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989
[[circled]]5[[/circled]]

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are shown by lines of blue dots and dashes.

Relief is shown by contour lines in brown, which on a few maps are supplemented by shading showing the effect of light thrown from the northwest across the area represented, for the purpose of giving the appearance of relief and thus aiding in the interpretation of the contour lines. A contour line represents an imaginary line on the ground (a contour) every part of which is at the same altitude above sea level. Such a line could be drawn at any altitude, but in practice only the contours at certain regular intervals of altitude are shown. The datum or zero of altitude of the Geological Survey maps is mean sea level. The 20-foot contour would be the shore line if the sea should rise 20 feet above mean sea level. Contour lines[[/upside-down]]
of 20 to 250 feet.
The aerial camera is now being used in mapping. From [[this]] information recorded on the photographs, planimetric [[maps]] which show only drainage and culture, have been made for some areas in the United States. By the use of stereoscopic [[plotting]] apparatus, aerial photographs are utilized also in the making [[of]] the regular topographic maps, which show relief as well [[as]] drainage and culture.
A topographic survey of Alaska has been in progress [[since]] 1898, and nearly 44 percent of its area has now been [[mapped.]] About 15 percent of the Territory has been covered by [[maps]] on a scale of [[1 over 500,000]] (1 inch = nearly 8 miles). For most of [[the]] remainder of the area surveyed the maps published are on scale of [[1 over 250,000]] (1 inch = nearly 4 miles). For some areas of particular economic importance, covering about 4,300 square miles the maps published are on a scale of [[1 over 62,500]] (1 inch = nearly 1 mi or larger
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