Topographic Maps and Digital Elevation Models

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Chapter 6 Topographic Maps and Digital Elevaton Models 103 angular separation between them (the magnetic declination) also is given. When stating the magnetic declination of a map, it is always necessary to indicate whether the arrow pointing to the magnetic pole is east or west of the geographic pole. If it is east, the declination is stated as so many degrees east, for example, 212° E. Most maps also have an arrow pointing toward G.N., the location of the grid north direction for the Universal Transverse Mercator (UTM) grid system (see Fig. 6.9). Symbols Standardized symbols and colors are used on government maps to designate various features. On USGS maps, cultural features (those made by people) are generally drawn in black; forests or woods are shown in green (they are not always represented); blue is used for bodies of water; brown shows elevation (contours), some mining operations, and beaches or sand areas; and red is used for the better roads and some land subdivision lines. See Figure 6.10 for symbols and Figure 6.9 for some examples. Note that when USGS topographic maps are revised, any new features (e.g., roads, suburbs, strip mines) that appear in an area are colored purple. Symbols for Canadian government maps are shown on the backs of the maps. Mexican map symbols are generally on the front. Working with Topographic Maps Now that you understand contours, coordinate systems, and scale, we are ready to cover some ways of working with topographic maps. We'll start with the basics of how topographic maps are produced. Making Topographic Maps Making a topographic map requires accurate points of elevation in the map area. A bench mark is a point whose elevation and location have been precisely determined by government surveyors; its location is marked by a small brass plate. Bench marks are designated on maps by the symbol B.M. (Fig. 6.9). Spot elevations are somewhat lessprecisely determined elevations used in the construction of topographic maps. They are shown at many section corners, bridges, road intersections, hilltops, and the like and may be marked with an "x" (examine Fig. 6.9). Bench marks and spot elevations are used in conjunction with aerial photographs to construct topographic maps. Two aerial photos, taken from different points but overlapping the same area, provide a three-dimensional view of the land surface when viewed through a stereoscopic viewer. By orienting the photos properly, two beams of light from different sources can be focused at any elevation. If the superimposed beams are moved around a hill, for example, they will trace a line at a precise elevation. The numerical value of this elevation can be determined from known elevations within the area (e.g., bench marks). Aerial photographs are discussed further in Chapter 7. If you are a landscaper or an architect, for example, you may want to make your own detailed topographic map of an area. You can start by tracing any important features (drainages, coastlines, buildings, etc.) from an air photo obtained from the USGS or from your state. Then, starting from the lowest spot on the property, take a series of hikes uphill with a 5-foot staff and a spirit (bubble) level that allows you to site horizontal lines from the top of your staff. These allow you to plot successive elevation increments of 5' on your map (Fig. 6.l1A). Now add the contours to reflect the landscape by following these steps: I. Select a contour interval that will show the level of detail you need. Too many contours can be confusing. 2. If your staff was a convenient length (e.g., 5 feet), simply connect those points that correspond to multiples of the contour interval. If the c.I. is 20 feet, you would connect dots marking 20, 40, 60, etc., feet. 3. Draw fairly smooth, fairly parallel contours, but be sure to bend them upstream when crossing drainages and gullies (Fig. 6.l1B). Adding extra wiggles implies you know more than you do. Draw the lines to the edge of the map. Label each contour or index contour with its elevation. Topographic Profiles A topographic profile shows the shape of the land surface as it would appear in a cross section; it is like a side view. Topographic profiles portray the shape of the land surface along a particular line of profile. They are useful for many practical purposes, such as planning roads, railroads, pipelines, hiking trails, and the like, or