Special Publications, Edição 23

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U.S. Government Printing Office, 1915
 

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Página 13 - Triangulation. — The method of surveying called triangulation is based on the trigonometric proposition that, if one side and the angles of a triangle are known, the remaining sides can be computed.
Página 22 - Of course, measurements can not be made when the temperature of the air is below the freezing point. The trough is mounted on a carriage which runs along a portable track. The successive bar lengths are referred to microscopes placed on posts set along the base.
Página 38 - ... at various points with precise level bench marks, and the trigonometric levels are adjusted to fit the precise leveling between those points. When this has been done, the resulting heights are of sufficient accuracy to serve as control points in contouring for all except very detailed topography. Tests of the accuracy of the trigonometric leveling indicate that it is an even chance that the difference between the two points determined by this method is correct within one or two inches to the...
Página 13 - A line, forming a side of one of the triangles and called the base, is measured with extreme care. The angles of each triangle are measured and the distances between the points are then computed, one from another, through the successive triangles, proceeding in regular order from the measured line or base.
Página 10 - ... Tests of the accuracy of the trigonometric leveling indicate that it is an even chance that the difference between the two points determined by this method is correct within one or two inches to the mile when vertical angles have been observed in both directions over the line. Gravity Measurements. The study of the force of gravity as a part of the geodetic problem has received attention for many years, and the Survey has developed methods and instruments with which the work can be done with...
Página 7 - ... be multiplied many times. To this must be added the shore line of Porto Rico, the Hawaiian Islands and the Philippine Islands. The length of the general shore line of only 14 of the principal islands of the latter group exceeds 11,000 miles.
Página 44 - The second series is known as the general charts of the coast. They are on a scale three times as large as those of the first series, and embrace more limited areas, such as the Gulf of Maine, etc. They are intended for coastwise navigation when the vessel's course is mostly within sight of land, and her position can be fixed by landmarks, lights, buoys, and soundings. The third series comprises the coast...
Página 44 - ... statute miles, a scale sufficiently large to give the features of the topography and hydrography with great clearness, portraying the appearance of the coast and the irregularities of the bottom with a detail quite close enough for the navigation of the principal harbors. The fourth series consists of "harbor charts" on large scales, intended to meet the needs of local navigation. On the Pacific coast the first series is similar to that on the Atlantic coast, and extends from San Diego, Cal.,...
Página 23 - Plan for the Reorganization of the Survey of the Coast, as adopted by a Board convened on the 3Oth of March, 1843, by direction of the President of the United States," explicit provision is made for the making of " all such magnetic observations as circumstances and the state of the annual appropriations may allow.
Página 7 - Europe to arrange for the construction of the necessary instruments and standards, some of the most important of these being made after his own design. The outbreak of the War of 1812 seriously interfered with his commissions, their completion being thereby delayed until the close of 1815, and in consequence actual field work was not possible until 1816. The work was suspended in 1818 and resumed in 1832. For the purpose of furnishing geographic positions and other data to State surveys the scope...

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