North Seattle Community College's
PHYSICAL GEOLOGY 101
Instructor:  Tom Braziunas

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TOPOGRAPHIC MAP EXERCISE

@1996 -- The information contained in this document is copyrighted.
No reproduction may be made without prior approval from the author.

 

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I. A Tutorial

A topographic map is a very accurate graphic representation of the three-dimensional configuration of the earth's surface. This map is generally used by surveyors and engineers as well as hikers and back packers. It establishes both horizontal and vertical control. A topographic map may or may not include shaded relief to help visualize steep slopes.

Horizontal control gives correct location, scale and directions on a map. A geographic grid system of latitude and longitude (in degrees) is accurately established by field surveys.  Each degree is divided into 60 minutes (60'),and minutes are divided into 60 seconds (1' = 60"). Two types of scales are shown: (1) a precise representational fractional scale expressed as a ratio, for example, 1:24,000 means one inch on the map represents 24,000 inches in reality; and (2) a linear (bar) scale is an approximate practical scale that helps one measure distances (for example, a bar showing that 1 inch equals approximately 2 miles).

Vertical control uses contour lines to show correct elevation and the shape of the terrain. A contour line connects all points of equal elevation. It is determined by fixing certain elevations in the field and measuring up or down from these points. These control points are established permanently by using a metal marker set in concrete with the elevation engraved in it -- called a bench mark (BM). Contour lines are always at right angles to the direction of the slope and, as stated, all points along each of these lines are at the same elevation.

This exercise begins your introduction to the "vertical control" of contour lines in, hopefully, an entertaining way by using topographic maps for the Seattle area.  You will gain some experience with the "horizontal control" of latitudes and longitudes from your second lab exercise this week.     

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II. Virtual Lessons:

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Here are a few external web sites with interesting and educational information on topographic maps (such as the Egyptian topographic map above).

PLEASE CAREFULLY READ THE INFORMATION ON TOPOGRAPHIC MAPS FROM THE INFORMATIVE WEBSITE BY MARK MCNAUGHT AT MOUNT UNION COLLEGE IN ALLIANCE, OHIO:
Mark's Topographic Map Tutorial
 

THE UNITED STATES GEOLOGICAL SURVEY WEBSITE ALSO HAS A CHART SHOWING THE COMMON SYMBOLS ON TOPOGRAPHIC MAPS: http://topomaps.usgs.gov/ .

In addition, here are a couple other interesting websites to check out:

 

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III. Online Map Exercises:

Answer the following fifteen questions on paper or in a word processor, then copy and e-mail your responses to me on the appropriate submission form (for Week 2 Lab Homework Part 1) in your In Box.  To avoid losing work because of a computer crash or Internet connection failure, do not compose your answers directly in the submission form.  Total points possible are 15. 

 

in your lab manual lesson to be extremely useful in order to answer these questions. 

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CARKEEK PARK QUESTIONS #1 - 6:

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To the left is a small portion of a topographic map showing a road and a creek (blue-green line) in Carkeek Park in Seattle.  North is always toward the top of topographic maps such as this.

Q.1: Which way is the creek flowing -- and why?

Hint: You will find the section on "Streams and Stream Valleys" in Mark's Map Tutorial to be especially helpful for deciding whether the creek is flowing north-to-south or south-to-north.

Q.2: What do the little black boxes denote?

Hint: There is a guide to "Topographic map symbols" on the U.S.G.S. website mentioned above. These squaares don't represent gas wells but instead are the same as the rectangles in the guide.

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More of Carkeek Park is displayed on the portion of topographic map shown below. Note that every fourth contour line is heavier and labeled with heights in feet (these heavier lines are "index contours"). The spacing between adjacent contour lines (all contour lines including the lighter ones) is called the "contour interval".

Q.3: What is the contour interval of this map?
      Hint: Carefully count the number of contour lines between index contours.
Q.4: What is the elevation difference between the two red arrows?
 
Q.5: Which direction does Pipers Creek flow, and why?
Hint: Use the elevation difference information you have just calculated.
 
Q.6: What might the large "25" on the map stand for?
Hint: Read about the Public Land Survey System at the Idaho State University GeoSTAC website and/or in the Wisconsin State Cartographer's website.

 

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GREEN LAKE QUESTIONS #7 - 9:

The contour interval on this map is 25 feet.

Q.7: What is the approximate elevation of Green Lake?
           Hint: Find the closest contour line and determine its elevation.
 
Q.8: What is the approximate elevation of Bagley School?
          
Q.9: Which corner of the map shows the highest elevation (and what is the elevation)?
           Hint: Look for the index contour line closest to the corner and then determine the elevation of the contour line in the very corner.
 

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BLAKE ISLAND QUESTION #10:

The next map shows Blake Island in Puget Sound. By now, you have learned how to read that the highest point on Blake Island is between 250 and 275 feet. The contour interval of the map is 25 feet and the index contours are every 100 feet. The numbers "25" and "36" are related to the Public Land Survey System. The next two maps will illustrate some additional interpretations of contour lines.

Q.10: From which direction would it be easiest to climb from the shore to the top of Blake Island?
  • a.  From the Northwest
  • b.  From the Southwest
  • c.  From the Southeast 
  • d.  From the Northeast
WHY?: __________________
 

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GRAND CANYON QUESTIONS #11 - 12:

This topographic map shows the Isis Temple, a high-elevation (7012 foot) butte in the Grand Canyon. You will notice that the contour lines are closer at some elevations (the steep cliffs) and more broadly spaced at other elevations (the gentler slopes).

Q.11: What is the contour interval, and how far apart are "index contours"?
         Hint: Remember that the contour interval is the elevation difference between contour lines.
                  The "index contours" are the darker, labeled ones.

Q.12: Approaching from the south, the steepest cliff occurs between which adjacent INDEX contours?

 

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FLORIDA QUESTIONS #13 - 14:

This map shows a sinkhole region in Florida. Note that hachure marks mean that the contour lines drop in elevation instead of rise. Some of these sinkholes are deep enough to be filled with water to become lakes.

 

Q.13: What is the contour interval, and how far apart are the "index contours"?

Q.14a: What is the elevation at the tip of green arrow #1?
Q.14b: What is the elevation at the tip of green arrow #2?
Q.14c: What is the elevation at the tip of green arrow #3?

 

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SNOQUALMIE PASS QUESTION #15:

The next question links a topographic map and a (beautiful) visual image of the landscape. Shown is one of the peaks near Snoqualmie Pass. Study the contour lines on the map and the slopes of Mt. Guye in the photograph.

(Photograph and photo retouch by Frances Brooks for Tom Braziunas)

 

Q.15: Which arrow (A, B, or C) points to the side of Mt. Guye facing the viewer in the photograph, and why?
         Hint: We are looking at a steep face of the mountain and there is another peak to the left in the photo.

 
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