Manual of Topographic Methods

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U.S. Government Printing Office, 1906 - 88 Seiten
 

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Seite 52 - After learning how to make good observations the observer should place the utmost confidence in them, and never yield to the temptation of changing them because they disagree with some preceding observations. Such discrepancies are in general an indication of good rather than poor work. Stations or tripods which have been unequally heated by the sun or other source of heat usually twist more or less in azimuth. The rate of this twist is often as great as a second of arc per minute of time, and it...
Seite 11 - Plane tables, with telescopic alidades of the best type, used for secondary triangulation and height measurements. Plane tables of simple form, with ruler alidades, compasses, or solar attachment, used for traversing and minor triangulation.
Seite 60 - Each triangle, therefore, furnishes an equation of condition, which is known as an angle equation. The number of angle equations in any figure is equal to the number of closed triangles into which it can be resolved. But since certain of these are a consequence of the others, the number of angle conditions which it is desirable to introduce is less than the number of triangles. The number of angle equations in any figure is equal to the number of closed lines in the figure plus one, minus the number...
Seite 53 - ... occur. For the best work, observations should be made only when the air causes small or imperceptible displacements of signals. In applying this rule, however, the observer must use his discretion. Errors of pointing increase rapidly with increase of unsteadiness, but it will frequently happen that time may be saved by counterbalancing errors from this source by making a greater number of observations. Thus, if signals are fairly steady it may be economical to make double the number of observations...
Seite 55 - ... by exactly — — . A difference of half a degree either way is unimportant as respects periodic errors, and it is advantageous to have the minutes and seconds differ for the different settings. Field notes should be clear and full. The date, place, name and number of instrument used, and the names of observer and recorder should be recorded at the beginning of- each day's work at a station. The positions of the instrument and signals observed should be defined either by a full statement or...
Seite 32 - ... which break is recorded on the chronograph sheets at both stations. The breaks are repeated at every two seconds for at least one full minute. The operation is then reversed by the observer at the second station making the breaks which are recorded at both stations as before. The differences of time between the chronometers at the two stations are read from the chronograph sheets at each station and corrected for error of the chronometers. The results from the two chronograph sheets will differ...
Seite 15 - ... two seconds. The observer at the instrument is provided with a telegraph key, which may also be put in the circuit with the chronometer and chronograph, and as a star near the meridian crosses a thread in the telescope he records that fact by pressing on the key, which makes a record upon the chronograph along with the record of the chronometer. An illustration of the form of chronograph in use by the United States Geological Survey is shown in fig. 1. FIELD WORK. Since the observations for latitude...
Seite 64 - COMPUTATION OF GEODETIC COORDINATES. The next step is the computation of the latitude and longitude of the stations and the azimuth or direction of the lines connecting them. Initially, the latitude and longitude of some point are determined by astronomic observations, and this point is connected with the triangulation.
Seite 60 - The measured angles of each triangle should equal 180° plus the spherical excess. Each triangle, therefore, furnishes an equation of condition, which is known as an angle equation. The number of angle equations in any figure is equal to the number of closed triangles into which it can be resolved. But since certain of these are a consequence of the others, the number of angle conditions which it is desirable to introduce is less than the number of triangles. The...
Seite 17 - ... changed or not. This constitutes the observations upon a single pair of stars. For the determination of latitude twenty such pairs of stars should be observed each evening, if possible, and the same pairs of stars should also be observed on other evenings, assuming it to be possible.

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