Instead, these lines are continuous, although they can continue off the edge of the map. The elevation value of the minor contour lines can be determined through the counting process from one index contour to another. The value written on the index line is the elevation. In addition to obeying the set contour interval for a map, contour lines should never branch (split) or simply end inside of the mapped region. On a topographic map, a contour line that is thicker than the rest and usually labeled with the elevation that it represents. Based on the reference map, an index is one that is bolder, darker contour line and has a value written on the line. Benchmark locations will normally be found in the area between contour lines. For example, a benchmark of 236ft will be found somewhere between the 230ft and 240ft contour line (if the contour interval is 10ft), or between the 235ft and 240ft contour line (if the contour interval is 5ft). These benchmarks are rarely located on a contour line and instead are usually identified by a black “x” or identified with the letters “BM” and with the elevations included in black numbers (as opposed to the brown numbers on index contours). An index contour is one of the ways that vertical dimension, or vertical scale, is demonstrated on a topographical map. These surveyed points are exact elevations above sea level and are commonly used to mark the elevations of mountains, hilltops, road intersections, and airport runways. Index contour lines make it easier to read elevation levels, much like chapters in a book. So, at this example of Streap, where the. Index contour lines are usually identified every fourth or fifth line on a topographic map. In addition to contour lines, topographic maps will also have benchmarks (actual surveyed points) in various locations on your map. So, if the index contours are every 50m then between each index contour there should be four skinny contour lines. Remember to use the contour lines to determine the elevations, and the bar scale on your map to measure the distance.
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