The meaning of symbols and the symbolic presentation of data is not clear until explained and correlated to a key.
The essence of maps is to present features (points, lines, polygons) or phenomena based on their classification, count, or amounts. Without a legend that explains what the representation means the user will be clueless as to what they are looking it.
Maps that show operational data almost always need a legend to explain the meaning of this content. This is especially true when the app is being displayed in full map or partial map layout. Typical visualization types that require legends are choropleth, unique symbols, counts and amounts (size/color).
Display a legend that shows the key to the currently active data content only; there is no need to show every symbol used on the map. Relate map symbols with the values that are used to calculate the size or color. Place the legend in a container that is floating on the map, mostly positioned in a corner adjacent to the map navigation.
- A legend does not need to be titled “Legend” and often don’t need a title at all. The only exception are choropleth maps where the title is important to depict the meaning.
- For quantities consider also showing the units if they are not evident already.