the files, copy them, go back to your project folder and paste them there.
Adding a Shapefile To an ArcMap Map
Open ArcMap and select BLANK MAP.
Click the plus sign in the toolbar to Add Data.
To access the contents of a folder, you must Connect to Folder to make that folder or directory visible to ArcMap. It is usually easiest to just connect to your desktop so that you can access all folders on your desktop.
Then navigate to your project directory and Select the .shp file for the data you want to map, and this will display the data.
You will want to zoom into your area of interest if you are not mapping the entire area. Use the zoom tool that looks like a magnifying glass with plus sign in it.
Save Your Workspace
Save your workspace file to your project directory. ArcMap is an old, buggy program that crashes frequently and at inopportune times, so you should save your work frequently to avoid wasting irreplaceable hours of your life.
Workspaces are saved as .mxd files. This file contains the information ArcMap needs to create the map.
Unlike a word document or excel spreadsheet, the .mxd file only points to where data is, it does not contain the data needed to create the map. If you just send someone a .mxd file without the data they will not be able to view the map. You also need to send them the data files.
Data sets are large and frequently stored in databases rather than in simple files like shapefiles. Also, there may be many different maps or versions of maps referring to the same data. Therefore it would be waste of system resources to store multiple copies of the same data for each map. While this practice of separating data from how it is rendered can cause confusion, this is an efficient and flexible means of handling data when procedures are in place for controlling data.
Changing the Projection
By default, ArcMap will display the data as if it were drawn on a grid of longitudes and latitudes. Because of the shape of the earth, this tends to stretch the width of areas as you move farther north or south from the equator.
To make the shapes look more realistic or accurate we use something called a projection.
The top element in the table of contents (the panel on the left side of the screen) is the data frame that contains all elements for this map.
Right click on the data frame and go to properties and the Coordinate System tab.
The Continental projections under the Projected Coordinate Systems list contains common projections used when creating maps of specific continents.
For this map of North America, we choose the continent of North America and use the North America Albers Equal Area Conic projection. This commonly-used projection minimizes distortion of area although it distorts the shape of the country into curves.
Save your work.
Coloring The Polygons With A Categorical Variable
To vary the colors of states according to the data in the shapefile, right-click on the layer and select Properties.
On the Symbology, choose the way the features will be displayed. In this case, we are using the categories of Democratic and Republican, so we use the Categories, Unique Values symbology.
The Value Field select the variable used to control the symbology. For this example, we use the the winner of each states electoral votes in 2012, which is called, appropriately, WIN2012.
Click Add All Values to fill the legend.
Double click the patches if you want to change them to an appropriate color. In recent years it has become common to map Democrats in blue and Republicans in red.
Click off All Other Values so those are not included in the legend.
When you click OK, you see a choropleth.
Save your work.
Rename the Layer
We usually want the layer to have a more meaningful name than the name of the data file.
Modify the Properties of the layer, and On the General tab, give the layer that meaningful name, in this case 2012 Electoral College.
Thus far, we have been viewing the map in Data View, which as the name indicates, is the best view for working with data.
To setup how the map will look when it is exported to a figure, switch to Layout View. This view shows us what our final map will look like when it is exported to an image file or printed.
To create a figure that you will embed in a document, you should adjust the Page and Print Setup so your layout will appear at a similar size to the map you paste into your document.
Unclick Use Printer Paper Settings, then give the figure a custom width of six inches width and 4 inches height, which usually works fairly well for graphics in reports on letter size paper.
You should also select Scale Map Elements Proportionately To Changes in Page Size so the default map elements are scaled in size to fit the layout.
Drag the data frame to the edges to fill out the page.
Use the Zoom Tool (the magnifying glass with a plus sign) to zoom into the area of interest.
For simplicity with this example, we will just map the contiguous 48 states.
Save your work.
For a choropleth you need to Insert a Legend that indicates what the colors in the choropleth mean.
Give the legend a one-point wide border and a white background so it will be cleanly separated from the map.
Default legends have alot of extra elements in them that are not needed for a simple choropleth like this.
To further configure the legend, right-click the legend and select Properties.
On the General tab, click off Show Title since it is obvious this is a legend.
Select Items, Style, Properties and select the Horizontal Single Symbol Layer Name and Label style.
Other options may be more appropriate for different types of maps.
Also right-click on the data frame, change its Properties, and remove the Frame border from the data frame, which we won't need for a figure.
Save your work.
Exporting A PNG Image File
To embed this figure in a document, you need to File, Export the map to a PNG file. PNG stands for portable network graphics. This is a file format that usually looks good and works with a wide variety of online web apps and desktop software.
Change the resolution to 300 DPI, which will keep the image nice and sharp when it is inserted into the document. Using the default will make your map image fuzzy.
Put this file in your project directory and give it a meaningful name.
Save your work.
Inserting The PNG File Into A Document
In a word processing program or online document app like Google Docs or Office 365, Insert, Picture the PNG file you exported from ArcMap.
Center the image and give it a caption with a meaningful name and the source of the map data.
Save the file as a PDF file, and open the file to verify it looks the way you planned.
If you are sending a document like this to someone via e-mail or on a website, PDF is the preferred file format over .docx word documents because word documents can render differently depending on the version of the software the reader is using.
Double click the file in the Windoze Explorer to view the file in the PDF viewer installed on your machine.
Coloring The Polygons With A Quantitative Variable
It is also possible to make a choropleth that maps a quantitative, where numeric values cover a range.
In layer Properties, Symbology change to Quantities.
For this example the values we will map are the percent of the vote that went Democratic in 2012.
The default map may have a number of issues. For one, the default colors are not particularly suited to this variable.
For choropleths of quantitative variables, you generally want to use shades of a single color or shades moving between two colors so it is clear which are the higher and lower values.
Since Democrats are commonly represented as blue, we choose a Color Ramp that gives shades of blue.
As you can see, this choropleth shows that most states have large numbers of voters for both parties. While the social divisions in the country are real, they are not always as spatially explicit as we might imagine from a categorical map like the one we previously created.
Save this as a new workspace so you can recover your earlier categorical choropleth if you need to make changes to it.
Formatting Quantitative Legends
The default legends in ArcMap are often not appropriate for some variables. In this example, the extra decimal points in the ranges are unnecessary, and it would be nice to have percent signs.
Format the layer properties, right click on the labels and Format Labels.
Choose Percentage and under the numeric options, remove the decimal places. This gives a legend where the precision more clearly reflects the accuracy of the divisions between categories.
Because you have changed what this layer shows, you should change the layer name, which will also change the legend title.
You should also generally avoid overlapping the legend with the map content unless absolutely necessary. For this we zoom out and then pan the states over a bit.
Save your work.
Inserting The File Into A Document
As before, export a PNG and import it into a document that you can save as a PDF if needed.
Saving Your Project To a .zip Archive
If you are on a public computer, when you are done with your project you should archive the files so you can recover the project in case you want to make changes to it, or in case you want to start a new project based on this one.
Right-click on the folder and select Send To, Compressed (Zipped) Folder. This will create a new .ZIP file which you can rename if you desire.
You can then save that file to a flash drive or upload it to cloud storage, like Google Drive.
In case you need to restore your files on another machine, you can then download the .zip file and unzip it.