Interpreting the Topographic

Interpreting the topographic map, like interpreting poetry, takes some practice and some imagination. The map's contour lines are like steps, with a fixed height of "risers." The distance between the contour lines may be 20 feet or 100 feet; the interval is chosen to present a readable representation of the detail involved on the particular map, and the interval is designated on the map.

Contour Interval Drawing

River valley and hills (upper drawing) and topographic map of same area (lower drawing). The river flows into a bay partly enclosed by a sandspit. The hill on right has a gradual slope; the one on left rises steeply above a tableland. From the improved valley road, a dirt road takes off right to a church and two houses. Contours on the map are 20 feet apart. (From U.S. Geological Survey's "Topographic Maps.")

One way to understand the idea of contours is to visualize a view of the land from an airplane. (If one flew over an area on which the actual contour lines were marked on the landscape, he would, in effect, be seeing a contour map, but contour lines seldom occur naturally.) Imagine a series of stakes driven on a hill: first a series at every point with an elevation of 5 0 0 feet above sea level, then another row at 5 1 0 feet, and so on to the top of the hill. From the air these rows of stakes would appear to form irregular circles some distance apart.

The same thing could be accomplished by creating a giant lake with its surface 5 0 0 feet above sea level. Then the shoreline of the lake would be the 500-foot contour line on the aerial map or on a contour map. Raising the lake level 10 feet would establish the 510-foot contour line. In relatively flat country such a 10 foot rise would extend the shoreline hundreds of feet horizontally; the contour lines would be far apart on the map. Conversely, a lake wedged between almost vertical cliffs would show very little increase in size as it rose. Its contour lines would be almost on top of each other.

Each contour line, therefore, represents a series of points of like elevation that form an irregular circle. A hill is a stack of closed loops; a valley is a V-shaped or U-shaped series of lines that cross the valley; and a cliff face is a punched-together mass of lines. If the cliff is exactly vertical, the lines coalesce into one line. Contour lines may run off the map, but if the map were big enough it would be seen that they eventually form a closed loop.

Every fourth or fifth contour line, depending on the scale, is printed darker and carries a figure of the elevation in feet. The elevation of certain prominent objects such as mountain peaks, bench marks, or lakes is given in black. A bench mark (BM on the map) is a real marker placed on the earth's surface that records one of the thousands of points whose elevation has been precisely determined by survey. Usually it is a metal plate on a concrete post. These markers should never be marred or disturbed.

Depressions are also marked with contour lines. Short ticks (hachures) at right angles to the contour lines point down the slope. They are added when there is possibility of confusion between a small hill and a shallow depression.

Topographic maps are periodically revised, especially those where urban growth has caused major changes in manmade structures. Sometimes, however, the collector will wish to find an old fossil location by an obsolete description, such as "one-half mile south of Jones's Ferry on the west bank." Jones's Ferry is long gone. An old topographic map, however, may be found in a library or museum, and it may carry the old site description. By such research, old collecting sites can be rediscovered. The date of a topographic map appears in the lower right-hand corner.

Geological Survey Quadrangle Maps

The quadrangle maps, known as the national series of the U.S. Geological Survey (and those of the Canadian Survey), are based on the meridians of longitude and the parallels of latitude. The meridians of longitude run north and south, dividing the earth into wedge-shaped pieces east and west of the prime meridian at Greenwich, England; the parallels of latitude run east and west and divide the surface north and south of the equator. The United States is in the north latitudes and west longitudes.

The maps are printed on large sheets of paper. Each of the large-scale l-to-24,000 maps covers 7h minutes of latitude and longitude; the medium-scale l-to-62,500 maps cover 15 minutes of latitude and longitude; and the l-to-250,000 maps cover 1 degree of latitude and 2 of longitude. In the United States 15 minutes, which is a quarter degree, is about 18 miles of latitude and 15 miles of longitude. The latitude and longitude of the map are recorded on the corners and sides. Tick marks on the margin divide the map into nine rectangles, which can be designated for locating a point as in the NW, NC, NE, WC, C, EC, SW, SC, and SE rectangles, or by the position in degrees, minutes, and seconds of latitude and longitude.

All continental land surfaces of the United States except the eastern founding states and Texas are divided into 6-mile squares known as townships. Each township, in turn, is divided into 36 sections, each a mile square containing 640 acres. Townships are numbered north and scuth from a base line (symbol T) and into ranges (symbol R) east and west from a principal meridian. A township might be Township 3 North (T3N) and Range 2 West (R2W). Sections are numbered from right to left on the odd-numbered lines and left to right on the even-numbered lines, starting in the upper right-hand corner and continuing to the bottom right-hand corner. Sections are divided into quarter sections, primarily for land-title purposes, so that a legal description might be SW;1 of NEJ of Section 17 of the township described above; its full description would be SWJ of NE{, Sec. 17, T3N, R2W. (See accompanying example). By continuing the quartering, an exact location of a single house (or fossil site) any place in the United States where the township system is used can be described in less than one line.

