The thin brown lines snaking around a topographic map are called contour lines. All points along the same contour line are at the same elevation above sea level. Think of a contour line as a closed loop. By following a contour line on the ground, you would travel neither uphill nor downhill, eventually ending up back at your starting point. A line marked "6500," for example, means that point on the map is 6,500 feet above sea level.
Contour lines allow you to infer general terrain characteristics from their patterns. For example, lines crowded close together mean steep sections. Lines spaced widely apart indicate more gentle slopes. When studying topographic lines, be sure to take into account the contour interval found in the legend.
Be careful, however: Because contour lines are measured in regular intervals, you can find a wide elevation variation in the terrain between two contour lines.
Contour Lines vs. The Real World
As reliable as a topographic map may be, it can't tell reveal every detail of the terrain. A 20-foot-high cliff or a 30-foot deep ditch could lie between two 40-foot contour lines ~ and you wouldn't know it because it won't be indicated on the map in any way.
A route that climbs up a gentle creek to the ridgetop may look ideal on the map ~ only to be an impassable tangle of downed trees left by a winter storm when you try to hike it. Flexibility, then, is the key to navigation using any map.
Index Lines vs. Interval Lines
You'll notice both thin and thick contour lines on a topo map. The thick lines are known as "index lines" and are labeled with a number revealing the elevation. The thinner, unmarked contour lines between the index lines are called "interval lines." Use the elevation marked on index lines to calculate the elevation of interval lines.
For example, an index line marked "6500" means everything along that line is 6,500 feet above sea level. On a map with a 40-foot contour interval, the interval line to the inside of the index line would be at 6,540 feet above sea level. The interval line to the outside of the index line would indicate 6,460 feet above sea level.
The following examples show how some commonly encountered terrain features might look on a topographic map: