It’s possible to have two maps with the same roads, borders, labels, and symbols and still have one show the viewer what they’re supposed to look at and the other have the viewer looking everywhere. Visual hierarchy is the arrangement of map elements by significance; accomplished via contrast, color, size, line weight, and placement.
One way to begin developing your visual hierarchy is to draw a grayscale sketch of your map with only three values. Choose the darkest value for the theme/route/layer you want to emphasize most, a mid-tone value for secondary layers, and a light value for the base layer. Don’t add any embellishments at this stage. Look at the sketch from far away or zoom out on your screen. You should be able to see your main theme before you can read the labels or see any minor roads.
Line weight affects visual hierarchy quite a bit. You might have a major road use a heavier or darker line than local roads. You might decide to use a lighter or dashed line for administrative boundaries. If all lines have the same weight, then primary and secondary elements become difficult to distinguish from each other. It can also cause boundaries, rivers, and roads to blend together despite being colored differently.
Color can play a role as well. Bright, saturated colors draw the eye, especially when they fill large spaces. Use bright colors only for important themes, and use muted colors for the background geography. If you’re using a color ramp, test whether your hues remain distinct in grayscale. Doing so will help you determine if your visual hierarchy relies only on the colors you’ve chosen, and it can also help you identify categories that are too visually similar to each other.
Symbol size and label placement are important, too. Large symbols could obscure nearby roads or boundaries. Large place names could overpower your thematic data. Create a consistent scale for symbols based on importance or quantity, and create a set of label styles that correspond to the category of name. Move labels off of key intersections, and don’t use boldface type for all place names because emphasis loses its power when everything is emphasized.
Examine your map once it’s complete, without looking at the title. Ask yourself where your eye goes first, second, and third. Does it follow the order you want it to? Maybe it’s your theme, then major cities, then the rest of your base map. If something like a minor road, a north arrow, or a boundary gets in the way, reduce its contrast until it plays a secondary role again.