Topographic maps are made today from aerial photographs from which three-dimensional projections can be made to give the mapmaker the data on contours, drainage, forested areas, etc. Maps were formerly made from surveys sketched by hand in the field. Today some such surveys must be made to provide basic points for interpretation of the aerial photographs

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Townships are laid out along a principal meridian and a base line (drawing at top). Ranges are measured east and west from the meridian, townships north and south from the base line. Each township is six miles square. Each township is divided (lower left) into 36 sections, each a mile square, numbered from right to left and from left to right alternately. Each section is then divided into quarter sections (drawing lower center), and each quarter into forties (40 acres) (drawing lower right). In designations T stands for township, R for range, and N E S W for the directions.

and to get information about areas, such as a heavy forest, where the detail would be hidden from the camera's eye. Man-made features are also described from field data.

The user of a topographic map will occasionally wish to determine his own position on it. This he can do by taking compass readings on two visible landmarks that appear on his map. He can then draw in the lines on his map, and the point where they intersect will be on his position.

Hydrographic maps are topographic maps of the bottoms of lakes or seas. They are made from soundings taken with an instrument that projects a signal toward the bottom and then registers the interval of time before it rebounds.

Topographic quadrangle maps for areas east of the Mississippi river may be ordered from the Washington Distribution Section, U.S. Geological Survey, 1200 South Eads Street, Arlington, Virginia 22202. Those for areas west of the Mississippi river may be ordered from the Denver Distribution Section, Geological Survey, Federal Center, Denver Colorado 80225. Indexes of the available maps may be obtained from these centers free of charge. These indexes also list dealers in principal cities who stock the maps. Maps are designated by the names of a town or major natural feature appearing on them. Many other agencies of the federal government issue maps, for example, there are Forest Service maps of national forests. They may be ordered from the Forest Service, Department of Agriculture, Washington, D.C. 20250. Orders should be placed sufficiently far ahead to allow time for the order to be filled and the maps returned.

For students, the Geological Survey has made available a set of twenty-five maps showing typical topographic features, such as mountains, glaciers, and faults.

GEOLOGIC MAPS

Geologic maps are topographic maps with added information of interest to collectors, prospectors, miners, and specialists in such subjects as land use and water supply. A topographic map itself conveys a lot of geologic information to anyone able to read it. Ridges on the map probably stand for bodies of hard rocks, such as dikes, upturned strata, and dense limestone and sandstone; valleys indicate areas of eroded soft rocks, such as shale. Geologic maps, however, carry this type of information much further. They are especially useful to the fossil collector because they will tell him where to find sedimentary rocks, the source of most specimens.

Geologic maps locate and identify specific rock masses that lie on or near the surface with respect to other topographic features. Such maps grew out of the ones that Baron Cuvier and Alexandre Brongniart drew of the Paris basin and those that William Smith made of England in the first two decades of the last century.

The basic geologic laws that these men helped discover still guide map-makers today. These include the law of superposition—that in any pile of undisturbed sedimentary rocks the youngest rocks are at the top, the strata underneath are progressively older, and the oldest are found at the bottom. Another law affirms that water-laid sediments — the happy hunting ground for fossils — continue horizontally in all directions until they thin out at the original shorelines on the edge of the basin. To this belongs the corollary that where a sedimentary stratum disappears abruptly, it has either been removed by erosion or displaced by a fault.

In preparing a geologic map, cartographers assume that similar rocks showing the same succession of strata and not too far separated presumably belong to the same time, and that an unidentified stratum probably can be traced along until it interfingers with a more familiar one that will help in placing it.

Field geologists trench, drill, and sample outcrops to identify the surface rocks and interpret their relationships. A geologic map may stop with surface data, or it may become a study in depth —a geologic column — which is like a vertical cut through the rocks at one or more selected points in the map. Such a detailed map may require elaborate laboratory work. Geologic columns showing the pile of strata and relative thickness are usually printed on the margins of the map. Formations (groups of strata) are the basic units of the geologic map, and each must be extensive enough to be meaningful to the user of the map. Folds, faults, and tilted formations are also indicated. For collecting purposes, the relatively simple geologic map that identifies surface rocks is the most useful.

Dip and Strike

Just as on topographic maps, the means employed to convey the map's message are symbols, textures, and colors. One of the most perplexing symbols on the map, that for dip and strike of rock masses, deserves some explanation because it is commonly used and is especially significant. Many rocks, such as sedimentary ones that were laid down in horizontal strata, have been tilted and otherwise disturbed. They may have been raised into anticlines (hills) or depressed into synclines (valleys), which may later have been eroded.

Dip and strike are more difficult to define than to understand. If a common 3 by 5-inch file card is rested on one long edge on a horizontal surface so that it stands vertically, the compass direction along the top edge of the card is its strike. Strike is a compass direction. Now if the vertical card is tilted to one side the angle of the tilt with the horizontal surface will be the dip. Dip is the inclination of the stratum from the horizontal, measured in degrees, and strike is the direction of the line of intersection along the horizontal plane. The symbol is \25, in which the longer line gives the compass direction and the short line is the dip, expressed in an accompanying figure in degrees.

On geologic maps, contacts of formations are shown in black lines which are solid if the contact is visible and dashed if it is not. Faults are shown by heavy black lines, and the symbols U or D show the direction of movement of the fault walls. Arrows show horizontal fault movement.

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Responses

  • ALESHA CRAIG
    How to use quadrangle maps fossils?
    7 years ago
  • milen
    How to determine the surface rocks on topographical maps?
    8 months ago

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