Page 271 - Laboratory Manual in Physical Geology

HOW TO USE A POCKET STEREOSCOPE

North Imaginary plane
Distance between between two halves
eye pupils of user of stereogram

50 60

Left line Right line
of sight of sight

Stereogram
First have a partner measure the distance
between the pupils of your eyes, in mm. Set
that distance on the stereoscope as shown
above. Then center the stereoscope over the
stereogram, as above. Look through the
To view this stereogram without a stereoscope, cross your eyes until a stereoscope, and move it around slightly until
third white dot appears between the first two. The image should then the image becomes three-dimensional.
appear three-dimentional.

FIGURE 9.9 Stereogram (left) of Mount Meru region and how to view it. There are two methods used to view a stereogram so

it appears to be three-dimensional: use a pocket stereoscope or cross your eyes. The tallest feature in this stereogram is Mount Meru, a
4,566-meter-high volcano located in east Africa (Tanzania), about 70 km (44 mi.) west of Mount Kilimanjaro. Both images in the stereogram
TM
are 20 km wide and 37 km tall (courtesy of NASA/PJL/NIMA). To view Mount Meru In Google Earth , search: 03 14 36S, 36 45 41E.
distortion is then removed, and the exact elevations of the layered digital maps called “US Topo” maps ( FIGURE 9.11 ).

contour lines on the map are “ground truthed” (checked The map layers can be turned on or off, including an aerial
on the ground) using very precise altimeters and GPS. photograph layer called an orthoimage . The digital products
The final product is a topographic map like the one in can be downloaded free of charge (no registration required)
FIGURE 9.10 . or they can be ordered as printed paper maps.

Notice how the contour lines in FIGURE 9.10 occur Aerial photographs (taken from airplanes) are usually

where the landscape intersects horizontal planes of specific taken at angles oblique to the landscape, but topographic
elevations: 0, 50, and 100 feet. Zero feet of elevation is maps are representations of the landscape as viewed from
sea level, so it is the coastline of the imaginary island. You directly above. Orthoimages are digitized aerial photographs
can think of the contour lines for 50 and 100 feet above or satellite images that have been orthorectified, corrected for
sea level as additional water levels above sea level. An “x” distortions until they have the same geometry and uniform
or triangle is often used to mark the highest point on a scale as a topographic map. Therefore, an orthoimage cor-
hilltop, with the exact elevation noted beside it. The high- relates exactly with its topographic map and reveals visual
est point on the map in FIGURE 9.10 is above the elevation attributes of the landscape that are not visible on the topo-

of the highest contour line (100 feet) but below 150 feet graphic map. The topographic map, orthoimage, and other
(because there is no contour line for 150 feet). In this orthorectified “layers” of data can be added or removed to
case, the exact elevation of the highest point on the island give the viewer extraordinary perspectives of the landscape.
is marked by spot elevation (“x” labeled with the elevation All of this can be done at US Topo, courtesy of the USGS
of 108 feet). and their partners. One can display features like hydrography
(water bodies), roads, and UTM grid lines on a topographic

US Topo Maps and Orthoimages base ( FIGURE 9.11A ), or display the topographic map layer

on an orthoimage base ( FIGURE 9.11B ). All layers can be

Historic USGS map series ( FIGURES 9.1C and 9.3 ), and enlarged with outstanding resolution ( FIGURE 9.11C ). To learn

those of most other countries, are one-page paper maps. more about obtaining and using US Topo products, watch a
However, the latest series of USGS topographic maps are 6-minute USGS video ( http://gallery.usgs.gov/videos/663 ).
Topographic Maps and Orthoimages ■ 241