Data Literacy: Top Five Tips for Interpreting Maps

It’s data literacy time again, and this month, instead of talking more about how to interpret data when it’s laid out in tables or cited in text, we’ll go over some ideas of what to do, and what not to do, when interpreting data that’s presented geographically. That’s right: maps.

There are those among us who are in, or who are entering, fields where creating and interpreting maps will be professionally or academically relevant. But even for those of us for whom that’s not the case, map use is a life skill.

The obvious use is for navigation. Walk-through navigational tools available on your phone or built into your brand-new car are great, but if you forgot to charge your phone, or if your car is not actually brand-new, being able to read a road or transit map with minimal panic turns out to be really helpful. Also, thematic maps, or maps that illustrate information other than or in addition to navigational aids, crop up frequently in news and media articles.

So to help you with interpreting both navigational and thematic maps like a pro, we’ve curated our top five cartographic tips.

1. Read the legend.
This may seem like an obvious place to start, but it’s hard to overstate how important this is. The legend tells you what you’re looking at, by providing a key to what the map’s visual elements (symbols, colors, overlay patterns, etc.) actually mean. In the case of thematic maps, it also shows you the units that are being displayed.

Consider the difference between population (number of people) and population density (number of people per defined unit of area). Both of these are mappable concepts, and it’s important to be clear on which you’re looking at. A hypothetical neighborhood might have 500 residents – or it might have 500 residents per acre. Depending on the size of the neighborhood, these are clearly very different things.

Similarly, a map can show population growth, which can be measured numerically (e.g., 0-100 new residents, 101-500 new residents, 501-750 new residents), or by percent (e.g., 0-5% growth, 6-10% growth, 11-15% growth). Which method is best suited to a given map depends on a lot of things, but as the reader, you’re not responsible for that decision – you’re just responsible for checking the legend to see which measurement you’re looking at.

2. Pay attention to scale.
Scale is important for both navigational and thematic maps. When you’re reading a map intended for navigational purposes, the scale (usually shown in a scale bar) connects the distance portrayed on a map with real-world distance: whether one inch on the map equals one mile, five miles, 10 miles, and so on. This will give you an idea of not only how far away your destination is, but also how long it might take you to get there.

Thematic maps generally also have scale bars, and it’s important to consider physical scale there as well. But on thematic maps, scale also connects back to reading the legend. For example, a population map might be presented with a color saturation scale, where more saturated shades correspond to a greater population. The scale in the legend will confirm if that’s what the map is showing, and will tell you by how much the population is greater in one area than another.

3. Don’t enter with assumptions.
Thematic maps can use color, saturation, patterning, and intensity, in addition to other visual elements, to communicate their data. And some of these, as in the example above of increasingly saturated shades of the same color showing a larger and larger population, act as pretty intuitive visual cues. Another example is a color scale that progresses from cool to warm colors (e.g., blue, green, yellow, orange, red). On thematic maps that illustrate things like poverty, unemployment, or days per year of poor air quality, which can be generally understood to be things that it’s better to have less of, not more, the cool to warm scale has automatic connotations about the intensity of the measured problem in a given area. Following this model, an area mapped in red generally has more days per year of poor air quality than a different area mapped in orange, and many more than a third area mapped in green. But however intuitive the visual cues seem to be, do not assume that the map is portraying what it looks to you like it is. The person who created the map may not have the same assumptions and visual cues as you do. Aside from that, a map may have a color saturation scale or a cool-to-warm scale for operational reasons that are totally unrelated to its content: because it fit the overall design of the document it’s within, because the person who made the map made an arbitrary selection, or because an ink cartridge in the printer was empty. In cases where it was an operational decision rather than a substantive one, the information on the map might not be communicating increase or intensity, and may not even be quantitative at all. This, again, circles back to Tip #1 – take a minute, double-check, and read the legend.

One more assumption not to make: look for the north arrow. The default assumption is often that north is up, and that a north arrow, if present, would point to the top of the page. However, in the case of maps like site plans or other relatively small or specific areas, that’s not always true. We recommend finding the north arrow at the outset, when you’re looking at the legend and the scale. It’s a quick step that can save you a lot of confusion later.

4. Enter with focus.
If you’re using a map for a specific research or fact-finding purpose, keep in mind the question you started with. Large, complex, or highly detailed maps can contain an overwhelming amount of information. This can be true of topographic maps, detailed site plans, or even navigational maps. It’s easy to make the mistake of thinking that, in order to get any useful information out of the map, you need to process all of the information that’s in the map. But if you go to a topographic map looking for the elevation of the northernmost point in Tolono, you don’t need to understand the comparative elevations of the whole county. If you’re reading a site plan looking for the height of a section of fencing, you don’t need to interpret the entire contents of the site plan. And if you’re looking at a road map for the best route for a specific trip, you don’t need to know the course of every road in this part of the state. Remember the question you started with and the information you’re actually after, and it can help you be less overwhelmed, distracted, or intimidated by the big picture.

5. Filter with caution.
There are lots of online mapping tools out there, with lots of different data sources, topics of interest, and publishers, and many of them allow you to work through their interface to customize your area of interest and filter the available data to show just what you’re looking for. And this is great.

However, data portrayed on a map is the same as data shown in a table, particularly when what’s being shown are estimates: filtering to greater levels of detail often means that you’re filtering to smaller sample or population sizes, which can result with poorer-quality data (e.g., larger margins of error). So even though you may be interested in a single Census block, and even if the data appears to be available at such a fine-grained level, we recommend the following. First, check the data source. Second, check whether the data displayed is a count or an estimate. Finally, check how margins of error are handled, if applicable, to make sure you aren’t sacrificing overall data quality for geographic detail. In some cases it may be better to look at a slightly larger area in order to get significantly more reliable data.

So to wrap up, maps are useful, informative, and, yes, still relevant. Just take a few minutes when you first start looking at a map to check key factors of units, scale, visual elements, and data source. With that, you should be able to avoid major interpretation issues (and maybe even get around without your GPS).

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