Every fifth line in the sequence is a thicker line referred to as an index line, and somewhere along that line is the exact elevation. The lines work by connecting points on your map that represent the same elevation. Spaces with no lines indicate flat ground. Contour lines close together represent a steep increase or decrease in elevation, while lines further apart represent a gentle slope. The vicinity of contour lines to one another indicates the elevation lost or gained on any slope aspect.
The cornerstone for all topo maps is the contour line.
