628 GEOLOGICAL SURVEY OF THE TEEKITORIES. 



At Denver and Colorado Springs, the triangles are connected with 

 very accurate astronomical stations, located for us by the kind coopera- 

 tion of the United States Coast-Survey. Geodetic latitudes and longi- 

 tudes of forty-eight stations were reduced trigonometrically from Den- 

 ver, and plotted on a projection. From these points, the secondary 

 triangulation and topography was plotted with protractors by the topog- 

 raphers. The accuracy of the triangulation is determined by the clos- 

 ure of the triangles, whose observed angles should sum up to 180° plus 

 the spherical excess. The forty-seven triangles, completed in 1873 and 

 used in the final adjustment of the scheme, have a mean error of closure 

 of 10".3. During the season of 1874, a check-base will be measured in 

 the San Luis Valley, and we shall then be able to judge more closely 

 of the probable errors of measurement by this triangulation. At pres- 

 ent it appears as if they would not exceed two feet per mile. Twelve 

 thousand square miles were completed last summer, and more than 

 twice that amount partially finished. 



The secondary triangulation is done by the topographers with the 

 gradienter, whose circle is small, but its telescope powerful. The error 

 of closure of these triangles is about 2'. As they rest for their bases 

 upon the primary triangulation, the errors do not accumulate over any 

 large area. The topography is sketched from the trigonometric sta- 

 tions, and the principal points of the sketch located by triangulation. 

 The important roads are meandered. The magnetic needle is not used 

 for angular measurement, except for minor details. 



The vertical measurements of the survey are based on determinations 

 of the height of the trigonometric stations above Denver on the follow- 

 ing plan : 



The D. & S. P. E. E. had run a line of levels from Denver, whose 

 elevation is about 5,000 feet, up to Fairplay, at about 10,000 feet, and I 

 had these levels extended at our expense to the summit of Mount Lincoln, 

 about 14,000 feet high. These points are about sixty miles west of Den- 

 ver. On Mount Lincoln and at Fairplay we established barometric sta- 

 tions, and at Denver the station of the United States signal-office was 

 used for reference. Mnety miles to the south, at Caiion City, on the D, 

 & E. G. E. W., we had another barometric station at an elevation of 

 about 5,000 feet. The heights of the base-barometers above Denver 

 being thus known by railroad-levels, the surveying-parties, using mercu- 

 rial barometers (Green's mountain-barometer) observed at their stations 

 and camps synchronously with the observations at the permanent sta- 

 tions. 



The height of any point where observations were taken was deter- 

 mined by referring them to the permanent station nearest in altitude. 

 From the trigonometric stations, angles of elevation and depression 

 were taken to a great number of points to assist the topographer in 

 sketching the contours of the ground. These are, of course, referred to 

 the elevation of the trigonometric station, as determined barometrically. 



As it was impossible to proceed with our final maps until the elevation 

 of Denver was known,! have undertaken to put together into connected 

 chains all the various lines of railroad-levels that unite Denver with 

 the sea. It has been a labor of many months. The details and results 

 of the investigation are given in a sei^arate report. 



