TopographicMapsLab_StudentHandout_v4 (1)
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Name __Logan Ondrak_
GEOL 121
Lab Section _Lab 21_
Topographic Maps
Lab
Why Use a Map?
With the click of a mouse, Google Earth
TM
and NASA World Wind provide satellite images of
any point on the planet. These and other tools allow us to build models of the landscape, draw
topographic profiles, measure straight-line distances and the length of meandering streams, and
estimate slope steepness.
Why are geologists interested in maps? We use them as the fundamental tool for
communicating information about the distribution of rock units and landforms. If you ever go to a
national park, you will find topographic and geologic maps of the park area prominently displayed at
the visitor center. In fact, you can even download an electronic version of the map showing your
neighborhood from the
US Geological Survey website
.
Figure 1.1.
Topographic map of the Colorado State University campus at Fort Collins.
Reading a Topographic Map
In the United States,
topographic quadrangle maps
are produced by the United States Geological
Survey (USGS). A quadrangle is a rectangular area of Earth bounded by north-south and east-west
lines. Usually the quadrangle is named after a prominent geographic feature (e.g., a mountain or a
town) in the map area. Quadrangles use many symbols for various natural and artificial features of the
landscape; these symbols are explained in a booklet published by the USGS and available online at
https://pubs.usgs.gov/gip/TopographicMapSymbols/topomapsymbols.pdf.
Each topo map includes the following features:
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GEOL 121 – Topographic Maps Lab
Map name and date
: On a topographic quadrangle, the name is printed in the lower right corner of the
map, and under it is the date when the map was compiled. Note that the name is also printed in the
upper right corner.
Scale:
This explains how large an area the map covers. Specifically, scale is the ratio of a linear
distance on the map to the corresponding distance on the surface of the earth. For example, if your map
scale is 1:50,000, it means that 1 inch on the map equals 50,000 inches on the face of the earth, or 1
centimeter on the map equals 50,000 centimeter on the face of the earth. There are four ways to
express scale.
As a ratio: 1:50,000
As a fraction: 1/50,000
Verbally: 1 inch equals 50,000 inches (or 1" = ~ 4167')
Graphically: Using lines marked in kilometers, meters, miles, or feet (scale bars)
Note that if you reduce or enlarge a map, the original fractional scale will no longer be valid, but a
graphical scale will still apply.
Contour lines:
Elevation (or altitude) is the vertical distance between a given point and a reference
elevation.
In most cases, the reference is sea level. Because a map is a flat sheet, some method is
needed to show different elevations on the map. Topographic maps show elevation by using
contour
lines
, which connect points of equal elevation. At every point on the 100-foot contour line, for
example, the elevation is 100 ft. If you walked along that line, you would not go either uphill or
downhill. The
contour interval
refers to the vertical difference in elevation between adjacent contour
lines. The contour interval for a given map is usually specified at the bottom of the map along with the
scale. Every 4th or 5th contour line is shown as a heavier line and labeled with its elevation. This is
called an
index contour
. Some rules for contour lines are listed below and illustrated in Fig. 1.2.
Contour lines never divide or split.
Contour lines never simply end; they either close or intersect the edge of the map.
A contour line must represent one and only one elevation.
A contour line can never intersect another contour line.
The contour interval must remain constant within a given area.
Closely spaced contour lines indicate steep slopes, and widely spaced contour lines show
gentle slopes.
When a contour line crosses a stream, it forms a V-shape that points upstream.
Figure 1.2.
This map has three index contours, a hill with a top elevation between 110 and 120
meters, and a stream flowing south. Note how the contours trace V-shapes as they cross the
stream. The elevation of point Q is 100 m; point R, 110 m; and point S, 65 m.
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GEOL 121 – Topographic Maps Lab
Part I: Visualizing Topography using Contour Lines
Overview
In this part of the exercise, you learn to read topographic maps by manipulating topography
using an AR Sandbox and studying an example of a topographic map from Colorado.
Learning Objectives
Visualize 3-dimensional topography using 2-dimensional contours lines
Investigating Topography
In this section of the lab you will use the Augmented Reality (AR) Sandbox to investigate how contour
lines reflect the topography of a land surface.
Take a minute to play around with the sandbox. Move the sand around to create different features in
the landscape. Notice how the contour lines appear as you change the landscape.
1. Flatten out the sand in the sand box. Notice how the contour lines appear. Now, create a steep slope
in the sand. How did the contour lines change when you created the slope?
as we flatten out the sand on the sandbox the contour lines are are very small there are slight
differences in height. After we make a steep slope in the sand there will be a center contour ring and it
will be leaning more to a side and will have a more lengthy slope on the opposite side of the steep
slope.
2. Create a hill in the sand. Draw the shape of the contour lines below.
3
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GEOL 121 – Topographic Maps Lab
3. Create a valley in the sand. Draw the shape of the contour lines.
4. Reconstruct the map below as best you can using the AR Sandbox. Imagine there are people
standing at each of the indicated points. Who can see who in this scenario? Explain your answer.
Assume a contour interval of 20 feet, answer, the people can use binoculars, and there is no vegetation.
Point A can see B, D, and C
Point B can see A, D, and C
C can see both A and B but cant see D as B is in the way of D
And D can see only A and B because B is in the way of C
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GEOL 121 – Topographic Maps Lab
5. In the diagram below, match the numbered contour lines on the left to corresponding landform on
the right
1B
2 E
3 D
4 C
5 F
6 A
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GEOL 121 – Topographic Maps Lab
Part II: Using Topographic Maps to Investigate Geologic Questions
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GEOL 121 – Topographic Maps Lab
Overview
On May 18, 1980, Lawetlat’la (Mt. St. Helens) in the state of Washington exploded in a cloud of ash,
plus lava and mud flows. What had been a beautiful symmetrical snow-covered mountain with heavily
forested slopes became a startling landscape of ash, mud, and downed trees surrounding a broken,
irregular peak. The power of the initial blast was directed upward and laterally, snapping off trees for
miles in the blast zone. In the years since 1980, many people – geologists, biologists, environmentalists
– have been observing and studying how the landscape recovers after a major volcanic eruption.
Screen shot from the Mt. St. Helens
WebCam
on February 8, 2023.
Learning objectives
Describe the shape of Lawetlat’la (Mt. St. Helens) before and after the eruption.
Draw two topographic profiles across the volcano and determine their vertical exaggeration.
Use your profile to estimate the dimensions of the material removed by the eruption and
calculate the volume.
Compare your result with published values and identify sources of error in your work
A Brief Chronology of the Eruption
In the month of March 1980, gas-rich magma rose beneath the volcano, causing the ground to
rise ~300 feet (like blowing up a balloon).
Small earthquakes were detected beneath the mountain, as
magma began moving toward the surface.
Older eruptions here had produced silicic lavas, so
geologists warned that there was a high likelihood of an explosive eruption. At 8:30 AM, on May 18,
two strong earthquakes caused by movement of magma triggered a large landslide.
This in turn
released the pressure on the magma and set off the eruption.
The side of the mountain burst
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GEOL 121 – Topographic Maps Lab
horizontally in a lateral blast of volcanic ash, and almost simultaneously, the top blew up.
There was
an enormous force to the blast (~25 megaton H bomb).
The mountain's symmetry was destroyed, with
a large crater forming at the top, and trees were knocked over for large distances.
The ash reached 16
km up into the atmosphere and darkened skies as far away as western Montana.
Ash was carried by
the jet stream all the way to the eastern part of the United States.
Hot ash melted the snow on the
mountain, and the mixture of ash and water produced destructive mudflows that extended miles from
the mountain.
In the subsequent years, Lawetlat’la (Mt. St. Helens) has occasionally produced small
earthquakes and ash eruptions, but nothing comparable to 1980.
Within the crater, a small bulge
has developed, called a lava dome, which indicates the mountain is still active, only biding its time.
Instructions
There are two topographic maps of Lawetlat’la (Mt. St. Helens) that accompany this exercise (separate
file). The first map shows the arrangement of contour lines around Lawetlat’la before the eruption,
while the second map shows contours after the eruption.
1. Study the first map of Lawetlat’la (before the eruption), and examine the contour lines closely.
Describe the shape of the volcano before it erupted, including its outline, general topography,
symmetry, and the slopes of its sides.
The first imagine of the volcano it has stable topography and has a center peak that is well
defined on the map Contour lines all make a ring and don’t open up ring now. Glaciers are
present and more visible in the first map.
2. Now study the second map of Lawetlat’la (after the eruption), and examine the contour lines closely.
Describe the shape of the volcano after it erupted, including its outline, general topography,
symmetry, and the slopes of its sides.
The contour lines now form a V instead of a ring and the outline of the bulge that was once
present is now a gaping crater that is forcing the topography of the mountain to break apart.
3. These maps show many glaciers surrounding the peak, some of which are labeled with their names.
Which of these named glaciers were totally destroyed by the eruption, as shown by comparing the
before and after maps? Which glaciers survived the eruption but were cut off from their source at
the top of the mountain?
Wishbone, Lesch, and Loowit were all the glaciers that got destroyed. And the ones that survived
were Talus, Toutle, forsyth, Nelson, ape, Shoestring, Swift, and dryer were all the glaciers that
survived the blast and still remained.
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GEOL 121 – Topographic Maps Lab
.
You will now use topographic profiles to estimate the amount of rock that was removed from the
volcano during the eruption.
What is a Topographic Profile?
A topographic profile is an illustration of the shape of the ground surface between two points. If
a topographic map provides a top view (i.e., what a bird would see if looking straight down), a
topographic profile provides a side view (i.e., what you might see along the edge of a cliff).
Applications of topographic profiles include engineering projects such as roads and pipelines and
scientific programs such as studying landforms or hydrology of an area. Geologists use topographic
profiles as a basis for geologic profiles, which indicate the position and orientation of subsurface rock
layers.
Figure 2.1.
Topographic profile across Antelope Peak. Screenshot from Google Earth.
Constructing a Topographic Profile
A
topographic profile
is a cross-section (vertical slice) that illustrates the topography between
two points. The profile traces the variations in elevation along the line between the points. To construct
a profile, follow the steps below. The process is illustrated in Fig. 2.2.
1. Draw a line across a topographic map connecting the two end points of the profile
line.
2. Mark the places where the line intersects contour lines.
3.
Project the elevation of each intersection to the correct level on the graph.
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GEOL 121 – Topographic Maps Lab
4.
Connect the points on the graph with a smooth curve. If two consecutive points lie
at the same elevation, bring the profile line slightly above or slightly below that
elevation in between the two points.
Figure 2.2.
Procedure for drawing a topographic profile.
4. Use the graph page (separate file) to draw a topographic profile along a line from point A to point A’
on the first topographic map (Lawetlat’la before the eruption).
5. Now draw another topographic profile along line B-B’ on the second map (Lawetlat’la after the
eruption), using the same graph. A and B are the same point and A’ and B’ are the same point. So when
you graph the second profile, start from the south end (at B’), and the profile should match the first one
up to the 2500-meter contour.
6.
If 1 cm = 150 m on the Y-axis of the profile graph, what is the fractional vertical scale of your
profiles? Hint: change 150 meters to centimeters.
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GEOL 121 – Topographic Maps Lab
1:15,000 is our vertical scale profile.
7. If 3.5 cm = 1 km on the map, what is the fractional horizontal scale of the profiles?
Hint: change 1 km to centimeters, then divide both sides of the equation by 3.5. Round your answer
to the nearest 1000.
1:100,000 is our vertical scale.
8. Topographic profiles are often drawn with a vertical scale that is much larger than the map scale.
Geologists draw profiles this way in order to bring out detail in the topography. This is known as
vertical exaggeration, and can be calculated by dividing the fractional vertical scale of the profile by
the fractional horizontal scale of the profile.
What is the vertical exaggeration of your topographic profile?
Hint: divide the
denominator
of the horizontal scale by the
denominator
of the vertical scale. You
can remember this as dividing the
larger
number by the
smaller
number, so that your answer is
always greater than 1. Round your answer to the nearest whole number
1:29,000 is our vertical exaggeration.
Compare Topographic Profiles
Look again at your two topographic profiles. In this section, you will estimate how much
material was removed from the top of the mountain by the explosion and eruption. Assume that the
upper part of the volcano (the part that was blown away) was a perfect cone. You will draw a
horizontal line across the profile to represent the base of the cone. Of course, the part of the mountain
removed by the eruption was not a perfect cone, so you will have to choose the best position for the
base line.
In the sketch below, the black line represents the original profile of Mt. St. Helens, and the red
line shows its profile after the eruption.
11
GEOL 121 – Topographic Maps Lab
Representative sketch of the topographic profiles and possible choices for
the base of the conical section removed by the eruption.
If you use the highest point of the after profile as the base of the cone (blue line), this would
correctly account for the area colored blue, which was removed by the eruptions. However, it would
omit rock below the blue line and above the red line, so that the calculated volume would be too small.
But if you use the lowest point of the after profile as the base of the cone (yellow line), this
would imply that everything shaded blue
and
yellow was removed, including all rock between the red
and yellow lines. Because these lower rocks were not removed, the calculated volume would be too
large.
You must choose a location for the base of the cone such that
the amount that is included but
should not be
is approximately the same as
the amount that is not included but should be
!
Representative sketch of the topographic profiles and one possible
choice for the base of the cone.
Vertically ruled area ≈ horizontally ruled area. Measure
h
using the scale
on the Y-axis and
d
using the scale on the X-axis.
10. Draw a horizontal line across your topographic profile to represent the base of the cone-shaped
mass of rock removed by the eruption. Measure the diameter
d
from side to side on the "before"
profile, using the scale printed at the bottom of the graph. What is the approximate value of
d
for the
cone on your profile?
D = 13 cm.
13 cm is our Diameter which equals the cone on mt st Helens is 377,000cm
(13x29,000)
3.77 KM is the height
d= 3.77 km
12
d
h
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GEOL 121 – Topographic Maps Lab
11.
Given the diameter
d
you found in the previous question, what is the radius of the cone? Hint:
d
=
2
r.
13 = 2r.
13/2 = 6.5 cm
3.77 km = 2r
r = 1.885 km for the radius
12.
What is the approximate height
h
of the cone? Measure the height from the highest point of the
mountaintop on the "before" profile to the base line that you drew for the cone.
Height cone is 90,000cm
(90,000/100,000)
H = 0.9
13. Make sure that
r
and
h
are expressed in the same units (both meters or both km). Use the formula
for the volume of a cone:
V
=
1/3
(
r
2
h).
What is your calculated volume for the cone that
represents material removed by the eruption? Show your work.
V
=
1/3
(
1.885
2
0.9).
1/3 (10.04651) =3.34
V = 3.34km^3
14. Open the fact sheet on Mt. St. Helens
http://pubs.usgs.gov/fs/2000/fs036-00/
, which summarizes
the 1980 eruption. Compare numbers on the fact sheet for the volume of mountain removed with the
value you found in the previous question. You will have to convert between metric and English
units. Is your calculated volume greater than, less than, or about the same as the number given?
My number is less then the perceived outcome of volume lost from the eruption. My estimate was 3.34
km^3 while in actuality the volume removed from the explosion was 4.2km^3
15. What are some possible sources of error in your calculation?
Some calculation errors could be wrong measuring size, not on point accurate numbers and also
calculation errors with finding out how tall or how wide the mountain originally was. Meniscal errors
can cause sources of error within my calculations.
13
GEOL 121 – Topographic Maps Lab
Part V: Reflection
1. What aspect(s) of this lab were easiest for you in this lab?
Understanding how topography works and how to graph topography and understand what the contour
lines mean.
2. What aspect(s) of this lab were hardest for you in this lab?
Nothing of this lab was hard for me.
3. What questions do you still have about topographic maps or contours?
One question that I still have about this lab would have to be Why was there a bulge on Mt. St Helens?
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