Introduction to Topographic maps
Topographic maps are two-dimensional representations of the earth's surface,
showing the shape, size, and relative position of natural and man made features at a
greatly reduced scale.
Topography is the shape of
the land surface. Differences
in elevation are shown on
topographic maps by means
of contour lines, which are
lines connecting points of
equal elevation. The number
of contours used depends on
the relief (total difference in
elevation) of the area. A
mountainous area will
require many more contours
than a flat area of little relief
(if the contour interval is the
same for both).
All topographic maps use mean sea level as their reference line. Imagine an island in
the Atlantic Ocean. The water line on the beach would appear as a contour line
labeled "0". The rising land surface would be mapped by contour lines showing
increasing elevation and spaced accord to the contour interval (see figure 1 below).
When contouring, keep in mind the following points:






Contour lines never cross or intersect one another (but may merge to form a
single contour line where there is a vertical cliff).
The closer the contour lines are together, the steeper the slope.
Contour lines for most areas should be smooth, curving lines.
Every contour line closes on itself, either within or outside the limits of the
map.
Contour lines point upstream as they cross a valley.
Basins have hachure marks on the downhill side.
All standard topographic maps should have an orientation arrow (north), a legend,
a scale, and a method locating things geographically. A very common method of
geographic location is the latitude-longitude method. The earth is nearly a sphere
and therefore may be considered to be divisible into 360 degrees. By measuring
latitude and longitude in degree, minutes (1/60th of a degree), and seconds (1/60th of
a minute), it is possible to pin-point any spot on the earth's surface. Any point on
earth can be accurately defined in terms of a) its distance from the Prime Meridian
which extends N-S through Greenwich, England and the poles, and b) its distance
north or south of the equator. Lines connecting the poles are knows as longitude
lines or meridians; however, these lines are not parallel as they converge at the
poles. Lines parallel to the equator are know as parallels of latitude.
Topographic Profiles
Profiles or cross-sections are constructed across a contour map to provide a threedimensional view of the topography. The profile is actually a silhouette of the hills
and valleys as seen against the sky. Topographic profiles are easily made using
graph paper and the method depicted in this linked figure. Note that profiles need to
have both a horizontal scale and a vertical scale.