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Show 84 were made. Since a large tract of country was covered by the party, the topographical stations were made on an average of one every seven miles. Points located T> y intersections from these stations or computed from the measured angles in connection with the bases measured in 1874 are taken as the frame- work or skeleton, for the map now being constructed by Mr. Spiller; and the minor details, roads, streams, &&, filled in from meander- lines and sketches and bearings made along the trails; the distances being measured by odometer, checked by frequent three- point stations, and the bearings from a topographer's transit. Each of the meander- stations, which numbered about seventeen hundred, was also a barometric station, so that continuous profiles are secured over the entire route of the party. The method of covering large areas by sketches alone from high stations, these stations made on an average of from 8 to 12 miles apart, is defective- 1st. Because it can only be applied in a mountainous country, where well- defined points exist which may serve as vertices for the triangles and for commanding positions from which the necessary sketches may be made; at best, then, it is only applicable to limited areas. 2d. Except in very exceptional districts not more than one- half the points Bighted from one station can be recognized or seen from another; particularly is this true of the lower points sighted. 3d. The details are good only in the immediate neighborhood of the points occupied, t. e., in the most inaccessible and practically the least important portions of the territory to be mapped. Any one at all cognizant with mathematical drawing can at once understand the difficulty and the absurdity of attempting to represent the details included in from 49 to 144 square miles of territory from sketches and bearings made from a single station ; of representing properly the sinuosities of roads and streams from occasional sights at prominent changes of direction, & c, and of representing by accurate contours the slopes of the country from such data as can be obtained from so few primary barometric bases, upon such a large scale as required for detailed topographical maps. In a partially mountainous region, watered by numerous streams, traversed by roads, and quite well settled, as parts of New Mexico and Arizona, where peaks above timber-line are rare, and well- marked points are not very numerous, the method would prove a failure as far as detailed maps are concerned, for, in addition to the objections to the system where commanding points can be obtained, the foundation of the system itself would be insufficient for its requirements, where such stations do not exist. In level districts or on the rolling plains it is worthless. The mountain- stations are, as they have always been used on this survey, essential to a good map; they are necessary for securing the mathematical accuracy needed in the location of points; for gaining a knowledge of the headwaters and upper drainage of minor streams which cannot be meandered, and for impressing upon the topographer's mind a good general ides, of the topography and the relations of great topographical features to each other; but to rely upon the knowledge gained from them for an accurate detailed map of the artificial as well as the natural features of the country is folly, reasoning as topographical features and stations are and must be, and not theoretically as they should be, found. As previously stated, the high stations were made during the past season as numerous as have ever been practiced by the advocates of this method, and its insufficiency showing itself in the lack of practical details in the lower and inhabited and traveled regions, dose odometric meanders of all the roads and principal streams and careful profiles were made for furnishing the information absolutely necessary for the work, and which could not possibly, without immense physical labor and expenditure of time and money, be secured from topographical stations. The odometer, if checked by the three- point problem every 8 or 10 miles, will give results which will compare very favorably with actual chaining, at a great gain of time and expense, and the bases measured by it serve well for the location of all points within a few miles of the trails followed, and which may not be better located from triangulation or topographical stations. Some form of this instrument must necessarily always form part of the outfit of a party engaged upon geographical work in unsettled regions, its importance increasing with the number of marked topographical stations decreased. In nearly every portion of the West, however, which still remains to be mapped, there is always to be found a sufficient number of marked natural objects to serve as vertices for a system of triangles, which may be located easily and at not too great expense, to be used as checks upon odometer- meanders to prevent the accumulation of errors by false measurements. Where the camps could not be located trigonometrically, latitude- checks were introduced by me; but these were seldom necessary in the mountainous region traversed by the party the past season. BAROMETRIC WORK. Cistern- barometers and thermometers were used at the hours prescribed by the printed instructions, in camps and on the march, at culminating points of the trails. |