# What are Contour Lines

Contour lines are lines drawn on a map
connecting points of equal elevation. If you walk along a contour
line you neither gain or lose elevation.

Picture walking along a beach exactly where the water meets the land (ignoring tides and waves for this example). The water surface marks an elevation we call sea level, or zero. As you walk along the shore your elevation will remain the same, you will be following a contour line. If you stray from the shoreline and start walking into the ocean, the elevation of the ground (in this case the seafloor) is below sea level. If you stray the other direction and walk up the beach your elevation will be above sea level (see diagram at right).

The contour line represented by the shoreline separates areas that have elevations above sea level from those that have elevations below sea level. We refer to contour lines in terms of their elevation above or below sea level. In this example the shoreline would be the zero contour line ( it could be 0 m, 0 ft., or something else depending on the units we were using for elevation).

Contour lines are useful because they allow us to show
the shape of the land surface (topography) on a map. The two
diagrams below illustrate the same island. The diagram on the left
is a view from the side (cross profile view) such as you would see
from a ship offshore. The diagram at right is a view
from above (map view) such as you would see from an airplane flying
over the island.

The shape of the island is shown by location shoreline on the map. Remember this shore line is a contour line. It separates areas that are above sea level from those that are below sea level. The shoreline itself is right at zero so we will call it the 0 m. contour line (we could use m.,cm., ft. in., or any other measurement for elevation).

The shape of the island is more complicated than the
outline of the shoreline shown on the map above. From the
profile it is clear that the islands topography varies (that is some
parts are higher than others). This is not obvious on map with
just one contour line. But contour lines can have elevations
other than sea level. We can picture this by pretending that
we can change the depth of the ocean. The diagram below shows
an island that is getting flooded as we raise the water level 10 m
above the original sea level.

The new island is obviously smaller than the original
island. All of the land that was less than 10 m. above the
original sea level is now under water. Only land where the elevation
was greater than 10 m. above sea level remains out of the water.
The new shoreline of the island is a contour line because all of the
points along this line have the same elevation, but the elevation of
this contour line is 10 m above the elevation of the original
shoreline. We repeat this processes in the two diagrams below.
By raising water levels to 20 m and 30 m above the original see
level we can find the location of the 20m and 30 m contour lines.
Notice our islands gets smaller and smaller.

Fortunately we do not really have to flood the world
to make contour lines. Unlike shorelines, contour lines are
imaginary. They just exist on maps. If we take each of the
shorelines from the maps above and draw them on the same map we will
get a topographic map (see map below). Taken all together the
contour lines supply us with much information on the topography of
the island. From the map (and the profile) we can see that
this island has two "high" points. The highest point is above
30 m elevation (inside the 30 m contour line). The second
high point is above 20 m in elevation, but does not reach 30 ft.
These high points are at the ends of a ridge that runs the length of
the island where elevations are above 10 m. Lower elevations,
between the 10 m contour and sea level surround this ridge.

With practice we can picture topography by looking at the map even without the cross profile. That is the power of topographic maps.

**READING ELEVATIONS**

A common use for a topographic map is to determine the elevation at a specified locality. The map below is an enlargement of the map of the island from above. Each of the letters from

AtoErepresent locations for which we wish to determine elevation. Use the map and determine (or estimate) the elevation of each of the 5 points. (Assume elevations are given in feet)

Point A = 0 m

Point A sits right on the 0 m contour line. Since all points on this line have an elevation of 0 m, the elevation of point A is zero.

Point B = 10 m.

Point B sits right on the 10 m contour line. Since all points on this line have an elevation of 10 m, the elevation of point B is 10 m.

Point C ~ 15 m.

Point C does not sit directly on a contour line so we can not determine the elevation precisely. We do know that point C is between the 10m and 20 m contour lines so its elevation must be greater than 10 m and less than 20 m. Because point C is midway between these contour lines we can estimate the elevation is about 15 feet (Note this assumes that the slope is constant between the two contour lines, this may not be the case) .

Point D ~ 25 m.

We are even less sure of the elevation of point D than point C. Point D is inside the 20 m. contour line indicating its elevation is above 20 m. Its elevation has to be less than 30 ft. because there is no 30 m. contour line shown. But how much less? There is no way to tell. The elevation could be 21 m, or it could be 29 m. There is now way to tell from the map. (An eight foot difference in elevation doesn't seem like much, but remember these numbers are just an example. If the contour lines were spaced at 100 m intervals instead of 10 m., the difference would be a more significant 80 m.)

Point E ~ 8 m.

Just as with point C above, we need to estimate the elevation of point E somewhere between the 0 ft and 10 ft contour lines it lies in between. Because this point is closer to the 10 m line than the 0 m. line we estimate an elevation closer to 10 m. In this case 8 m. seems reasonable. Again this estimation makes the assumption of a constant slope between these two contour lines.