Lab 5: Topography and Topographic Maps

Transcription

1 SJSU Geol 4L Planet Earth Lab Lab 5 p. 1 of 8 Lab 5: Topography and Topographic Maps In this lab, you ll learn some of the tools available to help geologists recognize and interpret the topography of Earth s surface. Topography refers to the 3-dimensionality of the surface of a place hills, valleys, plains, streams, etc. For instance, the topography of Kansas is boringly simple because the area is so flat, but the topography of the Bay Area is highly varied. A topographic map is a two-dimensional (flat) representation of three-dimensional topography. Like a road map, a topographic map shows features laid out according to the chief compass directions N, S, E, and W, but it also shows information about the height of the land surface by using contour lines (commonly called contours). **A contour is simply a line connecting points of exactly the same elevation.** Topographic maps are essential tools for geologists, geographers, backpackers, forestry crews, surveyors, land planners, military personnel, engineers, emergency response teams, and anyone else who deals with features on the Earth s surface. I. Visualizing Topography Topography is notoriously difficult to portray in words or using two-dimensional surfaces like paper. However, we can use computer representations of topography that, although technically two-dimensional, fool our brains into "seeing" in three dimensions. The Gallery of Virtual Topography was developed by Professor Steve Reynolds at the University of Arizona. The site has many cool images and movies, but they take a while to load so PLEASE follow the instructions below (and those of your instructor). Click on the following links in the following order: Simple Hill Choose QTVR Movie Rotate the image to see how the contours reflect the real shape of this hill. Complex Valley Choose QTVR Movie Rotate the image to see how the contours reflect the real shape of this river valley (formally called a stream drainage). New River Mesa, Arizona Choose Movie with contours and water plane Drag the cursor up and down to cause sea level to rise and fall in this region. While 3-D visualizations help us imagine how contours express the true shape of part of the Earth's surface, most topographic information is expressed on 2-D digital or paper maps.

2 SJSU Geol 4L Planet Earth Lab Lab 5 p. 2 of 8 II. Latitude and Longitude The coordinates of a point on Earth s surface are its latitude and longitude. Latitude is expressed in degrees north (N) or south (S) of the equator, which has latitude 0. The north pole has latitude 90 N. Longitude is expressed in degrees east (E) or west (W) of the prime meridian the 0 longitude line that runs through Greenwich (GREN-itch), England. Degrees of latitude and longitude are subdivided into 60 minutes ( ), and each minute is divided into 60 seconds ( ). In other words, 1 = 60 = The coordinates of a point are its latitude and longitude. In the diagram at right, point A has latitude 40 N and longitude 30 W, generally just given in the form 40 N, 30 W. Point B s coordinates are 20 S, 15 E. 1. What are the coordinates of points C and D in the far-right diagram? point C: point D: 2. On the far-right diagram, place a dot labeled E at 30 N, 60 W. 3. On the far-right diagram, place a dot labeled F at 70 N, 45 E.

3 SJSU Geol 4L Planet Earth Lab Lab 5 p. 3 of 8 Glossary of terms and features you ll find on maps in this lab Map name: The name of the quadrangle is shown in the upper right corner. Map publisher: The publisher of the map is indicated in the upper left corner; in the U.S., topographic maps are published by the U.S. Geological Survey. Latitude and longitude: These are marked at the four corners of the map (in the white area just outside the map); intermediate values are shown at a few points along the map edges. Township, range, and sections: The U.S. government developed this method of subdividing land in the late 1700s so it could more easily distribute farmland to settlers. Townships are 1- mile-high horizontal subdivisions, ranges are 1-mile-wide vertical subdivision, and sections are the squares formed by the intersection of a township and range roughly 1 mile on a side. All township, range, and section information is shown in red ink on U.S. topographic maps. Scale: Every map is a scale model of the real world, so it must specify the ratio by which the real world was reduced to fit on the map. This ratio scale usually is printed just below the map. The most common scale for U.S. topographic maps is 1:24,000, which means that 1 unit of measurement (inch, cm, foot, etc.) on the map equals 24,000 of those same units in the real world. Maps also include one or more graphic (bar) scales in the bottom map panel, which you can use with a ruler to calculate distances between points. Contour lines: Imaginary lines that connect points of exactly the same elevation above sea level. A contour line separates areas above its elevation from areas below its elevation. On most U.S. maps, every fifth contour line is a bit thicker than other contour lines and is called an index contour. The difference in elevation of two adjacent contour lines is the contour interval, which is specified in the bottom map panel beneath the bar scales. Relief: The difference in elevation of two points. For instance, if a hill is 500 feet high and an adjacent valley floor is 100 feet high, then the relief between them is 400 feet. Control stations (aka benchmarks): Some points on the map have been surveyed precisely, and their elevations are shown as specific numbers, generally printed next to an X or a triangle. Symbols: Every map uses numerous symbols to convey information, but they aren t always explained on the sheet. Symbols are used for roads, bridges, railways, waterways, mines, vegetation type, buildings, and many other features. Magnetic declination: Lines of longitude run exactly from the north pole to the south pole, but magnetic compasses point to Earth s north magnetic pole, which currently lies about 700 km (450 mi) from the north pole. The angle between the the directions of true geographic north and magnetic north is called the magnetic declination, and is usually shown in the bottom map panel.

