Topographic Jlllaps. Qualities of Topographic ltltaps. featui,s;tiii;ii6:inei;:, Earth ,:

Transcription

1 Topographic Jlllaps one of most frequently used types of maps in earth science is topographic map, which depicts features of an area through use of contour lines. Qualities of Topographic ltltaps topographic map shows topography tuh-pahg-ruh-fee) of an area, that is, its natural and human-made surface features. t indicates A landforms such as cliffs, mountains, and beaches; bodies of water; and streets and buildings. To show landforms, maps must show relief highs and lows) of Earth's surface. Relief can be shor,rrn in manyways, such as by shading or coloring. Topographic maps are unique and especially useful because y use contour lines to show relief. The United States Geological Survey USGS) has mapped entire United States in more than 5,000 maps. The USGS updates map information regularly, using information from aerial photography and fleld checks byvolunteer members of Earth Science corps. Because,making topographic maps is so expensive, most revisions do not involve radical changes. nstead, y involve updating boundary changes or adding features such as new areas of city grovvth) that have appeared since last revision. A common USGS topographic map is 7.S-minute map. t shows an area called a quadrangle that spans 7.5 minutes of latitude and 7.5 minutes of longitude. t has a scale of l:24,000. Because one inch represents 2000 feet, se 7.5-minute maps give detailed information about areas y depict. Some maps even indicate individual houses. Topographic maps of Alaska are different from those for continental United States. called ls-minute maps, y cover areas of 15 minutes of latitude and 20 to 6 minutes of longitude. USGS ls-minute maps have a scale of 1:6,60 one inch on this map equals one mile). natural featui,s;tiii;ii6:inei;:,..,'...",' featu*ii ci 6tett,,hudirfu. :.'.::.,' :'" :.'..,' GYVocABUTffy.. topographic mep. topography. contour lines o contour interval. slope. matnetic declination HKERS, such as this woman exploring New Yorkl Adirondack Mountains, are among people who find topographic maps invaluable. Chapter Models of Earth 5-- -,:

2 '-) vt''c Contour line.-...' ' t'=..,, :,.ki: A TOPOGRAPHC MAP The graphic above is a section of topographic map of Portland, Maine, area' The city is shown in pink. You will use line marked "A-B" in minilab on page 56...,.; ",- [ -'' #t,\; Contour Lines By Unlike road maps, topographic maps show shape of Earth's surface. reading a topographic map, you can get a mental picture of shape of area it depicts if you know how to interpret contour lines. contour lines are narrow brown lines on topographic maps that indicate landscape's elevation, or height above sea level. Contour lines are drar,rm to connect points of same elevation in an area. They are map' key to understanding what you are looking at on a topographic On a contour map of Maine seacoast, for example, first contour line is found atzero feet above sea level at sea level). The next contour line appears 10 feet outside first one. That means that all features that this line intersects are 10 feet above sea level. Anlthing inside line is is more betr,veen zero and 10 feet above sea level, and anlthing outside it two than 10 feet above sea level. The difference in elevation between consecutive contour lines is called contour interval' The line above 1O-foot line indicates features at 20 feeu next line above that indicates 0 feet, and so on, in l0-foot increments. Most of interval fifth lines on a topographic map are not marked., but every fourth or will be a hear,y line, similar to one you see at sea level, with a number on it 100, 200, etc.) that serves as a reference point se are called index contours). Not all maps begin at sea level, of course, and some have intervals greater than 20 feet. However, by looking for prominent index contours, you will be able to judge elevation of area map depicts. some maps include information on depths of water y show; that y are called topographic-bath)'t netric maps' Like contour lines of show elevations on land, blue lines called isobaths show form land beneath water's surface. 54 Unit 1 lnvestigating Earth

3 Slope and Elevation he distance between one contour line and next shows you slope,, r steepness, of landscape. f contour lines are very far apart, -.ndscape is relatively flat. f y are close toger, it is steep. To :etermine slope of an area, divide change in elevation by :-stance covered. The figure that results will be average slope. For example, say you want to find slope of a mountainside. The : rint you start measuring is at 1400 feet; summit is at 165 feet. - re distance you're measuring is two and a half miles. Subtract 1400 from 165 to get 1765 feet change in elevation). - -ride 1765 feet by 2.5 miles, and you get an average slope of 706 feet per =-le. The actual slope maybe different if some parts of mountainside :-. steeper than ors. n mapmaking, surveyors find exact elevation of many points in,.p area. These may be shor,rm on map in a number of ways. A bench,-.:rk is a location whose exact elevation is knornm. At survey location, it, roted on a brass or aluminum plate, which is permanently set into.: -,und. On a map, a bench mark is shor.jtr by abbreviation BM that :::ears next to a cross, an X, or a triangle. Numbers next to.::reviation and symbol give elevation to nearest foot. Spot elevations are elevations of road forks, hilltops, lake surfaces, : or points of special interest. These points are usually shor,rm on -.: by small crosses. Numbers giving elevations checked by surveyors are ". :-ted in black. Unchecked elevations are printed in bror,rrn. Water depths. -. shor,rm in blue, with blue lines called isobaths indicating shape of - = COAST OF MANE The topography of Maine can be complex, as this photograph taken near Portland shows. -and below water. r g r Chapter Models of Earth 55

