Topographic Map Contour Lines

Contour lines are the greatest distinguishing feature of a
topographic map . Contour lines are lines drawn on a map
connecting points of equal elevation, meaning if you
physically followed a contour line, elevation would remain
constant. Contour lines show elevation and the shape of
the terrain. They're useful because they illustrate the
shape of the land surface -- its topography-- on the map.
Here's a cool way to understand how to interpret contour
lines: Take an object like a ball or a pile of laundry , and
shine a red laser pointer along the object's side. The line
you see will look like a contour line on a topographic map.
In order to keep things simple, topographic maps show
lines for certain elevations only. These lines are evenly
spaced apart. We call this spacing the contour interval. For
example, if your map uses a 10-foot contour interval, you
will see contour lines for every 10 feet (3 meters) of
elevation -- lines at 0, 10, 20, 30, 40, and so on. Different
maps use different intervals, depending on the topography.
If, for example, the general terrain is quite elevated, the
map might run at 80- to even 100-foot (24.4- to 30.5-
meter) intervals. This makes it easier to read the map --
too many contour lines would be difficult to work with.
Look in the margin of your map to find out its contour
interval.

To make topographic maps easier to read, every fifth
contour line is an index contour. Because it's impractical to
mark the elevation of every contour line on the map, the
index contour lines are the only ones labeled. The index
contours are a darker or wider brown line in comparison to
the regular contour lines. You'll see the elevations marked
on the index contour lines only. To determine elevations,
pay attention to the amount of space in between lines. If
the contours are close together, you're looking at a steep
slope. If the contours have wide spaces in between -- or
aren't there at all -- the terrain is relatively flat.