单项选择题
The reference datum used in determining the heights of land features on most charts is（）.
A.mean sea level
B.mean high water
C.mean low water
D.half-tide level
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The purpose of the “set” hand on an aneroid barometer is to（）.
A.adjust the barometer
B.indicate any change in the reading of the barometer
C.provide a correction for height above sea level
D.provide a correction for temperature changes -
单项选择题
The error in the measurement of the altitude of a celestial body，caused by refraction，increases as the（）.
A.horizontal parallax decreases
B.observer's height above sea level increases
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单项选择题
The boom stops on a pedestal crane prevent the boom from（）.
A.being raised too high
B.swinging at sea
C.overloading when not in use
D.being lowered too low
