Chapter 8 Studying the Earth's Surface

Chapter 8 Studying the Earth's Surface - © 2009 Allan...

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Unformatted text preview: © 2009 Allan Ludman and Stephen Marshak W.W. Norton & Company CHAPTER 8 STUDYING EARTH’S LANDFORMS PURPOSE • To understand the different ways in which geologists study the surface of the Earth. • To be able to select the most appropriate image for studying an aspect of Earth’s surface • To practice interpreting landform models MATERIALS NEEDED • A clear plastic ruler marked in millimeters and/or tenths of an inch and • a circular protractor. • A globe and maps provided by your instructor 8.1 INTRODUCTION It is easier to study Earth’s surface today than at any time in history. To help understand surface features, we can now make detailed surface models from satellite elevation surveys and download images of any point on the planet at the click of a mouse using Google Earth® and NASA’s WorldWind. Geologists were quick to understand the scientific value of satellite imaging technology and adapt new methods as quickly as they are developed. You will use images in this manual that even researchers didn’t have at their disposal a decade ago. The study of the Earth’s surface is almost as dynamic as the surface itself! And you don’t have to be in a college laboratory to use this technology because much of it can be downloaded free of charge from the Internet. Google Earth® and NASA’s World Wind show satellite images of the entire globe and can be used to get three-dimensional views. Microdem® can build realistic digital elevation models of the landscape, draw topographic profiles, measure straight-line distances and the length of meandering streams, and estimate slope steepness. Other resources are relatively inexpensive: National Geographic’s TOPO! provides continuous topographic map coverage of each state for about $100 and also draws profiles and measures distances. 8.2 IMAGES USED TO STUDY EARTH’S SURFACE 1 The best way to study landforms would be to fly over them to get a birds-eye view and then walk or drive over them to understand them from a human perspective. That’s not practical for a college course, so we will have to bring the landforms to you instead. To do so, we will use traditional tools like maps (Figure 8.1a) and aerial photographs (8.1b), a new generation of Landsat and other satellite images (8.1c), digital elevation models (DEMs; 8.1d), and Google Earth and World Wind images that were science fiction when we began planning this book. Figure 8.1 shows a part of eastern Maine using four of these methods. A topographic map (Figure 8.1a) uses contour lines to show landforms (see Chapter 9). Topographic maps used to be drawn by surveyors who measured distances, directions, and elevations in the field. They are now made by computers from aerial photographs and radar data. Aerial photographs (Figure 8.1b), including United States Geological Survey (USGS) Orthophotoquads, are photographic images taken from a plane and pieced together to form a mosaic of an area. Landsat images (Figure 8.1c) are made by a satellite that takes digital images of Earth’s surface using visible light and other wavelengths of the electromagnetic spectrum. Scientists adjust the wavelengths to color the image artificially, to emphasize specific features. For example, some infrared wavelengths help reveal the amount and type of vegetation. Digital elevation models (DEMs; Figure 8.1d) are computer generated three-dimensional views of landforms made from radar satellite elevation data spaced at 10- or 30-meter intervals on the Earth’s surface. A new generation based on 1-m data is now being released and provide a more accurate model of the surface than anything available to the public 5 years ago. Each type of image is particularly helpful for different types of study and each has its drawbacks, so that no one image is the right one. The latitude and longitude of the corners of a topographic map, DEM, or Landsat image are normally listed and there would be a scale for measuring distance on the map and DEM. North is assumed to be at the top of all images unless otherwise indicated. 