Topographic Mapping
What is a topographic map?
A
map is a way of
representing on a twodimensional surface, (a paper,
a computer monitor, etc.)
any real-world location or
object
Most maps only deal with
TWO dimensions.
 Topographic
maps
deal with the third
dimension by using
contour lines to show
elevation changes on
the surface of the
earth or below the
surface of the ocean.
Maps
that show
the surface of a
body of water
are called
bathymetric
maps.
 Bathymetric
map of Hawaii
How do I read a
topographic map?
The
concept of a
topographic map is,
on the surface,
fairly simple.
Contour
lines placed
on the map represent
lines of equal elevation
above (or below) a
reference datum.
(more on that coming
up…)
Contour lines look like
this…
To
visualize what a
contour line represents,
picture a mountain (or
any other topographic
feature) and imagine
slicing through it with a
perfectly flat, horizontal
piece of glass.
 The
intersection of the
mountain with the glass is
a line of constant
elevation on the surface of
the mountain and could
be put on a map as a
contour line for the
elevation of the slice
above a reference datum.
The darker, thicker contour
lines are…
…index
contour lines.
These have elevations
printed on them,
periodically, over their
length.
Between
each index
contour are four
intermediate
contours that are
thinner lines than
the index contours.
Important!!!!!!
Contour
lines will
NEVER cross or
touch!!!! They might
LOOK like they’re
touching, but they’re
not.
Okay…going back…what was
that “reference datum” stuff…
A
reference datum is
a known and
constant surface
which can be used to
describe the location
of unknown points.
On
Earth, the normal
reference datum is sea
level.
On other planets, such
as Jupiter or Mars, the
datum is the average
radius of the planet.
Topographic
maps
come in quadrangles.
The title of the
quadrangle is printed
in the upper and
lower right corners of
the map.
The
titles of
adjacent
quadrangles are
printed around the
edges and at the
corners of the map.
The
legend and
margins of
topographic
quadrangles contain
a myriad of other
useful information.
Township
and range
designations, UTM
coordinates, and
minute and second
subdivisions are
printed along the
margins of the map.
 The
Universal Transverse
Mercator (UTM) is a geographic
coordinate system. (Don’t worry
about it right now…)
The
elevation change
between the
intermediate
contours is what is
given in the map
legend.
So, if
the contour interval
listed in the map legend
is forty feet, each
intermediate contour
represents forty feet and
the elevation change
between index contours
is 200 feet.
Some
maps will
have supplementary
contour lines
representing smaller
vertical distances.
They
will be dashed
lines and the
supplemental contour
interval will be listed
below the regular
contour interval in the
map legend.
A
final type of contour
that may appear on a
topographic map is a
line representing a
closed depression (such
as a sinkhole or a crater
at the top of a volcano).
These
contours will
be hachured (they
will have small tic
marks perpendicular
to the main contour
line), with the tic
marks pointing
downslope.
On a map, it looks like this…
These are the most important
lines you need to know…
Grid systems

A grid system allows the location of a
point on a map (or on the surface of
the earth) to be described in a way
that is meaningful and universally
understood.

Projecting the earth’s surface (or a
portion of it) allows for a
representation of an area on a flat
piece of paper.
Different types of grid systems…
Geographic:
Uses
degrees of latitude and
longitude. This is the
one we will be using.
UTM: Preserves shape,
allows for precise
measurement.
State
Plane: Used
for local surveying.
Public Land Survey:
Used in Colonial
America; not very
accurate.
Latitude
Lines of latitude are
also called parallels.
Latitude lines run
east to west or,
horizontally.
Each
degree of
latitude is
approximately 69
miles (111 km) apart
To remember latitude,
imagine them as the
horizontal rungs of a
ladder ("ladder-tude").
Degrees
latitude
are numbered
from 0° to 90°
north and south.
Zero
o
(0 )
degrees
is
the equator, the
imaginary line which
divides our planet
into the northern
and southern
hemispheres.
90°
north is
the North Pole
and 90° south
is the South
Pole.
Longitude
Lines
of longitude are also
called meridians.
Longitude lines converge
at the poles and are
widest at the equator
(about 69 miles or 111
km apart).
Zero
degrees
longitude is
located at
Greenwich,
England (0°).
The
degrees
continue 180° east
and 180° west
where they meet and
form the
International Date
Line in the Pacific
How do they work together?
To
precisely locate
points on the earth's
surface, degrees
longitude and latitude
have been divided into
minutes (') and seconds
(").
There
are 60
minutes in each
degree. (like
minutes in an hour)
Each minute is
divided into 60
seconds.
Seconds
can be further
divided into tenths,
hundredths, or even
thousandths.
For example, the U.S.
Capitol is located at
38°53'23"N ,
77°00'27"W
YOU
would read that
as 38 degrees, 53
minutes, and 23
seconds north (of the
equator) and 77
degrees, 0 minutes and
27 seconds west (of
the Prime Meridian).
Creating a topographic profile
A
topographic
profile helps
understand what
topographic maps
represent.
A
topographic
profile is a crosssectional view along
a line drawn through
a portion of a
topographic map.
 If
you could slice through a
portion of the earth, pull away one
half, look at it from the side, the
surface would be a topographic
profile.

http://geology.isu.edu/geostac/Field_Exerci
se/topomaps/topo_profiles.htm