Quick and Dirty Thematic Mapmaking for Everyone

A thematic map (wiki link) is a map that illustrates a particular theme.  Most of the "informative" maps we see are thematic maps: weather maps, maps of political results, maps of bicycle paths, etc.

In this article, we'll take a step-by-step look at how to make a thematic map  - a choropleth - using free source graphics and free software. A choropleth, for those of you who aren't geography geeks, is just a map where information is presented by shading or coloring different sections of the map. A choropleth is a great way to visualize data about specific geographic areas: states on a national map, counties on a state map, or neighborhoods on a city map.

There are basically three things you'll need to create a simple thematic map:

1. Basic graphic design / image editing software
2. Base map of the area(s) you wish to display
3. A schema for actually drawing the map - for example, which states get which colors

Let's look at each of these in more detail before we start making our first map.

Software

The basic process for creating a quick and dirty thematic map is to find a source map to use as a foundation, and then edit it and customize it using traditional graphic design / image editing software.  We don't need any special mapping or geographic software, nor any special design skills: the point of doing things quick and dirty is that we can do this using software we already have, without having to go out and buy Photoshop or something similar.

For the purposes of this article, we'll be using MS Paint, the simple graphics program that has been bundled for free on every Windows computer for the past 20 years.  If you're using a Mac, you'll have to find an equivalent free or cheap program depending on what software was bundled with your computer - some simple graphics programs ranging from free to inexpensive include Poster Paint, Seashore, the drawing program in AppleWorks, and PhotoEdit.  For Linux users, there are a wide variety of free graphics programs - for something like this, I recommend kolourpaint.

If you've absolutely never worked with graphic design software before, you might need to spend some time in the Help files/documentation for your particular software.  In general, what we'll demonstrate in this article is very simple stuff that anyone can do with a bit of practice. 

Base map

Since we don't want to have to draw a map of all of our states, or counties, or townships by hand, we'll want to find a base map to use as a source - something which we'll turn into a customized thematic map.  We've got a few concerns here when we're looking for our base map:

• scope - the map has to be appropriate for the geographical area we're mapping; for example, we can't make a decent map of the local tri-county area's community centers when we're starting with a map of the entire United States.
• appearance - we don't want to use, for example, a Google Map: the default Google Map has road symbols and highways and other extra features that will only complicate our mapmaking.  We're looking for a simple map, as visually plain as possible.
• availability/license - maps are copyrighted material, so we need to be careful that we're not violating copyright by unintentionally pirating a proprietary map.  This means we can't just scan in a map from our favorite atlas or encyclopedia and start drawing on top of it.

Luckily for us, there are several places to get suitable base maps for many regions of the world (including virtually all of the U.S. and Canada).

The first site to look at is the free media repository at Wikimedia Commons.  The Commons, operated by the same nonprofit foundation that runs Wikipedia, is a repository of images, audio, and video that are all freely available for us to use under permissive copyright licenses.  The Commons maintains an entire section just for maps, known appropriately as the Atlas.  When looking for a specific area, I find it's helpful to just use the search feature to search for a specific phrase, such as "Maps of Illinois" or "Counties of Ohio."

Many maps are available at Wikimedia Commons, including physical, topographical, political, and socioeconomic maps for regions around the world.  There are extensive collections of township, county, and state maps for the entire US, as well as many regions of Canada.

The US government provides a National Atlas, which includes a printable map generator.  The maps provided by the National Atlas are a bit "rough around the edges" aesthetically when compared to some of the attractive and polished maps available on Wikimedia Commons.  Most of these rough spots can be polished up using the techniques mentioned later in this article. Because the National Atlas is provided by the government, all of the content available on it is in the public domain and freely available for us to use.

Despite having a few graphical rough spots, the National Atlas generator is still helpful because of its ability to display many political boundaries and other features for the entire US.  For instance, only some states have congressional district maps on Wikimedia Commons, but you can generate those maps for the entire country using the National Atlas.  In addition to being a source for base maps, the National Atlas is a very valuable resource in its own right because of the many data sources that can be integrated into its maps.

For some projects, especially those on a more local level, base maps might not be easily accessible online.  If you're seeking township or sub-county maps, it might be helpful to contact your local county surveyor's office or clerk to see if any maps are publicly available online or in person.  The geography departments of local universities, as well as extension / agricultural coop offices, are also good sources of map data.

For the purposes of this article, we'll be using a map of Illinois counties from Wikimedia Commons.

