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supervise such office computations. Since the creation in 1903 of the separate section bearing the above title, all office work connected with the reduction and publication of the results of spirit leveling has been in charge of the chief of that section.

The methods and instruments employed in the astronomic and trigonometric work, as well as in the spirit leveling, of the Geological Survey have developed as the work has progressed. This development has been in the direction of economy and efficiency as well as accuracy, so that at the present time, with a diminished expenditure for both field and office work, a much higher degree of accuracy is obtained than was possible at the outset.

Triangulation. In early years the instruments used in triangulation were vernier theodolites reading to 10 seconds, with circles 6, 7, 8, 10, and 11 inches in diameter. In 1889 there were substituted for these, 8-inch theodolites reading by microscope to 2 seconds, and these have since been exclusively employed in the primary triangulation. It is believed that the instruments at present used are of as high a grade as those in use in any other part of the world, and that, by employing sharper signals, by exercising more care in selecting the times for observing, and by taking a larger number of measurements of angles than heretofore, results can be obtained with them equal to the best secured in geodetic work.

The signals used differ with the differing facilities afforded by the various localities. The commonest form that generally used where sawed timber can be obtained-is the ordinary quadripod and pole. In the Rocky Mountain region a monument of stones is generally used. Each station is marked with a bronze tablet cemented in solid rock or in a dressed-stone post set in the ground, and sometimes by an iron bench-mark post. One or more witness marks are always left.

Base measurement.—In the early years of the work base lines were measured with secondary base bars. These were used until 1887, when steel tapes 300 feet in length, under constant tension, were substituted and have since been employed. They have been found more advantageous for the following reasons: The base can be measured more rapidly, and, owing to the diminished number of contacts, with quite as great accuracy. By making the measurements on cloudy days or at night, the correction for temperature can be determined nearly or quite as accurately. Longer bases are measured, thus simplifying the expansion; and because of the cheapness and speed of this method bases are measured more frequently, thus affording a greater number of checks on the triangulation. After the base line has been fully prepared for measurement and the men handling the tape have had a little training, a speed of a mile an hour can be attained, so that under favorable conditions a base 5 miles in length can be measured twice in one day.

Bull. 227-04- -5

The precision of measurement is represented by a probable error of from 1/100000 to 1/300000, sufficiently accurate for all primary triangulation not required in the solution of geodetic problems.

Primary traverse.-Where it is inexpedient, because of great expense, to procure primary control by means of triangulation, as in a heavily forested and level country or in the prairies of the Central West, the means adopted is the running of traverse lines of a high degree of accuracy. The instruments used are, a good transit reading by vernier to 30 seconds, a 300-foot steel tape, a 100-foot steel tape, 2 standard thermometers, 4 hand recorders, 2 flag poles, and for azimuth observations a good watch and electric hand lamps. In order that the effect of errors of angular measurement may be minimized, astronomic azimuths are observed at intervals of 8 to 10 miles. The effect of errors in both azimuth and distance is practically eliminated by closing the traverse back on itself or on some other adjusted primary traverse, or, better still, by connecting with primary triangulation stations or astronomic positions.

The initial and terminal points of the primary traverse, and intermediate points on the line of the traverses which may be used as tie points for primary or secondary control, are indicated by permanent marks bronze tablets cemented in masonry, or bench-mark posts set in the ground.

In localities over which the United States land surveys have been extended township and section corners serve as such permanent marks. Spirit leveling.-Beginning with the season of 1896 the field parties engaged in the topographic mapping have conducted careful spirit leveling on a scale more extended and of a quality more accurate than had ever before been attempted in this country. This was in pursuance of the act of Congress providing for the sundry civil expenses of the Government for the fiscal year 1896–97, a paragraph of which is as follows:

* * *

For topographic surveys in various portions of the United States Provided, That hereafter in such surveys west of the ninety-fifth meridian, elevations above a base-level located in each area under survey shall be determined and marked on the ground by iron or stone posts or permanent bench marks, at least two such posts or bench marks to be established in each township, or equivalent area, except in the forest-clad and mountain areas, where at least one shall be established, and these shall be placed, whenever practicable, near the township corners of the publicland surveys; and in the area east of the ninety-fifth meridian at least one such post or bench mark shall be similarly established in each area equivalent to the area of a township of the public-land survey.

It was realized by the framers of the above act that it would be impossible to establish and mark on the monuments their exact height above sea level. Any attempt to do this would have necessitated the running of thousands of miles of precise levels in order to connect with sea level the initial points within the various areas under topo

graphic survey, and this would have cost immense sums and occupied several years, during which the topographic surveys would have continued without being accompanied by spirit leveling and the establishment of the bench marks required. The act was therefore so framed as to permit of the acceptance of some point within each area under topographic survey as a central datum point for that area, and the elevation of the initial bench mark established there was determined as nearly as practicable from existing elevations adjusted through by railway levels or other levels brought from the sea. In consequence, though all the elevations connected with the same central datum point agree one with the other, they may not be reduced to mean sea level because of the differences between the primary elevations upon which leveling in the various localities is based. However, prior to and since 1896 precise-level lines have been extended by this Survey and other organizations to more accurately determine inland elevations above sea level, and the elevations originally determined have been corrected from time to time in publications, so that at the present time nearly all the central points have been reduced to mean sea-level datum, carrying with them all the levels resting thereon.

It was decided in 1896 that in addition to determining the elevations of these bench marks they should be instrumentally connected with the horizontal measurements taken in the course of topographic surveys, but only with such accuracy as would permit of their being properly plotted on the resulting maps, the location and elevation of these bench marks being published in two ways-first, by a symbol on the atlas sheets, the letters B. M., and the elevation to the nearest foot; second, by lists of bench marks in the annual reports or bulletins of the Survey, with a full description of each bench mark and its exact elevation above sea level to the thousandth of a foot, as adjusted and referred to the various central datum points, these lists to be corrected in publications from time to time as better connections are made with sea level.

It was thought that these bench marks should be of the most attractive and substantial character consistent with reasonable cost, and after an examination of the various forms used by other Government and city surveys, two standard styles were selected. First, a circular bronze or aluminum tablet, 3 inches in diameter and onefourth inch thick, appropriately lettered, having a 3-inch stem to be cemented into a drill hole in the vertical walls of public buildings, bridge abutments, or other substantial masonry structures. The second form, to be employed where masonry or rock is not accessible, consists of hollow wrought-iron posts, 4 feet in length and 34 inches in outer diameter, split at the bottom and expanded to 12 inches, so as to prevent both the easy subsidence of the post and its being maliciously pulled out of the ground. The iron is heavily

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