Geodetic Surveying and the Adjustment of Observations (Method of Least Squares)

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1911 Excerpt: ...the computed true time for the same set. 102. Time by Equal Altitudes. In this method the clock time is noted at which the sun (or a star) has the same altitude on each side of the meridian, from which the clock time of meridian passage (upper or lower transit or culmination) is readily obtained. By comparing the clock time with the true time of meridian passage the error of the observer's clock is at once made known. The advantages of this method over the method of single altitudes are as follows: the results are in general more reliable; the computation is simpler, as it does not involve the solution of a spherical triangle; no correction is required for refraction, parallax, semidiameter, nor instrumental errors; the latitude need not be known at all for star observations, and only very approximately for solar work. The observations may be made with a transit or a sextant (with artificial horizon), the latter being the most accurate. In either case several observations ought to be taken in immediate succession, as described below, and the average time used in the reductions. The probable error of the result should not exceed about two seconds with the transit nor about one second with the sextant. The actual error may be greater on account of the uncertainties of refraction. The method evidently assumes that the refraction will be the same for each of the equal altitudes, but on account of the lapse of time between the observations this is not necessarily true. The observation is commonly made with the sextant and on the sun. 102a. Making the Observation. As with the previous method, the best time for making an observation on the sun is between 8 and 9 o'clock in the morning and between 3 and 4 o'clock in the afternoon. The observations may be taken enti...