2 Figure 8.1: Representations of an area in eastern Maine in different kinds of images a. Topographic map b. Aerial photograph c. Landsat 7 image (artificial color) d. Digital elevation model (DEM) The images in Figure 8.1 show the same area but look different because each emphasizes different aspects of the surface. Which is best? For learning the names of mountains or lakes, the topographic map (Fig. 8.1a) is the obvious choice; for studying the network of roads in the area, the DEM would be useless. For analyzing landforms and their evolution, geologists can choose the image that works best for their particular project. EXERCISE 8.1: WHICH IMAGE WORKS BEST? a. Examine the images in Figure 8.1 and rank them (1-4) in Table 8.1 by how well they show the map elements indicated (1 would be the most effective, 4 the least effective. Ties are allowed) b. Which of the images enables you to recognize the topography most easily? Why? 3 Table 8.1: Evaluating landscape images Topographic map Location Direction Elevation Changes in slope Distance Aerial photograph Landsat image DEM Names of features c. Which is least helpful in trying to visualize the hills, valleys, and lakes? Why? d. Erosional agents often produce a “topographic grain”, an alignment of elongate hills, ridges, and valleys. Which images show the topographic grain in this area most clearly? Once you’ve seen it on those images, can you recognize it on the others? e. Which images show highways most clearly? f. Which images show unpaved lumber roads most clearly? g. Which image do you think is the oldest? The most recent? Explain your reasoning. h. Which image(s) would you want to have if you were planning a wilderness hike? Why? 8.2.1 Map projections These images are flat, two-dimensional pictures but Earth is a nearly spherical three-dimensional body. The process by which a three-dimensional sphere is converted to a two-dimensional map is called making a projection. There are many different projections and each produces maps that look different and are used for different purposes (Figure 8.2). Some, like the Mercator projection, preserve 4 accurate directions between points; others preserve the true areas of features, and some the true distances between points. All projections must to some extent distort 3-D reality to fit on a 2-D piece of paper. Figure 8.2 Four common map projections and their different views of the world a) Orthographic b) Mercator c)Polyconic d)Sinusoidal 8.3 MAP ELEMENTS All accurate depictions of Earth’s surface must contain certain basic elements: location, a way to show precisely where the area is; a way to measure the distance between features; and an accurate portrayal of directions between features. It is also important to know elevations of hilltops and other features, and the steepness of slopes. 8.3.1 Map Element 1: Location Road maps and atlases use a simple grid system to locate cities and towns, e.g. Chicago is in grid square A8. This is not very precise because many other places may be in the same square, but is good enough for most driving. More sophisticated grids are used to locate features on Earth precisely. Maps published by the USGS use three grid systems: latitude/longitude, the Universal Transverse Mercator (UTM) grid, and, in most states, the Public Land Survey System. The UTM grid is least familiar to Americans but is used extensively in the rest of the world. 8.3.1a Latitude and longitude The latitude/longitude grid is based on location north or south of the equator and east or west of an arbitrarily chosen north-south line (Figure 8.3). A parallel of latitude connects all points that are the same angular distance north or south of the Equator. The maximum value for latitude is 90°N or 90°S 5 (the North and South Poles, respectively). A meridian of longitude connects all points that are the same angular distance east or west of the Prime Meridian a line that passes through the Royal Observatory in Greenwich, England. The maximum value for longitude is 180°E or 180°W, the International Date Line. Remember: You must indicate whether a point is north or south of the Equator, east or west of the Prime Meridian. “44º Latitude” could be in the northern or southern hemisphere. Latitude and longitude readings are typically reported in degrees (º), minutes (’), and seconds (”) where there are 60’ in a degree and 60” in a minute, e.g. 40°37’44”N, 73°45’09”W. For reference, one degree of latitude is equivalent to approximately 69 miles (111 km), one minute of latitude is about 1.1 mile (185 km), and one second of latitude about 100 feet (31 m). The same kind of comparison can only be made for longitude at the equator because the meridians merge at the poles and the distance between degrees of longitude decreases gradually toward the poles (Figure 8.3b). hand-held Global Positioning System (GPS) receivers and those used in cars and planes can locate points to within a second. EXERCISE 8.2: LOCATING CITIES USING LATITUDE AND LONGITUDE a. With the aid of a globe or map, determine the latitude and longitude of your geology laboratory as accurately as you can. How could you locate the laboratory more accurately? b. If you have access to a GPS receiver, locate the corners of your laboratory building. Draw a map showing the location, orientation, and distances between the corners. c. Locate the following U.S. and Canadian