4 Drawing a Profile Materials. graph paper ftre map on pages has a line from A to B marked on it. Place bottom edge of graph paper on that Rt each point where line crosses a contour, make a mark on edge of paper. Record height of each contour next to its naise point at highest contour close to top of page. Raise or pojnts to ir proper places in relation to highest Connect points to complete your profile. Analysis Did you connect points with straight lines or curved lines? Why? ls re any way to tell what elevation is between contour lines? Explain your answer. Understanding Topographic Map Symbols By understanding unique symbols topographic maps use, you can tell human-made features, such as buildings, from natural ones. You can tell water from land, and you can visualize landforms. Contour lines may form circles or ovals. These indicate hills, ridges, or mountains. The peak of a hill or mountain may also include a number, which indicates its elevation. The distance between contour lines will give you an understanding of slope of land. You may find a circle or oval with small lines jutting inward from main contour line. This symbol indicates a depression in landscape, and it is called a depression contour. The crater of a dead volcano would be shornm by a depression contour. Bodies of water are indicated in blue on topographic maps. f you know how to read contour lines, you can figure out which way rivers flow by using a topographic map. When a contour line crosses a river, it forms a point back toward river's source. On map on pages 54-55, you can see how rivers run do,,nm toward sea. Human-made features, as well as natural landforms, are depicted in black on topographic maps. Roadways, railway lines, bridges, gas and water tanks, dams, and individual buildings are often indicated. Thickly settled regions are depicted in pink. Maps on which areas have been revised indicate se updated areas in purple. Refer to page 697 for more information about symbols. Topographic maps have a special kind of compass rose that shows magnetic declination of area pictured. Compasses will point to magnetic north, not to true north. The magnetic decllnadon is angle by which a compass needle will vary from true north. Each topographic map gives angle of magnetic declination for area it represents. Using Topographic Maps 100 \l E\ t= -.\ l 6l l= -\" Approximate Mean Declination, 2001 TNAGNETC DECLNATON This symbol shows angle by which a compass point varies from true north. Topographic maps can be used to determine shape of land, direction of flow of rivers as mentioned above), and distance. t is easy to determine shape of landforms depicted on a topographic map. To do this, you draw a line across map. lvherever line meets a contour, exact height above sea level is knoum. Plotting se points on a vertical scale results in a profile of land. A profile is most easily made by placing bottom edge of a sheet of paper on top of line to be followed. At each point where line crosses a contour, make a mark on edge of paper. Record height of contour next to its mark on paper. \Mhen all points are marked, use vertical scale to raise each point to its proper height. Plotting is easier when graph paper is used.) A vertical scale is usually stretched out cotr elen c0tr apa 5 r i. : 1- ril olp id fffi iisx ie rua ros rri ff $&0 :fi 56 Unit 1 lnvestigating Earth --q,.-

5 compared to horizontal scale. This is to make differences in elevation more visible. An example of a vertical scale is inctr : 20 feet. of course, it is important to keep points same horizlontal distance apart as ywere on map. once elevated points have been plotted, y are joined to make profile. 80' 60' 40' 20' 0'-B PRoFltE DlAGRArur By transferring information from a topographic map to anor sheet of paper, it is possible to draw a landform! profile, o,,t "'p". To measure distance on a topographic map, you will use map scale. f scale of a map is given in words, distances on map may be measured with a ruler. when a graphic scale is printed on map, distance between two points can be marked offwith a straightedge, such as edge of a sheet of paper. A piece of string may also be used. The marked straightedge is held against scale for reading. zigzagdistances along roads or rivers may be marked offone after or on edge of a sheet of paper before measuring against graphic scale. Topographic maps are useful in a variety of ways. city planners use m to plan public works projects. Engineers use m when y plan civil engineering projects, such as roads. Researchers use m as y search for deposits of coal, oil, and natural gas. And everydaypeople use m too, as y hike, camp, orienteer, and orwise enjoy outdoors.. Section What do contour lines indicate on a topographic what would topography of an area be like if contour lines were close toger? lf y were far apart? O Describe how to measure distance using a topographic What human-made features might appear on a topographic map? o crlilcal TH NKNG Why do you think USGS cartographers made 15-minute maps for Alaska, instead of 7.5_minute maps? G) t ATHEMATCS Determine slope of an area, given following: start measuring at 700 feet. Measure a distance of four miles, and end at an elevation of 242 feet. O FARED ACTVTY Working with a partner, select part of topographic map of portrand, Maine, area, and create a profile. Chapter Models of Earth 57