Line weight defines how prominent a line object looks on a map. A fat, dark line stands out, whereas a fine or light-colored line recedes into the background. Lines can represent roads, administrative boundaries, and rivers, but they have different purposes. Assigning different line weights to each category enables users to quickly distinguish among them without reading the legend.
Produce a sample map with multiple road categories, one large administrative boundary, several smaller administrative boundaries, and at least two rivers. Create three copies of the sample map. In each copy, vary only the line weight. Retain the colors, extent, labels, and background in all copies. This makes it possible to compare line weights without other visual factors affecting your assessment. Check every copy in the output format at the output size. Lines that look acceptable at full zoom can become too subtle or heavy when exported.
Roads should look important enough without taking over the map. Thicker lines can be used for primary roads compared to secondary roads, but the difference doesn’t have to be dramatic. Some lower-level roads may have to be omitted instead of faded into obscurity on a small-scale map. Multiple levels of roads can exist on a large-scale urban map, as long as there’s a discernible gradation and the roads don’t detract from the main theme.
Boundaries can be especially difficult to style since a strong line can separate the map more aggressively than necessary. Large administrative boundaries might require a bold line, while smaller ones should usually have a more subtle appearance. A common mistake for new designers is to make every boundary bold, since boundaries can look impressive on their own layer. When all layers are combined, however, boundaries can overwhelm fills, text, and roads. Borders can be differentiated using dashed lines, but you should make sure the dashes are legible at the final map size.
Rivers shouldn’t look like roads or borders. Blue or blue-gray lines are a good way to differentiate them, but the line width can also play a role. Main rivers can be bolder than tributaries to demonstrate the natural hierarchy within the drainage basin. Not every river needs to be equally bold, particularly if there are numerous smaller rivers in the dataset. Too much hydrographic detail can clutter valleys and cities with lines.
Review the line styles without looking at the legend. Roads should be recognizable as a network of paths. Boundaries should be identifiable as separations of space. Rivers should be obvious as natural waterways. Examine areas where lines overlap or run near each other. If a river looks like a road, or a small boundary is dominating the map, or minor roads obscure the map’s primary message, then the line weight is incorrect. No amount of additional detail or color will improve a poorly styled line weight.
