Assignment+04+Histograms+%26+Measures+of+Spread

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This lab is being used and was modified with permission from Gary Jacobson of Grossmont Community College. GEOL101 Dynamics of the Earth Name: Laboratory 1: Topographic Maps . . . There’s Ups and Downs Section: Learning Outcomes: ● Explain what contour lines are and why they are useful on flat maps ● Apply the concept of contour lines to visualize elevational patterns over a region ● Use map scales to estimate distances within a given region Introduction to Topography and Contours Most maps focus on the horizontal “lay of the land,” but topographic maps are also designed to communicate local variations in elevation of the land surface using contour lines . Examine the three- dimensional perspective drawing of a volcanic island in Figure 1A. While you can identify higher and lower areas based on the perspective, the exact elevations above sea are not shown and only about half of the island is visible. In Figure 1B, contours lines that connect locations of the same elevation have been added to this perspective drawing, but the view is still incomplete. If you were walking along a given contour line, you would not increase or decrease in elevation. Note that the contour lines are regularly spaced every 10 feet and do not cross one another. In Figure 1C, the view has been rotated such that you are looking directly down on the entire island . . . while you lose the three-dimensional perspective in the previous views, you can now see the entire island and how each contour line closes on itself. Also notice that only every fourth contour line is bolded and labeled (i.e., 0’, 50’, 100’); these are known as index contours . Figure 1: A. Three-dimensional view of a volcanic island. B. The same volcanic island with contour lines drawn every 10 vertical feet. Notice that the lowest contour or zero contour is at the base of the island where sea level would be. C. Topographic map of the volcanic island with index contours labeled. The contour interval is the elevation difference between each contour line, and this difference is nearly always kept constant throughout an entire map. The contour interval in Figure 1C is 10 feet. Contour spacing visually conveys the relative steepness of an area: Widely spaced contours represent a shallow or gentle slope (Figure 2A and 2B, Box C), while closely spaced contours represent a steep slope (Figure 2A and 2B, Box D).

Figure 2: A. Topographic map of a cinder cone at Lassen Volcanic National Park in northern California. B. 3-D relief map with topographic overlay of the same locations as A. Notice that the contours in Box C are spaced farther apart than the contours in box D in both maps. A useful concept for interpreting contours is called the “the rule of V’s.” When a contour crosses a valley, it has to turn to maintain its elevation and then return on the other side of the valley. On the map, this turn and return of the contour produces a “V” that points towards higher elevations (i.e., uphill or upstream). Figure 3A is a topographic map of mountainous terrain with a stream (yellow line) flowing from the top to the bottom of the map. Figure 3B is the same area shown in a 3-D relief map view. Notice that the contours “V” in the upstream or uphill direction in both views Figure 3: A. Topographic map with a stream in the middle in yellow. Lines 1 and 2 are ridges. B. 3-D relief map with contours draped over the terrain of the same location as A. Lines 1 and 2 are the same ridges as in A. Conversely, when a contour crosses a ridge, it will turn along the ridge to maintain the same elevation and then return on the other side of the ridge. On the map, the contour would also make a “V” but this time it would point downstream or downhill. Line 1 on Figure 3A is drawn along the apex of a ridge. This same line in Figure 3B shows the same ridge in 3D relief. Notice that the contours “V” or point down elevation. Line 2 in Figure 3A and B is also along a ridge. Understanding the difference between a ridge and a valley or stream is very important when reading a topographic map.

Common Features of Topographic Maps Topographic maps typically include a variety of additional information within and outside of the map borders (See Figure 4). Inside the map, roads, buildings, and other human infrastructure may be shown if the map’s scale (discussed below) is fine enough. Outside the map borders, various descriptive information about the area and the nature of the map is provided along with magnetic declination and scale information, which are discussed in detail below. Magnetic declination : Next to the data and projection information in the lower left corner, an arrow pointing toward the geographic North Pole indicates “True North.” Usually there is another arrow which points toward the Magnetic North Pole. These two arrows are drawn from a single point, and the angular difference between them in degrees is known as the magnetic declination . Since the Magnetic North Pole moves slowly over time, the date of measurement of the declination is indicated beside the arrows. Magnetic declination is very important to know because 1° of latitude is equal to 69 miles on the Earth’s surface. Figure 4 is the USGS topographic map of La Mesa, CA. Notice in the enlargement that the magnetic declination is 11.5°. Ratio S cale : Of all the features of a map, scale is one of the most important and relates the distance on the map itself to the actual distance. This relationship is typically expressed as a ratio; for example, a 1:10,000 scale means that one unit of measurement on the map (say, 1 inch) is equal to 10,000 inches on the map (= 833 feet). One weakness of just showing the scale ratio by itself is that any reduction or enlargement of the map will introduce substantial errors in estimated distances; thus, most maps also include a bar scale with distance marks to minimize such problems. Bar scale : In this method a line or bar is drawn on the map. The actual length of the line or bar is the map distance. It is divided and calibrated according to the ground distance. On USGS maps more than one bar scale is usually shown, each one calibrated in different units-miles, feet, kilometers (Figure 4). To find the distance between two places, any piece of paper can be used, the map distance marked, and the ground distance read directly by placing the paper against the bar scale. This type of scale also has the advantage of remaining valid if the map should have to be enlarged or reduced for any reason, since the map distance and the length of the bar would both be changed equally.

Figure 4: USGS 7.5-minute quadrangle topographic map. Enlarged area showing magnetic declination, ratio scale, and bar scale.

Lab Activity Questions 1-7. Warm up – Match the contour map (1-6) to its corresponding mountain range (A – G) in the table provided. 1. 2. E 3. 4. 5. 6. A Choose your own adventure – See Map 1 at end of Handout You and a friend are hiking the Pacific Crest Trail (gray dashes) from Campo, California to Manning Park in British Columbia. You are hiking north and plan to go over Glen Pass (11,920’, blue star) tomorrow. You met a group of backpackers that just spent a couple days in Sixty Lake Basin (shaded purple) and highly recommend the detour. You are a day ahead of schedule and decide to take the recommendation. However, you would like to make it an adventure and do some off-trail hiking. Your goal for the day is to make it over Glenn Pass (blue star, lower left of map 1) and find a route into Sixty Lake Basin at the yellow star. Before you start route finding let’s get acquainted with the map. Use Map 1 (Mount Clarence King, CA) at the end of this handout to answer the questions below. Question 7. Complete the table below using Map 1 Contour interval (don’t forget the units) 80 feet Magnetic declination in degrees Direction of north on map Direction of south on map Direction of east on map Direction of west on map Highest elevation on map Lowest elevation on map

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