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| a two-dimensional (flat) representation (model) of a three-dimensional land surface (landscape). It shows landforms (hills, valleys, slopes, coastlines, gullies) and their relief (difference in elevation) by using contour lines to represent elevations of hills and valleys. The contour lines are the most distinguishing features of a topographic map. |
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a section of Earth's surface that is bounded by lines of latitude at the top (north) and bottom (south) and by lines of longitude on the left (west) and right (east).
Latitude and longitude are both measured in degrees. latitude is measured from zero degrees at the equator to ninety degrees North (at the north pole) or 90 degrees south (south pole). Longitude is measured in degrees east or west of the prime meridian, a line that runs from the north pole to the south pole through greenwich, england.
For finer measurements each degree can be subdivided into 60 equal subdivisions called minutes ('), and the minutes can be divided into 60 equal subdivisions called seconds (").
Quadrangle maps are published in several sizes, but two are most common: 15-minute quadrangle maps and 7.5-minute quadrangle maps. the numbers refer to the amount of area that the maps depict, in degrees of latitude and longitude. A 15-minute topographic map represents an area that measures 15 minutes of latitude by 15 minutes of longitude. Each 15-minute map can be divided into four 7.5-minute maps.
Because longitude lines form the left and right boundaries of a topographic map, north is always at the top of the quadrangle. this is called grid north (GN) and is usually the same direction as true north on the actual earth. Magnetic compasses are attracted to the magnetic north (MN), currently located northwest of Hudson Bay in Northern Canada, about 700 km (450 mi) from the true north pole. |
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| a compass-like symbol on the bottom margin of topographic maps, which shows the difference in degrees between compass north (MN) and true north (usually a star symbol). also shown is the declination between compass north (star symbol) and grid north (GN). the magnetic pole migrates very slowly, so the declination is exact only for the year listed on the map. |
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Lines of equal elevation (i.e., the coastline and 300-ft line).
topographic maps are miniature models of earth's three-dimensional surface, printed on two-dimensional pieces of paper. two of the dimensions are the lengths and widths of objects and landscape features. but the third dimension, elevation (height), is shown using contour lines. each contour line connects all points on the map that have the same elevation above sea level. |
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| the heavy brown contour lines. they have elevations printed on them (whereas the lighter contour lines do not). Index contours are your starting point when reading elevations on a topographic map. also notice that the index contours are labeled with elevations in the increments of 400 ft. this means that the map had five contours for every 400 ft of elevation, or a contour interval of 80 ft. |
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| specified on most topographic maps in feet or meters. all contour lines are multiples of the contour interval about the specific surface (almost always sea level). for example, if a map uses a 10-ft contour interval, then the contour lines represent elevations of 0 ft (sea level), 10 ft, 20 ft, 30ft, 40ft, and so on. most maps use the smallest contour interval that will allow easy readibility and provide as much detail as possible. |
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1. every point on a contour line is of the exact same elevation; that is, contour lines connect points of equal elevation. the contour lines are constructed by surveying the elevation of points, then connecting points of equal elevation.
2. interpolation is used to estimate the elevation of a point B located in line between points A and C of known elevation.
3. Extrapolation is used to estimate the elevations of a point C located in line beyond points A and B of known elevation. to estimate the elevation of point C, use the distance between A and B as a ruler to estimate in line to elevation C.
4. contour lines always seperate points of higher elevation (uphill) from points of lower elevation (downhill).
5. contour lines always close to form an irregular circle. but sometims part of a contour line extends beyond the mapped area so that you cannot see the entire circle formed.
6. the elevation between any two adjacent contour lines of different elevation on a topographic map is the contour interval. often every 5th contour line is heavier so that you can count by five times the contour interval. these heavier contour lines are known as index contours, because they generally have elevations printed on them.
7. contour lines never cross each other except for one rare case: where an overhanging cliff is present. in such a case, the hidden contours are dashed.
8. contour lines can merge to form a single contour line only where there is a vertical cliff or wall.
9. evenly spaced contour lines of different elevation represent a uniform slope.
10. the closer the contour lines are to each other the steeper the slope. in other words, the steeper the slope the closer the contour line.
11. a concentric series of closed contours represents a hill.
12. depression contours have hatch marks on the downhill side and represent a closed depression.
13. contour lines form a V-pattern when crossing streams. the apex of the V always points upstream (uphill).
14. contour lines that occur on opposite sides of a valley or ridge always occurs in pairs.
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| low-lying land bordered by higher ground. |
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| rounded elevation of land, mound |
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| linear or elongate elevation or crest of land. |
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| short ridge or branch of a main ridge |
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| low point in a ridge or line of hills; it resembles a horse saddle. |
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| low point/area in a landscape from which surface water cannot drain; contour lines with hatch marks. |
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| closely-spaced contour lines |
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| widely-spaced contour lines |
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| dashed contour line that crosses a solid; the dashed line indicates what is under the overhanging cliff. |
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| elevations of specific points on topographic maps (tops of peaks, bridges, survey points, etc.) sometimes are indicated directly on the maps besides the symbols indicated for that purpose. the notation "BM" denotes a benchmark, a permanent marker (usually a metal plate) placed by the U.S. Geological Survey or Bureau of Land management at the point indicated on the map. elevations are usually given. the "x" symbol labeled "BM463", indicating that "x" marks the location of the benchmark that was exactly 463 ft above sea level at the time of its placement. |
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the difference in elevation between landforms, specific points, or other features on a landscape or map.
regional relief is the difference in elevation between the highest and lowest points on a topographic map. the highest point is the top of the highest hill or mountain; the lowest point is generally where the major stream of the area leaves the map, or a coastline. |
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is a measure of the steepness of a slope. one way to determine and express the gradient of a slope is by measuring its steepness as an angle or descent (expressed in degrees). on a topographic map, gradient is usually determined by dividing the relief (rise or fall) between two points on the map by the distance (run) between them(expressed as a fraction in feet per mile or meters per kilometer).
for example, if points A and B on a map have elevations of 200ft and 300ft, and the points are located 2 miles apart, then:
gradient= 100 ft/ 2mi= 50 ft/mi |
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