![]() ![]() The latitude and longitude of the map are recorded on the corners and sides. In the United States 15 minutes, which is a quarter degree, is about 18 miles of latitude and 15 miles of longitude. 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. The maps are printed on large sheets of paper. The United States is in the north latitudes and west longitudes. 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. Geological Survey (and those of the Canadian Survey), are based on the meridians of longitude and the parallels of latitude. The quadrangle maps, known as the national series of the U.S. The date of a topographic map appears in the lower right-hand corner. By such research, old collecting sites can be rediscovered. An old topographic map, however, may be found in a library or museum, and it may carry the old site description. 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. Topographic maps are periodically revised, especially those where urban growth has caused major changes in manmade structures. They are added when there is possibility of confusion between a small hill and a shallow depression. Short ticks (hachures) at right angles to the contour lines point down the slope. These markers should never be marred or disturbed.ĭepressions are also marked with contour lines. Usually it is a metal plate on a concrete post. 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. The elevation of certain prominent objects such as mountain peaks, bench marks, or lakes is given in black. 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.Įvery fourth or fifth contour line, depending on the scale, is printed darker and carries a figure of the elevation in feet. If the cliff is exactly vertical, the lines coalesce into one line. 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. Its contour lines would be almost on top of each other.Įach contour line, therefore, represents a series of points of like elevation that form an irregular circle. Conversely, a lake wedged between almost vertical cliffs would show very little increase in size as it rose. 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. Raising the lake level 10 feet would establish the 510-foot contour line. Then the shoreline of the lake would be the 500-foot contour line on the aerial map or on a contour map. ![]() The same thing could be accomplished by creating a giant lake with its surface 5 0 0 feet above sea level. From the air these rows of stakes would appear to form irregular circles some distance apart. (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. ![]() One way to understand the idea of contours is to visualize a view of the land from an airplane. ![]() From the improved valley road, a dirt road takes off right to a church and two houses. The hill on right has a gradual slope the one on left rises steeply above a tableland. The river flows into a bay partly enclosed by a sandspit. River valley and hills (upper drawing) and topographic map of same area (lower drawing). 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. Interpreting the topographic map, like interpreting poetry, takes some practice and some imagination. ![]()
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