Physics of the Earth - II The Figure of the Earth Bulletin of the National Research Council (1931) / Chapter Skim
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Chapter XV. The Determination of Geographic Positions
Chapter XV. The Determination of Geographic Positions
Pages 221-242
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From page 221... ...
The latitude of a point is its angular distance north or south of the Equator. The longitude of a point is the angular distance measured alone, the Equator between the meridian through the point and the meridian through some reference station.
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From page 222... ...
The characteristic feature of this method is that instead of measuring the zenith v distance of a single star, the d~tterence In tile zenith distances of two stars is measured micrometrically as they pass the meridian, the two stars selected to -form a pair having approximately the same zenith distance but being on opposite sides of the zenith. The mean of their declinations, corrected for one half the difference of their zenith distances, would therefore be equal to the declination of the zenith or the latitude o:: the place of' observation.
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From page 223... ...
An instrument designed especially for determining astronomical latitudes by the Talcott method.
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From page 224... ...
While the primary purpose of this instrument is to determine time by observing the transit of stars across the meridian. latitude can be determined by the Talcott method if the instrument is equipped with the special sensitive levels which are shown in the picture.
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From page 225... ...
The probable error of the determination of the latitude at one of these stations from a single night's observing is seldom greater than 0".03. The Talcott method is used at these observatories, with a zenith telescope somewhat larger than the one generally employed in geodetic astronomy.
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From page 226... ...
The time required for the transmission of' the signals was eliminated by having part of the signals sent in one direction and part in the other. There are many cases in the work of the Coast and Geodetic Survey in which two observers determined the difference of longitude between each two of a number of stations form.in~, a circuit..
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From page 227... ...
3. Theodolite for first-order triangulation.
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From page 228... ...
The great advantage of the radio time signals in longitude work consists in the elimination of' one field observing unit and the flexibility of the method. Previously the field longitude stations had to be located alongside a telegraph line or a special wire had to be run to the field observatory.
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From page 229... ...
GEOGRAPHIC POSITIONS DETERMINED BY GEODETIC METHODS The errors inherent in Fair positions by astronomical methods are obviated by triangulation. A series of points several miles apart are selected, so located that the lines joining intervisible pairs of points will form a series of connected triangles.
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From page 230... ...
of the United States and because it was common to two great arcs of triangulation extending across the country, one along the 39th parallel of latitude and the other along the 98th meridian. A latitude and longitude were computed for the station Meades Ranch which made the sum of the squares of the differences between the astronomical latitudes and longitudes and the triangulation latitudes and longitudes a minimum.
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From page 231... ...
The probable error of the length of the Salt Lake base, as measured by the Eimbeck bars, was one part in five million. The various bars used in base measurements had different lengths but the usual one was :hve meters.
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From page 232... ...
Experiments made in 190~; by the Coast and Geodetic Survey in measurements of bases with invar tapes proved that measurements could be made satisfactorily even in sunshine. There is only one difficulty in using, invar tapes or wires in precise base measurements and that is due to the instability in length of the invar alloy.
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From page 233... ...
Extreme care was taken at the Bureau of Standards in determining the length of the eight tapes used in the fielcl, both before and after the field measurements, the Bureau certifying that the probable error of standardization of none of the tapes exceeded 1 part in 2,000,000. The probable error of the length of the base from, field measurements and observations was, + 3.40 mm Since the total measured length of the base was about 40,048 meters (the projected length being, ~ ~ v ~ _ 33,638 meters)
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From page 234... ...
The probable error of a direction resulting front the adjustment of a triangulation net is usually about +0"~. The theodolite is mounted on a wooden stand where the stations are located on mountain peaks or sharp hills or ridges; where the stations are in level country the instrument is mounted on a tower which is used to overcome the effect of the curvature of the earth and to lift, the lines above obstructions such as trees or buildings.
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From page 235... ...
~70\ OF 6~0~76 F037~3 233 FIG. 4.~ri~nguJ~i~ to~er~his steel tower can be quickly tacked down, moved by ~ Eagle tack to ^ 1~, ~ ~ ~ ~ ~ Ha.
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From page 236... ...
A Laplace azimuth is the astronomic azimuth of' a line between two triangulation stations corrected for the effect of the deflection of the vertical at the point of observation. The astronomic azimuth is obtained by measurin,, the horizontal angle between Polaris an:1 a triangulation station and correcting the measured angle by the angular distance of Polaris from true north at the time of' observation.
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From page 237... ...
axed. Ibis process usually continues until such time as ~ large area bus been r~tber co~pletel~ covered Aim ~ Ir~e-wolk of trisn~l~tion arcs.
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From page 239... ...
It is rather remarkable that the closing error for the perimetric circuit of 6,300 miles is only 33 feet or one part in 84S,000. It is expected that, by the end of 1930, the triangulation net coverin, the eastern half of the United States will be ready for a: readjustment similar to that which was made for the western half of the country.
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From page 240... ...
There are areas above the sea-level surface and other areas which are depressed below it. These areas have excesses and deficiencies of' mass, respectively, which deflect the plumb line, or direction of gravity, from the line that is normal to the ellipsoid surface.
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From page 241... ...
. Discusses the errors in determining time with held types of ustro Domic~I transit in Coast and Ccodetic Survcy and the closures obtuincd in longitude loops aside metric Dire circuits.
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From page 242... ...
. A statement and discussion of the principles of geodesy and of geodetic surveying.
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