BrodieBaileTopographicMapsLab

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School

Colorado State University, Fort Collins *

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Course

121

Subject

Geology

Date

Apr 3, 2024

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pdf

Pages

13

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Name __________Brodie_Baile______ GEOL 121 Lab Section _____________ 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: 1
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. 2
GEOL 121 – Topographic Maps Lab 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. 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? The lines are spaced out more, less evidence of elevation changes 2. Create a hill in the sand. Draw the shape of the contour lines below. 3. Create a valley in the sand. Draw the shape of the contour lines. 3
GEOL 121 – Topographic Maps Lab 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. B can see everyone, as they are the most elevated A would have a good chance of seeing everyone too, but depending how short b is, b might be excluded from a’s visuals Both d and c would not be able to see each other or b because of the elevation between them, depending how short a is, a might be excluded from c and d’s visuals 5. In the diagram below, match the numbered contour lines on the left to corresponding landform on the right. 1=b 2=e 3=d 4=c 5=f 4
GEOL 121 – Topographic Maps Lab 6=a Part II: Using Topographic Maps to Investigate Geologic Questions 5
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