cities as accurately as possible Nome, Alaska______________________________ Chicago, Illinois_____________________ St. Louis, Missouri_________________________ New York, New York_______________________ Saint Johns, Newfoundland __________________ Calgary, Alberta __________________________ Seattle, Washington_____________________ Los Angeles, California__________________ Houston, Texas __________________ Miami, Florida _________________________ Ottawa, Ontario_________________________ Victoria, British Columbia __________________ 6 Figure 8.3 The latitude/longitude grid 90° N. Latitude North Pole 90° W. Longitude Prime Meridian 0° E or W Longitude 60° N. Latitude 30° N. Latitude 30° 30° Equator 30° S. Latitude 60° S. Latitude 60° 75° W. Longitude 90° E. Longitude 60° W. Longitude 90° S. Latitude South Pole b. Latitude is measured in degrees north or south of the Equator a. Longitude is measured in degrees east or west of the Prime Meridian (Greenwich, England) 45° N. Lat., 15° E Long. 0° N or S Latitude 0 ° E or W 30° S Lat. , 75° W Long. c) Locating points using the completed grid d. Which of these cities above do you think is closest in latitude to each of the following world cities? Predict first, without looking at a map or globe, then check. Were you surprised by any? City Oslo, Norway Baghdad, Iraq London, England Paris, France Rome, Italy Beijing, China Tokyo, Japan Quito, Ecuador Predicted North American Latitude and Longitude Actual best match 7 Cairo, Egypt Capetown, South Africa 8.3.1b Public Land Survey System The Public Land Survey System was created in 1785 to provide accurate maps as America expanded from its 13 original states. Most of the rest of the country is covered by this system, except for Alaska, Hawaii, and Texas, and the southwestern states surveyed by Spanish colonists before they joined the Union. Points can be located rapidly to within an eighth of a mile in this system (Figure 8.4). The grid is based on accurately surveyed north-south (Principal Meridian) and east-west (Base Line) lines for each survey region. Lines parallel to these at six mile intervals create grid squares 6 miles on a side forming east-west rows called townships and north-south columns called ranges (Figure 8.4a). Townships are numbered north or south of the Base Line and Ranges east and west of the Principal Meridian (Figure 8.4a). Each 6-mile square is divided into 36 sections, each one mile on a side, numbered as shown in Figure 8.4. Each section is divided into quarter sections ½ mile on a side and each of these is further quartered, resulting in squares ¼ mile on a side. The location of the star in the red box in Figure 8.4 is described in the series of blow-ups: • T2S, R3E locates it somewhere within an area of 36 square miles (inside a 6 mi x 6 mi square) • Section 12, T2S, R3E locates it somewhere within an area of 1 square mile • SE ¼ of Section 12, T2S, R3E locates it somewhere within an area of ¼ square mile • SW ¼ of the SE ¼ of Section 12, T2S, R3E locates it within an area of 1/16 square mile 8 Figure 8.4 Locating with the Public Land Survey Grid R6W R5W R4W R3W R2W R1W R1E R2E R3E R4E R5E R6E Principal Meridian 6 miles 6 miles A T5N T4N T3N T2N T1N T1S T2S Base Line B T3S T4S T5S T2S, R3E 6 7 18 19 30 31 5 8 4 3 2 1 Section 12, T2S, R3E NW ¼ NE ¼ SE ¼ of Section 12, T2S, R3E 9 10 11 12 14 13 17 16 15 20 21 22 29 28 27 32 33 34 NW ¼ NE ¼ ¼ mile ½ mile 23 24 26 25 35 36 1 mile 1 mile SW ¼ SE ¼ SW ¼ SE ¼ ¼ mile SW ¼ of the SE ¼ of Section 12, T2S, R3E EXERCISE 8.3: LOCATING POINTS WITH THE PUBLIC LAND SURVEY GRID Determine the location of points A and B in Figure 8.4, additional points your instructor indicates on topographic maps. A_________ B ____________ Determine the location of points indicated by your instructor on topographic maps 8.3.1c Universal Transverse Mercator (UTM) grid The UTM (Universal Transverse Mercator) grid divides the Earth into 1200 segments, each containing 6° of longitude, and 8° of latitude (Figure 8.5). North-south segments are assigned letters (CX); east-west segments are called UTM zones and are numbered 1-60 eastward from the International 9 Date Line (180°W). Thus, UTM Zone 1 extends from 180° to 174° W Longitude, Zone 2 from 174° to 168° W Longitude, etc. The 48 conterminous United States lie within UTM zones 10-19, roughly 125º to 67º W Longitude. Because of the polar distortion in the Mercator projection evident in the sizes and shapes of Greenland and Antarctica in Figure 8.5, the UTM grid covers only latitudes 80°N to 80°S. Figure 8.9 The worldwide UTM grid To locate a point, begin with the grid box in which the feature is located. For example, the red box in Figure 8.5 is grid S22. UTM grid readings tell in meters how far north of the Equator (northings) and east of the