4 SJSU Geol 4L Planet Earth Lab Lab 5 p. 4 of 8 III. Working With Topographic Maps Topographic maps depict quasi-rectangular quadrangles. The top and bottom edges of the map are east-west latitude lines; the left and right edges of the map are north-south longitude lines. Work in pairs to answer questions about each of the following topographic maps. You can work on the maps in any order. You may have to wait to obtain a copy of a map. A. Lihu e (li-hoo-ay), Hawai i 1. What is the latitude and longitude of the northwest corner of this map? latitude: longitude: 2. What is the contour interval? 3. What is the highest point on the map? What is its elevation? 4. Find Lihu e airport. Like all airports, it was constructed on relatively flat ground. How does the spacing of the contour lines indicate the ground is flat here? 5. Imagine standing atop Keopaweo, a peak that is about 1 mile west of Kawai Point. In what direction would descent be the steepest and (probably) most dangerous? How does the spacing of the contour lines indicate this? 6. Find Aweoweonui, about 1 mile west of Kawelikoa Point. What is the elevation of the small pond (blue oval) between the A and w? Careful! This is a bit tricky. Hints: Follow the nearby contour lines closely. Note the small tick lines on some of the contours they aren t found elsewhere on the map, indicating something unusual about this spot. 7. What is the elevation of Kaua i Community College? 8. What is the straight-line distance between the lighthouse in Nawiliwili Bay and the tip of Kawai Point, in kilometers (to one decimal place)?

5 SJSU Geol 4L Planet Earth Lab Lab 5 p. 5 of 8 B. Lee Vining, California [The provisional edition of this map lacks some of the formality of the regular USGS maps. For instance, township, range, and section information is not shown in red ink.] 1. What is the elevation of Mono Lake? 2. What is the elevation of Rush Creek at its intersection with Walker Creek? 3. Towards what compass direction does Walker Creek flow? 4. Imagine driving northeast on Route 120 in Pumice Valley (bottom center of the map). To your east are the Mono Craters. Are they depressions (topographically lower than Pumice Valley) or mountains (higher than Pumice Valley)? How do you know? 5. What is the latitude of the town of Lee Vining, to the nearest minute? 6. What is the longitude of the eastern boundary of the map? C. San José West, California We have three versions of this map, dated 1953, 1961, and 1961 w/1980 photorevisions (the revisions are shown in purple). We only have a few copies each of the older maps, so please handle them carefully and replace them promptly when you re done with them. 1. What is the elevation of Duncan Hall? 2. Name the stream (river, creek, etc.) closest to SJSU. 3. What is the shortest distance from Duncan Hall to that stream, in kilometers? 4. Does that stream flow north or south? How do you know?

6 SJSU Geol 4L Planet Earth Lab Lab 5 p. 6 of 8 5. Did Highway 17 replace a surface street? 6. When was Highway 17 built? Be as specific as warranted by the maps. Now lay out side-by-side the 1953, 1961, and 1980 versions of the map. You should be able to clearly see a dramatic change in land-use patterns. 7. What percentage of the land surface on the San José West quadrangle is fairly flat, and thus suitable for either agricultural use or urban development? % 8. Estimate the percentage of fairly flat land used for agricultural and urban purposes on each map. [Note that the agricultural areas have a green pattern on some 1953 and 1961 maps, but lack patterns or coloring on others. Maybe the USGS was experiencing a green-ink shortage...] agricultural urban Find the following stretches of roadway on the 1980 map: a. Lawrence Expressway between Stevens Creek Blvd. and Graves Ave. b. San Tomas Expressway between El Camino Real and Campbell Ave. c. Highway 17 from Hamilton Ave. to Camden Ave. Now find the future locations of all three on the 1953 map. 9. What did these future locations have in common? 10. Why do you think humans selected those locations for roadways?

7 SJSU Geol 4L Planet Earth Lab Lab 5 p. 7 of Compare the changing patterns of land use as shown on the three San José quadrangles and on the excerpts of the 1899 and 1943 maps shown above. A. Did change in land use follow a particular compass direction or directions? If so, what? B. Did most of it happen earlier? later? C. Did the change expand smoothly? randomly? in some other pattern?

8 SJSU Geol 4L Planet Earth Lab Lab 5 p. 8 of 8 D. Test Your Understanding 1. Label all contours in the map below, and make the 1000 contour line thicker than the others. 2. In what compass direction does the Muddy River flow? 3. In what compass direction does the Raven River flow? 4. What is the minimum elevation of point B? 5. Is Point B the highest point on the map? Explain your reasoning.