According to About.com, interpreting the black dots and dashes on a map requires checking the map's legend. The legend details what the individual symbols mean and lets users interpret the information contained in the map.
Depending on the type of map, dots and dashes could mean a wide variety of things. Dots often represent cities or towns, but in a topographical map, they could represent peaks or other features. Similarly, dotted lines often represent geographical boundaries, but could also indicate the presence of footpaths or trails. The legend of a map identifies every symbol found on the map and helps users understand what they mean.