central meridian for each zone (eastings) a point lies. The central meridian for each UTM zone (the line of longitude that runs through the center of the zone; Figure 8.6) is arbitrarily assigned an easting of 500,000 m so that no point would have a negative easting. Points east of a central meridian will thus have eastings greater than 500,000 m, those west of the central meridian less than 500,000 m. Figure 8.6 UTM zones for the 48 conterminous United States The red line is the central meridian (105ºW) for UTM Zone 13; the blue line is the central meridian for Zone 18 10 Figure 8.7 shows how the UTM grid appears on USGS topographic maps. Blue tick marks along the border of every map define a grid containing boxes 1,000 m (1 km) on a side (Figure 8.7a). In newer maps, the grid lines are drawn (Figure 8.7b). Figure 8.7 Using UTM grid marks to locate features a)UTM tick marks (blue) along the sides and latitude/longitude (red) at the corner of a topographic map. 43 48 39º15’N 120º30’W 7 16 7 17 Grid labels across the top and bottom of a map are eastings (indicated by E), those along the side of the map northings (indicated by N). There are two kinds of labels, one a shorthand version, the other complete. In Figure 8.11b, 560000E means 560,000 m east of the central meridian for the UTM zone. Remember that each grid box is 1,000 m square. The marker immediately west of 42 5 60000E must be 559,000 m east of the central meridian but only every tenth value is written fully. Abbreviations like 559 are for the intervening grid markers. Northings are similar. The marker 81000 N means 4,281,000 m north of the Equator (4,281 km). And 4282, the marker, 1,000m north, is 4,282,000 m. b) UTM gridlines used to locate a road intersection UTM grid lines 11 To determine the location of the red star in Figure 8.8, measure proportionally the distance east of 559 and north of 42 82: 559450 m E, 4282182 m N. Rather than doing a lot of arithmetic, you can make a UTM tool for each major map scale (Figure 8.8) and determine location easily to within 10 m. Figure 8.8 Using a UTM grid tool UTM grid tool Tool chosen for correct map scale EXERCISE 8.4: LOCATING POINTS WITH THE UTM GRID a) Use the appropriate UTM grid tool from your tool kit to determine the location of the red star in Figure 8.7b and 8.8. _______________________ b)Give the UTM coordinates of the top of Grey Mountain in Figure 8.10c: ____________________ 8.3.2 Map Element 2: Direction Geologists use the azimuth method to indicate direction, based on the dial of a compass (Figure 8.9). The red-tipped compass needle in Figure 8.9a is pointing northeast -- somewhere between north and east but how much closer to north than to east? On an azimuth compass (Figure 8.9b) 0° or 360º is due north, 090º=east, 180º = south, 270º = west. The direction 045º is exactly halfway between north and east. The direction of the needle in Figure 8.9 can be read as 032º. 12 Use the circular protractor in your tool kit to determine the direction between any two points. Draw a line between the points, align the protractor’s registration lines in a N-S or E-W position and place its center point on one of the two points. (Figure 8.9c). The direction from that point to the other is where the line intersects the azimuth scale; in this case, the direction from A to B is 235º. Figure 8.9 Using the azimuth method to describe direction between two points 0º 0º 045º 315º 045º N 315º W E 270 090º 270 A 090º S a)A simple compass 225º 180º 135º 225º B 180º 135º C b)Compass with azimuth markings c) Using a circular protractor to determine the direction between points EXERCISE 8.5: GIVING DIRECTIONS Using the circular (azimuth) protractor in your tool kit, give the directions in Fig 8.9 from: a)from A to C _____º C to A _____º B to C _____º C to B _____º . b) from Point A to Point B in Figure 8.4 _________º 8.3.3 Map Element 3: Distance and scale A map of the entire world or your campus can fit onto an 8½ x 11” sheet of paper– if we scale the Earth down so it fits. A map scale indicates how much an area has been scaled down so we can relate inches or centimeters on the map to real distances on the ground. Figure 8.10 shows three maps of the same general area, made at different scales. The more we scale down an area, the more detail we lose; the closer the map is to the real size, the more detail we can see. The three map segments in Figure 8.10 are the same size on the page but the area each covers is different because of their different scales. The map in Figure 8.10a has been scaled down more than 13 four times as much as Figure 8.10c (1:100,000 vs 1:24,000) and therefore covers much more of the land surface on the same sized piece of paper. Figure 8.10 An area in eastern Maine mapped at three common map scales a)Scale = 