Topographic Maps
A Topographic map, or contour map is
used to show the shape of the Earth’s
surface.
Elevation is the height above or below sea
level. On a topographic map, contour lines
are used to represent the elevation. The
contour lines connect points of equal
elevation.
Reading Contour Lines
• The difference in elevation between two consecutive lines
(lines next to each other), is called the CONTOUR INTERVAL
• If the contour interval is not listed in the legend, then
subtract the difference in the value of two neighboring
contour lines. -OR• If every contour line is not
numbered, subtract the
difference in the value of
two neighboring contour
lines and divide that
number by the number of
spaces between the known
contour lines.
The “Have to Knows” about Contour Lines
• On every topo map, there is a directional
compass that shows you north, south, east and
west.
• There is also at horizontal distance located at the
bottom of the map. This is a map scale used to
show distances across the Earth’s surface.
• On most topo maps, every 4th or 5th contour line
is an index contour and is thicker or bolder.
Continued “The Have To Knows”
• When there is a series of contour lines that are successively
getting smaller and smaller in size, it means that the elevation
is increasing toward the top of a hill or mountain.
• When contour lines cross a
stream, they bend upward
toward the higher elevations
and “point” upstream.
Continued “The Have To Knows”
• The closer the contour lines are to each
other, the steeper the slope.
• Depression contours show that the
elevation is decreasing. These are
shown with small lines that look
like tooth combs called hatchers.
The first depression contour
line has the same elevation
value as the last regular
contour line.
• Often on topo maps, there is a location
labeled X or BM X with a elevation value.
This spot is the location of a bench mark
and the exact elevation.
Continued “The Have To Knows”
• The highest elevation of a hill
can be calculated by finding the
last (highest) contour line on
that hill, and then figuring out
the next line that would
be drawn. The highest possible
elevation of the hill is just below
the value of that next line. The
highest possible elevation of the
hill indicated by the arrow is
239 meters.
• The last line indicated on the map is 220 meters. The next line would be
240 meters (remember that the contour interval is 20 meters). Since there
is no 240 meter line, the hill cannot be higher than 239 meters!
How to Make a Profile
• What is a profile?
A profile is a side view of an area’s landscape. This
shows the upward and downward changes in elevation
across a horizontal distance on the Earth’s surface.
• When drawing a profile, you need to have a starting and an
ending location.
• On the topo map, mark off both of these locations with an
X and X’.
• Connect a straight line from X to X’. On this line, put a dot
where ever a contour line crosses it. On a separate piece of
paper, transfer these dots, including the starting and ending
locations. After the dots are transferred, make note of each
elevation according to the topo map for the corresponding
dots.
What is gradient and how
do we find it?
Gradient is the rate of change from place
to place within the field, also called slope.
The formula is in your reference tables
located on the front cover.
Gradient = change in field value
distance
(horizontal distance)