In 1975 Chinese survey teams
remeasured Mount Everest, the highest
of the Himalayan mountains. Like the
Line British in 1852, they used the age-old
(5) technique of "carrying in" sea level:
surveyors marched inland from the
coast for thousands of miles, stopping
at increments of as little as a few feet
to measure their elevation, and mark-
(10) ing each increment with two poles.
To measure the difference in elevation
between poles, surveyors used an
optical level—a telescope on a level
base—placed halfway between the
(15) poles. They sighted each pole, read-
ing off measurements that were then
used to calculate the change in eleva-
tion over each increment. In sight of
the peaks the used theodolites—
(20) telescopes for measuring vertical and
horizontal angles—to determine the
elevation of the summit.
The Chinese, however, made
efforts to correct for the errors that
(25) had plagued the British. One source
of error is refraction, the bending of
light beams as they pass through air
layers of different temperature and
pressure. Because light traveling.
(30) down from a summit passes through
many such layers, a surveyor could
sight a mirage rather than the peak
itself. To reduce refraction errors, the
Chinese team carried in sea level to
(35) within five to twelve miles of Everest's
summit, decreasing the amount of air
that light passed through on its way to
their theodolites. The Chinese also
launched weather balloons near their
(40) theodolites to measure atmospheric
temperature and pressure changes
to better estimate refraction errors. 32
Another hurdle is the peak's shape.
When surveyors sight the summit.
(45) there is a risk they might not all
measure the same point. In 1975
the Chinese installed the first survey
beacon on Everest, a red reflector
visible through a theodolite for ten
(50) miles, as a reference point. One
more source of error is the uneven-
ness of sea level. The British
assumed that carrying in sea level
would extend an imaginary line from
(55) the shore along Earth's curve to a
point beneath the Himalaya. In
reality, sea level varies according
to the irregular interior of the planet.
The Chinese used a gravity meter to
correct for local deviations in sea level.
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Q32
It can be inferred from the passage that refraction would be most likely to cause errors in measurements of a mountain's elevation under which of the following conditions? D
A. When there are local variations in sea level
B. When light passes through humid air
C. When theodolites are used relatively far from the mountain peak.
D. When weather balloons indicate low air temperature and pressure.
E. When sea level has been carried in to within five to twelve miles of the summit.
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发表于 2007-11-9 23:54:00
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