1:100,000 b)Scale = 1:62,500 Note the latitude and longitude values at the southwest corner of this map segment, and the UTM grid ticks along the bottom and western edge c. Scale = 1:24,000 14 8.3.3a Different ways to describe map scale Map scale may be expressed verbally, proportionally, or graphically. A verbal scale, used on many roadmaps, uses words like “one inch equals approximately 6.7 miles” to describe the scaling of map and real distances. A driver can estimate distances between cities but not very accurately. The most accurate way to describe scale is a proportional scale, one that tells exactly how much the ground has been scaled down to make the map. Proportional scales like 1:100,000 (one to one hundred thousand), mean that distances on the map have been scaled down to one 100,000th of the ground distance. The larger the number, the more the ground has been scaled down. A proportional scale refers to all units. Thus, one centimeter on the map equals 100,000 centimeters (one kilometer) on the ground, one inch on the map equals 100,000 inches (1.58 miles) on the ground, etc. The metric system is ideally suited for scales like 1:100,000,000 or 1:100,000 because it is based on multiples of 10: 1 cm=10 millimeters, 1 m=100 centimeters, and 1 km=1,000 meters. In the United States, we measure ground distance in miles but map distance in inches. Unfortunately, relationships among inches feet, and miles are not as simple as in the metric system. There are 63,360 inches in a mile (12 inches/foot x 5,280 feet/mile), so the proportional scale 1:63,360 means that one inch on a map represents exactly one mile on the ground. Old topographic maps use a scale of 1:62,500. For most purposes, we can interpret this scale to be approximately 1” = 1 mile, even though an inch on such a map would be about 70 feet short of a mile. Other common map scales are 1:24,000 (1” = __________ feet), 1:100,000 (see above), 1:250,000 (1”= _______ miles), and 1:1,000,000 (1” = ________miles). Map scale can also be shown graphically, using a bar scale (Figure 8.11) to express the same relation as the verbal scale. Depending on how carefully you measure, a bar scale can be more accurate than a verbal scale, but not as accurate as a proportional scale. 15 Figure 8.11 Scale bars used with three common proportional scales 8.4 VERTICAL EXAGGERATION—A MATTER OF PERSPECTIVE DEMs show the land surface in three dimensions and must therefore use an appropriate vertical scale in order to show how much taller one feature is than another. It would seem logical to use the same scale for vertical and horizontal distances, but we don’t usually do so because mountains wouldn’t look much like mountains and hills would barely be visible. Landforms are typically much wider than they are high, standing only a few hundred or thousand feet above or below their surroundings. At a scale of 1:62,500, one inch represents about a mile. If we used the same scale to make a three-dimensional model, a hilltop 400 feet above its surroundings would be less than one tenth of an inch high. A mountain rising a mile above its base would be only one inch high. We therefore exaggerate the vertical scale compared to the horizontal to show features from a human perspective. For a three-dimensional model of a 1:62,500 map, a vertical scale of 1:10,000 would exaggerate apparent elevations by a little more than 6 times (62,500/10,000= 6.25). A mountain rising 1 mile above its surroundings would stand up 6.25” in the model; a 400’ hill would be about half an inch tall –more realistic than the 0.1” if the 1:62,500 horizontal scale had been used vertically. 16 The degree to which the vertical scale has been exaggerated is, logically enough, called the vertical exaggeration. Figure 8.12 shows the effects of vertical exaggeration on a DEM. With no vertical exaggeration, the prominent hill in the center of Figure 8.12a is barely noticeable. One of the authors of this manual has climbed that hill several times and guarantees that climbing it is far more difficult than Figure 8.12a would suggest. On the other hand, Figure 8.12d exaggerates too much; the hill did not seem that steep, even with a pack loaded with rocks. Is there such a thing as too much vertical exaggeration? The basic rule of thumb: Don’t make a mountain out of a molehill. Vertical exaggerations of 2-5 X generally preserve the basic proportions of landforms while presenting features clearly. We will return to the concept of vertical exaggeration when we discuss drawing topographic profiles from topographic maps. Figure 8.12 DEMs of part of the area in Figure 8.1 showing the effects of vertical exaggeration (VE)(V.E.) No Vertical Exaggeration V.E. = 5X VE = 10X VE = 20X 17 ...
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