Map schema

Maps usually either ask questions or tell a story - thematic maps usually do the latter.  Our map might ask "are there any regional trends in state-by-state funding of after-school arts programs?"  Our map could also tell a story: "as you can see, our county ranks lower than every surrounding county in funding public health awareness programs."  Before we even open our mapping software or try to find a base map, we should know what question or story our map is intended to address. 

The data we have to work with, that we intend to display, dictates our choice of base map as well as the schema we use to draw our thematic map.  For example, if we're trying to illustrate county-level funding of a program, and we have data showing each county's funding level for the current year, we need to find a county-by-county base map and choose a coloring/drawing schema that shows the differences in funding.

Our data might be numerical or quantitative: for instance, a table of funding levels that we need to illustrate in a spectrum from low to high.  Our data might also be categorical or status-related: for instance, a list of states categorized by their adoption of a certain policy or program.  We can create a thematic choropleth map with either type of data.

Of the two types of data, it's easier to create a schema for categorical data.  One popular example is a map of election returns, sorting the states into the familiar red state / blue state categories depending on whether they voted Republican or Democrat in the previous presidential election.  Devising a schema for this type of data is as simple as choosing a color for each category and then coloring each division accordingly.

To map numerical data, we must first create a scale for the quantities involved, which depends on what question or story our map is to address.  For example, Illinois has 102 counties.  If we have a table of statistics for Illinois counties, we could evenly divide them up into three cohorts of 34 counties apiece, labeled "low," "medium," and "high."  We could alternatively devise a set of value ranges to sort the counties into - once we decide on those, we've basically turned the problem into a categorical map schema where the categories are the ranges of values.

One way of considering the schema for a thematic map is to think of what the map "key" would be.  For the election example above, it's very simple: red states voted Republican and blue states voted Democrat.  The USGS water withdrawal map, on the other hand, uses a spectrum of numeric values to determine an area's color, which illustrates the gradation of the numeric data.

Building the map

So at this point, we presume you know what your map should address.  You've gathered your data and you've found a suitable base map that you can use to start your thematic map.  It's time to fire up your image editing software and make a map!

For the example map in this article, we'll be randomly assigning colors to a map of counties in the Chicago metropolitan area.  There are a few maps of Chicagoland in the Wikipedia Commons, but I'm going to start with a county-by-county map of the entire state, because it's the largest and simplest.  (tip: graphics, including custom maps, look better if you draw them really big and then shrink them down to your ideal size when done.  this allows all of the rough spots to be smoothed out during the shrinking process.)

To start my map, I open this base map image in my image editor

and zoom in so that I can see it.  Since I'm only focused on the Chicago region, I use the select tool to draw a

box around the Chicago metro counties.  I then "crop" the image so that only that box remains, and everything outside of it disappears.

At this point, I "mask" the counties I wish to keep by using the "Fill" tool to completely fill them with a bright blue color, in a similar way to the green-screen or blue-matte-background video production technique used by weather broadcasters on TV. 

I then use the Fill tool to fill the extraneous outer counties with black, which essentially blends them into the background black of my image.  Then I recolor the counties I intend to map with white so that I have a plain template of the local counties.  (tip: this is useful to save as  an intermediate image so you can create similar maps in the future.)

At this point, it's time to implement our schema.  I'm going to use the Fill tool again, coloring each county the appropriate color that corresponds to my categories or numeric values.  For this example, I'm picking colors and counties at random :)

Now it's time to do some touchup work.  I Fill the background bright blue this time, which also ends up filling the border lines between counties with blue.  Then I zoom in and use the Line tool to reinforce all of the borders around and between counties with a thick black line.  This is easy if your counties are demarcated by straight lines.  If you have to trace rivers, or in this case, Lake Michigan, it helps to zoom in and be very patient :)

Once I've done my touchup work, I Fill the background with white to get rid of the blue, and then zoom back out.  At this point, my map is nearly done, but it's not very informative yet.  I need to add some explanatory text and some labels using the Text tool.  I also use the Square tool to draw some sample color patches for the map key, illustrating what each color on the map represents.  Finally, I copy the logo off of my organization's website and Paste it into my image so that everyone knows who made this map.

The result is an informative thematic map developed using only free or already-obtained data, software, and associated techniques!  I can now save this image and use it on my website, in publications, on T-shirts and coffee mugs, or wherever else it can help tell my organization's story!