TOPOGRAPHIC
MAPS AND
WATERSHED
BOUNDARIES
Some of the material in this presentation is
copyrighted by others and is used under the
doctrine of fair use for nonprofit
educational purposes.
READING TOPO MAPS
Topo maps show
lines of equal
elevation
Also show
landmarks,
roadways, high
points, and
waterways
READING TOPO MAPS
Lines close
together
represent rapid
changes in
elevation—steep
slopes
Lines that are
spread out
represent flatter
areas
READING TOPO MAPS
Lines close
together
represent rapid
changes in
elevation—steep
slopes
Lines that are
spread out
represent flatter
areas
READING TOPO MAPS
Lines close
together
represent rapid
changes in
elevation—steep
slopes
Lines that are
spread out
represent flatter
areas
READING TOPO MAPS
 Usually only the
major increment
lines are labeled
Contour Interval = 10
Major Increment = 50
READING TOPO MAPS
 Usually only the
major increment
lines are labeled
 Use the next labeled
contour to tell
whether adjacent
lines are higher or
lower
READING TOPO MAPS
 Use location related
to waterways to tell
if adjacent lines are
higher or lower
FOLLOWING DRAINAGE PATHS
Not all
waterways are
marked on the
map
“Upward-facing”,
pointed curves in
contour lines
usually represent
drainage paths
FOLLOWING DRAINAGE PATHS
Not all
waterways are
marked on the
map
“Upward-facing”,
pointed curves in
contour lines
usually represent
drainage paths
FOLLOWING DRAINAGE PATHS
Not all
waterways are
marked on the
map
“Upward-facing”,
pointed curves in
contour lines
usually represent
drainage paths
FOLLOWING DRAINAGE PATHS
Not all
waterways are
marked on the
map
“Upward-facing”,
pointed curves in
contour lines
usually represent
drainage paths
FINDING RIDGE LINES
Land ridges run
along higher land
dividing
waterways
Ridges show as
“downwardfacing”, rounded
bends in contour
lines
FINDING RIDGE LINES
Land ridges run
along higher land
dividing
waterways
Ridges show as
“downwardfacing”, rounded
bends in contour
lines
FINDING RIDGE LINES
Land ridges run
along higher land
dividing
waterways
Ridges show as
“downwardfacing”, rounded
bends in contour
lines
FINDING HIGH POINTS
High points or
peaks show as
closed ellipses
WHAT IS A WATERSHED?
 The land area that contributes runoff to a point of
interest (the outlet)
 Watershed boundaries defined by the high points and
ridges surrounding the stream network draining to
the outlet
WATERSHED DELINEATION
1.
2.
3.
4.
Identify the outlet location
Identify all drainage ways that lead to the outlet
(start at the outlet and work upstream)
Identify ridge lines and high points just outside the
drainage network
Connect your ridge lines and high points —stay
perpendicular to the contour lines
1. IDENTIFY THE OUTLET
 Following along with the example using the topo map
shown in your handout
 The outlet for this small watershed is shown with a
black dot
 In the “real world” you choose the outlet location to
suit your project
2. IDENTIFY THE DRAINAGE WAYS



Start at the outlet and work upstream
Sketch in marked and unmarked drainage ways
Remember– look for sharply -pointed, upwardfacing curves in the contour lines
3. IDENTIFY RIDGE LINES
 Look at the areas just outside your drainage network
and identify ridge lines
 Also note high points adjacent to your drainage
network
 High points will show as small ellipses
 Ridges show as downward facing, smooth curves in
the contour lines
4. CONNECT THE BOUNDARY
LINES
 Connect ridge lines and high points to complete the
watershed boundary
 Try to stay perpendicular to the contour lines
EFFECTS OF URBANIZATION
 Human activities can have marked impacts on the
flow of water across a land surface
 Many types of construction or land development can
alter drainage paths and watershed boundaries
EFFECTS OF URBANIZATION
Roadways,
railway
embankments,
and grading for
construction
can create
barriers to
water flow
EFFECTS OF URBANIZATION
Ditches and
other artificial
drainage ways
may move
water along
unnatural paths
EFFECTS OF URBANIZATION
Buried storm
sewers may move
water in
directions that do
not follow
topography
The city
maintains sewer
maps that show
drainage
direction
NEED FOR GROUND TRUTHING
 Topo maps give a first estimate of watershed
boundaries
 Areas that are developed should be field checked to
verify that the watershed has not been modified
 Presence of underground storm sewers
 Culverts under road or rail embankments
 Surface ditches that re-route flow
EFFECTS OF DATA RESOLUTION
 The number of elevation data points in the area of
interest impacts the topo map
EFFECTS OF DATA RESOLUTION
 How does this affect watershed delineation?
 The Mario example:
EFFECTS OF DATA RESOLUTION
The more refined the grid, the more detail. This
provides a more refined watershed boundary.
EFFECTS OF DATA RESOLUTION
The more refined the grid, the more detail. This
provides a more refined watershed boundary.
EFFECTS OF DATA RESOLUTION
Important to choose appropriate resolution for
watershed size
http://kel.tamu.edu/webwatershed/
DIGITAL AGE
 Can use computer software to determine contours
and watershed boundaries
 Requires background knowledge of software
 AutoCAD/ GIS (e.g. ArcView)
FINAL COMMENTS: SCALE

A scale of 1 : 24,000



Has no units!!
The map user selects the units
Could write this as:





1
1
1
1
unit on the map = 24,000 units on the ground
inch on the map = 24,000 inches on the ground
foot on the map = 24,000 feet on the ground
cm on the map = 24,000 cm on the ground
Counting squares

Engineering graph paper


Big squares are 1” x 1”
Small squares are 0.2” x 0.2”
FINAL COMMENTS: SCALE

Example:
1. Measure distance between
points on map
In this case we know one small
square on engineering paper is
0.2”
0.2”
0.2”
on the graph paper
FINAL COMMENTS: SCALE

Example:
1. Measure distance between
points on map
2. Multiply by ratio
0.2” on the paper x 24,000 =
4,800” on the ground
0.2”
0.2”
on the graph paper
FINAL COMMENTS: SCALE

Example:
1. Measure distance between
points on map
2. Multiply by ratio
0.2” on the paper x 24,000 =
4,800” on the ground
3. Convert to desired units
0.2”
4,800”x (1’/12”) = 400’
0.2”
on the graph paper
FINAL COMMENTS: SCALE

Example:
1. Measure distance between
points on map
2. Multiply by ratio
3. Convert to desired units
4,800”x (1’/12”) = 400’
0.2”
4. Calculate area
0.2”
on the graph paper
400’ x 400’ = 160,000 ft²