Marginalia
Why before where
Describe the theme or purpose of your map as the title and the location or region of the map as your subtitle or secondary clause.
In narratives, start with a topic sentence that introduces your research question or purpose. Make a promise or set up before you deliver or pay off.
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Scales of thought
Geographic thinking often requires people to think across scales.
Identify the scales of thought that your map should support. Rather than trying to make one map that helps people think at all of these scales, design separate maps for each scale of thought.
For example, when planning a trip, a person will need to compare attributes of different regions to figure out where to go. When they make the trip, however, they will need finer details, such as street names, to figure out how to get there.
Where in the world?
Many people suffer from geographic illiteracy. They do not do well when asked to situate a country, mountain range, or river on the globe.
Your audience should gain a sense of geographic context and understand what part of the world the map shows them. Show how your region of interest connects to a smaller scale map, or “locator” map.
Reference scale
Is it important for your audience to understand your map’s scale? Do they need to reason about distances? If so, did you include a scale bar or labeled distances directly onto connecting features like roads or provided some other means for directly representing the map’s scale? When choosing increments, did you use round numbers with intervals that reflect how people think about distances?
For many small scale maps, scale will vary across the map depending on the projection. In these cases, it is often best to omit a scale bar.
Dictionary definitions
A map key or legend essentially acts like a dictionary, providing definitions for symbols.
Organize your definitions as a dictionary: first state the term needing definitions, then state the definition. In legends, this translates to symbol on left, label on right. The symbol-labels should align horizontally on a row. Columns of symbol-label pairs should also be aligned: the symbols vertically, the labels left-justified.
Tobler meets Sullivan
When designing a legend, consider following a mixture of Tobler (near things are more related than far things) and Sullivan (form should follow function).
Organize your symbol-label pairs into groups that will help someone use your map. Identify the criterion or criteria that best relates the map features. Is it by type hierarchy (these thing are all kinds of roads), geometry (these things are all line features), or purpose of the map readers (there things are all about driving)? Keep in mind that there may be levels to your legend’s organization (first purpose, then geometry, then type hierarchy).
Define in context
Symbols on a map usually appear in context with other symbols and a visual background, yet legends often define symbols isolated from this graphical context.
Consider designing your legend so that symbols appear in graphical context. For example, show how the road looks against the background where you typically find the road. Or create more expressive graphical legends that act as diagrammatic models. For example, show roads, creeks and bridges together.
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