for estimating the volume of material that will need to be excavated or filled during road construction. Profiles are most easily made along straight lines, but they can also follow curved paths, such as a road or a stream. A topographic profile is made from a contour map using the following procedure (Fig. 6.12): l. Select the line or path along which the profile is to be made, such as line X-Y in Figure 6.l2A. 2. Record the elevations along the line as shown in Figure 6.12B. To do this, lay the straight edge of some scratch paper along the line of profile. Mark on the paper the ends of the profile line and the exact place where each contour line meets the edge of the paper. Label each mark on the paper with the elevation of the corresponding contour. Also mark the positions of any streams that cross the line of profile, because they will be low points on the profile. 3. Set up the graph on which the profile will be drawn (Fig. 6.l2C). First note the differences in elevation between the highest and lowest points along the line of profile; this will determine the range of elevations on your profile. Label the vertical axis with a range of elevations that extends beyond the profiJe elevations and conveniently allows each contour to be graphed. In Figure 6.l2C, the profile elevations range between 820 and 940 feet and are spanned by a vertical axis of 700 to 1000 feet. Horizontal lines on the vel1ical axis are 20 feet apart, which matches the contour intervaJ and makes graphing simple. CommonJy,
Topographic Map Symbols BOUNDARIES RAILROADS AND RELATED FEATURES COASTAL FEATURES National. . ......_-- Standard gauge single track; station.. Foreshore flat (shallow sediment). State or territorial _ Standard gauge multiple track . Rock or coral reef . County or equivalent. --- Abandoned . Rock bare or awash .. Civil township or equivalent. Incorporated-eity or equivalent. . .... . 1-_, _ . .r- - - - - Under construction Narrow gauge single track . . Group of rocks bare or awash. Exposed wreck......................•.. Park, reservation, or monument. .. .I-- . _ Narrow gauge multiple track . Depth curve; sounding . Small park . Railroad in street. . Breakwater, pier, jetty, or wharf. Juxtaposition........................•. Seawall .. LAND SURVEY SYSTEMS Roundhouse and turntable . U.S. Public Land Survey System: Township or range line. TRANSMISSION LINES AND PIPELINES BATHYMETRIC FEATURES Area exposed at mean low tide; sounding datum ._...../. Location doubtful. ... f- _ _ _ Power transmission line: pole; tower.. Channel . Section line f-----j Telephone or telegraph line. Offshore oil or gas: well; platform . o • Location doubtful. --- Above-ground oil or gas pipeline. .~__~ Sunken rock. Found section corner; found closing corner I- ~ ~_ Underground oil or gas pipeline . Witness corner; meander corner ~c1+ _ ~ RIVERS, LAKES, AND CANALS I MC, CONTOURS Intermittent stream . . ....---. . Other land surveys: Topographic: Intermittent river . ..... - ..:::::: Township or range line. Section line. Intermediate . Disappearing stream. .. .. ---< Land grant or mining claim; monument.. ..... +_ _ to Index . Perennial stream. Fence line .. Supplementary . Perennial river . Depression .. Small falls; small rapids . ROADS AND RELATED FEATURES Cut; fill .. I-."~"";,.j Large falls; large rapids . Primary highway.. f---~ Bathymetric: Secondary highway.. Intermediate. Masonry dam . Light duty road. Index. Unimproved road. Primary.. Dam with lock. Trail. . Index Primary . Dual highway. Supplementary. Dual highway with median strip . Dam carrying road. Road under construction . ~_~ MINES AND CAVES Underpass; overpass.. .~ Quarry or open pit mine . Intermittent lake or pond , . Bridge . I-__~ Gravel, sand.. clay, or borrow pit. . Dry lake .. Drawbridge. I-__~ Mine tunnel or cave entrance. Narrow wash. Tunnel. . 1-0"""'- Prospect; mine shaft. . Wide wash . ',' Mine dump .. Canal, flume, or aqueduct with lock . BUILDINGS AND RELATED FEATURES Dwelling or place of employment: small; large.. • _ Tailings . Elevated aqueduct, flume, or conduit. Aqueduct tunnel. School; church. •• SURFACE FEATURES Barn, warehouse, etc.: small; large. 0 ~ Levee Water well; spring or seep . House omission tint. Sand or mud area, dunes, or shifting sand. GLACIERS AND PERMANENT SNOWFIELDS Racetrack . Intricate surface area. . >'-
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Topographic Maps and Digital Elevation Models

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