Physics of the Earth - II The Figure of the Earth Bulletin of the National Research Council (1931) / Chapter Skim
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Chapter XIV. The Determination of Elevations
Chapter XIV. The Determination of Elevations
Pages 212-220
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From page 212... ...
S Coast arid Geodetic Survey The surface of the earth presents a varied succession of' plains7 hills valleys and mountains with an extreme cha.n~,e in elevation, in the United States, from 276 feet below sea-level in Death Valley to 14,496 feet above sea-level at the top of Mt.
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From page 213... ...
The determination of the relative elevations of widely separated points with a high degree of' accuracy requires the use of the most refined leveling instruments and the adoption of observing methods that will insure minimum accumulation of' the inherent leveling errors. The instrument and rods used for leveling of high precision in this country are described in Chapter ~II.
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From page 214... ...
Leveling that allows closure checks treater than the limit stated for third-order work, such as trigonometric leveling, ba.ron~etric leveling, or " flying " levels, shall be ~ o~sidered as belonging to the lower order of work. No bench marks established by leveling that is less accurate than that of the third order, as above described, shall be marked by standard bench-mark tablets, except that in mountainous regions inaccessible to ordinary
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From page 215... ...
A correction, called the orthometric correction, is also applied to the differences in elevation due to the fact that in this work level surfaces cannot be considered parallel surfaces. In ordinary leveling it is sufficient to assume that all level surfaces are parallel since the error in the results introduced by this assumption is small compared with the errors of leveling; but where great accuracy is desired it is necessary, especially if the level line is at a high altitude and runs in a north and south direction, to take account of the fact that level surfaces converge as the poles of the earth are approached.
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From page 216... ...
This work is perpetuated by bench marks consisting of' metal tablets, bearing an appropriate legend, set in buildings, masonry structures, and in the top of' stone or concrete posts. The bench marks about 2: miles apart on an average, are located in towns, at road crossings and in other places where they can be conveniently reached by surveyors or engineers who have need of them.
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From page 217... ...
DATUM Bel:'ore art elevation can be assigned to a bench mark it is necessary to adopt a reference plane or datum. For local leveling projects, where only relative elevations are used, this is usually any convenient arbitrary plane.
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From page 218... ...
VARIATION OF MEAN SEA-LEVF,L FROM A LEVEL SURFACE Although for datum purposes it is assumed that mean sea-level is everywhere at the same elevation on the open coasts, the results of leveling, between adjacent tidal stations have in a number of instances shown a larger difference between the elevation of the mean sea-level planes than earl be attributed to the systematic or accidental errors of the leveling observations. AYhen the tidal stations on the Atlantic coast at Fort Hamilton, N
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From page 219... ...
On the Pacific Ocean, starting at San D;e~,o, Calif., where mean sealevel is 0.40 meter higher than at Galveston, and proceeding, northward the elevation of mean sea-level at the following places is: San Peclro, Calif., +0.32 meter; San Francisco, Calif., +0.44 meter; Fort Stevens, Oreo., +0.79 meter; Seattle, Wash., +0.66 meter; and Anacortes, mash., +0.6a~ meter. These values seem to indicate that along the coast of the Gulf of Mexico there is a general slope downward in the mean sea-level surface front west to east, and that along the Atlantic and Pacific coasts there is a general slope upward from south to north.
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From page 220... ...
Mountain peaks do not in general have a trail leading to the top over which spirit levels can be run, and then, too, there are easier and less expensive methods for determining their elevations with an accuracy sufficient to make them valuable for many purposes. One of the most common methods for determining the elevations of mountain peaks is that of leveling by vertical angles, or, as it is usually called, trigonometric leveli.n~.
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