Standard 1.h: Read and interpret topographic
and geologic maps.
Topographic contours indicating surface relief. Image taken from US
Army training manual.
In this lesson, you will learn about the
ways to show three dimensions on a flat
map, how to read topographic maps and
other contour map types, and how to
draw contours.
Solid yellow lines represent surface isobars, line of equal air
pressure. Green dashed lines are geopotential heights, the
altitude at which 500 millibars of air pressure are attained. These
types of contour maps are used by meteorologists. The weather
system shown is the Storm of the Century, a powerful winter
storm called a nor’easter.
• Detailed maps showing
the elevations of hills
and valleys of an area.
• Use lines, symbols, and
colors to represent
changes in elevation and
features on
Earth’s surface.
Contour Lines
• Contour lines are used to
represent the three
dimensional surface of the
world on a flat map.
• Can also be called isolines
or isopleths.
• A contour line connects points of equal elevation.
• Elevation refers to the distance of a location above
or below sea level.
• Contour lines never cross.
Index Contours
• contour lines marked with elevation
Contour Intervals
• distance in elevation change between each
contour line
Uphill
direction
Rapid change in
elevation.
Contour lines
Close together.
Slow change in
elevation.
Contour lines
far apart.
The contour interval is 20 ft. Point elevations are:
A = 700 ft
B = 740 ft
C = 770 ft
D = 820 ft
Spacing between Contour Lines
• Widely spaced
contour lines
show a gentle
slope.
• When they are
close together,
the slope is
steep.
• When the contour
lines are close
together at the
top of a hill, the
hilltop is pointed.
• When the
contour lines are
widely spaced,
the hilltop is flat.
1. Contours are imaginary lines that join equal
values of whatever is being mapped. For
example, the 100 ft contour marks all areas on
a map that are 100 ft in elevation
2. Contour lines are defined by a contour interval.
The contour interval tells you the amount of
elevation change between contour lines.
3. The contour interval for a map is constant
unless otherwise noted.
4. When contour lines
are close together, the
change in elevation
increases very quickly
(steep).
5. When the contour
lines are spread apart,
the amount of
elevation change is
small (flat).
N
You are to hike to the top
of Blake Island.
Which side of Blake
Island would be the
steepest climb, the west
coast or the north coast?
(The west coast because the
contour lines are closely
spaced. You have a very
large amount of change in
elevation over a short
distance. Go to the north for
a more gentle climb)
W
Contour maps allow you
to interpret the “lay of
the land”. From
surveyors to soldiers, the
ability to read the
topography in a
topographic map is
essential. Review the
following topographic
maps and learn to
identify the features
shown.
The V’s point upstream
in a draw. Where you
see a draw in a
topographic map you
can reasonably assume
there is or has been
water flowing that led
to the erosion and
migration of the
contour lines.
Match the letter and number
•
•
•
•
•
•
•
Topographic (relief) maps
Bathymetric (sea floor elevation) maps
Isohyet (rainfall) maps
Isopach (rock or sediment thickness) maps
Isotach (wind speed) maps
Isobaric (air pressure) maps
Geopotential height (elevation of a given air
pressure) maps
• Very common topographic map
• Entire US covered by these
maps
• Size of each maps is 7.5
minutes by 7.5 minutes (30
minutes make 1 degree of
longitude or latitude)
• Scale of these maps is 1:24,000
(1 inch = 2000 ft/24,000 in)
• Also known as a topographic
quadrangle
Bathymetry map. Shows depth of seafloor.
Isobars (solid yellow lines)
on an isobaric map.
This is a strong low pressure
system, which produced
large amounts of snow and
wind across the eastern US.
In an isobaric map, where
isolines are close together,
winds are the strongest.
Winds are strongest in
Virginia, North Carolina,
and West Virginia.
Green dashed lines are
geopotential heights, the
altitude at which 500
millibars of air pressure are
attained.
Isohyet map. Rainfall
distribution on
August 17th – 20th,
2002 (isohyets are in
millimeters)
Isohyet map
showing predicted
rainfall in inches
Isopach lines (contour interval = 5 ft) show the
thickness of a rock layer known to produce oil.
The round symbols are oil well locations and
the depths (6000 – 7000 ft) drilled to reach oil.
Isopach map showing
thickness of the limestone
comprising the Florida Aquifer
Colored areas represent isotachs, lines encompassing areas of equal windspeed. Contour interval =
10 knots. The red area is a fast region of wind comprising of the polar jet stream. It flows from west to
east, something that you cannot infer from this map unless the mapmakers were to plot wind barbs,
arrows to indicate the direction of wind flow.
Green lines are geopotential heights (air pressure).
• Topographic maps and most
other maps include both
human-made and natural
features that are located on
Earth’s surface.
• These features are represented
by different symbols.
• A map legend explains what the
symbols on a map represent.
• When using a map, you need to know how to
measure distances.
• A map scale is the ratio between distances on a
map and actual distances on the surface of
Earth.
• There are three types of map scales: verbal scales,
graphic scales, and fractional scales.
– A verbal scale expresses distance as a statement, such as
“One centimeter is equal to one kilometer.”
– A graphic scale consists of a line that represents a certain
distance, such as 5 km or 5 miles.
– A fractional scale expresses distance as a ratio,
such as 1:63 500.
What does it mean if a map says
“Scale 1:100 000”?
This fractional scale means that one unit on the map
represents 100 000 units on Earth’s surface. For
example, one inch on the map would equal 100 000
inches on Earth’s surface.
This is a map of
geopotential heights.
Contour interval is 60
meters.
Note that the wind barbs
in this map convey the
direction of wind flow,
which is predominantly
west to east at this
altitude.