Page 273 - Laboratory Manual in Physical Geology
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A. Topographic map base (contour lines), UTM grid lines B. Orthoimage base with all other data layers: contour lines,
(WGS84, Zone 15S), hydrography, and transportation features. UTM grid lines (WGS84, Zone 15S), geographic names and
boundaries, hydrography, and transportation features.
FIGURE 9.11 US Topo (USGS) digital topographic maps and orthoimages of
quadrangles. The latest series of USGS topographic maps are digital maps with data layers for
planimetric features and an orthoimage. Choose layers displayed on a topographic map base (A),
or else combine the topographic base as one layer of data on an orthoimage base (B) that can
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be enlarged with high resolution (C). Use Adobe Reader (or Adobe Acrobat ) to print products
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in GeoPDF format. To learn about US Topo GeoPDFs, watch a 6-minute USGS video ( http://
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gallery.usgs.gov/videos/663 ). Using Adobe Acrobat , you can save products as JPEG images for
enhancement with any photo processing software. However, the location of all roads is ©TomTom.
C. Enlarged portion of B.
depression (a depression from which water cannot drain) contour lines of the same elevations as those crossed
( FIGURE 9.12 , rule 12). At the top of a hill, contour lines walking uphill.
repeat on opposite sides of the rim of the depression. On
the side of a hill, the contour lines repeat only on the Spot Elevations and Benchmarks
downhill side of the depression. Elevations of specific points on topographic maps (tops of
peaks, bridges, survey points, etc.) sometimes are indicated
Ridges and Valleys directly on the maps as spot elevations beside a small
FIGURE 9.15 shows how topographic contour lines triangle, black dot, or x -symbol at the exact spot of the
represent linear ridge crests and valley bottoms. Ridges elevation indicated. The elevations of prominent hilltops,
and valleys are roughly symmetrical, so individual con- peaks, or other features are often identified. For example,
tour lines repeat on each side ( FIGURE 9.12 , rule 14). the highest point on the ridge in the west central part of
To visualize this, picture yourself walking along an FIGURE 9.13B has an elevation of 266 ft above sea level.
imaginary trail across the ridge or valley (dashed lines in The notation “BM” denotes a benchmark , a permanent
FIGURE 9.15 ). Every time you walk up the side of a hill marker (usually a metal plate) placed by the U.S. Geologi-
or valley, you cross contour lines. Then, when you walk cal Survey or Bureau of Land Management at the point
down the other side of the hill or valley, you recross indicated on the map ( FIGURE 9.7 ).
Topographic Maps and Orthoimages ■ 243