A problem students often have when discussing typography related to cartography is the issue of readability vs. legibility. While this is not an attempt to cover the relevant literature, I do intent to provide examples to enable people to distinguish between the two.
Legibility can be defined as the ability a human reader to read something without effort. It can depend on many things. Often, the size of font chosen restricts legibility. For our purposes though, legibility is discussed in light of typeface choice.
Readability can be defined not on a letter by letter basis, but how he combination of letter are read within a larger body of text. In other words, readability is defined by the amount of effort one needs to make to read text, not single characters.
This may seem confusing at first, but looking an example of text below may help:
The above text is exactly the same in both cases, yet if one tries to read it, one finds some differences.The one on the left is a serif font (Times New Roman), while the one on the right is sans-serif (Helvetica). When reaching the end of each line, from my experience at least, it is easier to identify the correct next line in the text on the left. The one on the right creates some problems to read the line, even though letters are easier to understand. On the left we have what is called readable type, while on the right we have a legible type.
The use of lettering (and hence typography, legibility and readability) appears in many parts of a map: title, legend, labels and any other explanatory text (credits, projections, etc). Below are 4 examples from the major web mapping applications out there: Google Maps, MapQuest, Microsoft Live Search Maps and Yahoo! Maps. All four zoom in on a location on the University of Washington campus, showcasing labels for street features.
Which one is the best typographically is left to the reader. What is of note here though is that all labels, no matter what web mapping application you may use, come in only sans-serif fonts, encouraging legibility over readability. Labels not only need to provide a quick and easy way for us to identify specific features, but they also do not always follow traditional text in being horizontal. What is of note as well is the fact that the lettering is always pushed to fit within the width of the road features (not always successfully it seems). What most people note though is that labels of features tend to use non-serif fonts that allow readers to easily recognize names (as the purpose of labels is to quickly identify features by their name).
This can also be seen in the following two examples, even though they are much older and I focused on the title mostly. Here, the two maps are of Seattle, Washington, circa 1970 (USGS map) an 1917 (Automobile Blue Book, Volume 8 Section 2), both from the excellent Perry-Castañeda Library Map Collection at the University of Texas.
The above map is the USGS map for Seattle, Washington. The extracted version is of the southwestern most part of the map, to show the title and scale. Notice the map title is using a sans-serif font, as well as the scale and place names, like cities or airports. Serif fonts are used only in the names of islands and water features, like the Puget Sound or Fox Island. There is a mix of both types of typefaces that make distinctions that help the reader identify information.
The map above is the 1918 map of Seattle, WA cropped again to the southwestern most portion of the map to include the title and scale. Note that the title this time is serif along with other place names, while the street name and scale are using a sans-serif font. Again, a combination of the two that provides a visual hierarchy to the map reader that helps discern information faster and easier.
What sort of advice can I therefore give to a cartography student in regard to typography? Not much really. If we remember that cartography is an attempt to convey certain messages through visual hierarchies, then one needs to remember that sans-serif typefaces are more legible and therefore easier for spot reading, while serif typefaces are more readable and easier to read when one has long texts. As any cartographer knows though, a 72 points serif font on the map will be more legible than a 12 points sans-serif font, so when you choose your font, choose wisely: what typeface should you use, at which size and what weight, not independently from the rest of the lettering of your map. All lettering is related, and all together provide your visual hierarchy.
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A Typophile Discussion Thread about Cartographic Typefaces