2 A MANUAL OF TOPOGRAPHIC METHODS. 



put to use and will assist to a greater or less extent in the preparation of 

 the map. These maps and other material have been, or may be, adopted 

 by the Geological Survey. Their extent is represented upon the accom- 

 panying map, PI. i, as fully as 'possible, and they are enumerated, with a 

 brief description, as follows: 



SURVEYS UNDER THE UNITED STATES GOVERNMENT. 



The Survey of the Fortieth Parallel, from 1867 to 1872, under Mr. 

 Clarence King, embraced a zone of country 105 miles in breadth, extend- 

 ing from the meridian of 104° to that of 120° west of Greenwich, and 

 comprising an area of 87,000 square miles. The maps were made upon a 

 scale of 4 miles to an inch, with contours having a vertical interval of 300 

 feet. The work was controlled by triangulation, resting primarily upon a 

 base line measured by determining astronomically the latitudes of two 

 points, and the azimuth of the line connecting them; and, secondarily, upon 

 a base line extending nearly from the eastern to the western limits of the 

 work, the coordinates of the ends of which were determined astronomically, 

 the latitude by zenith telescope and the longitude by telegraphic time com- 

 parisons. Primary triangulation was done with theodolites reading to ten 

 seconds. Secondary triangulation and location were executed with minute 

 reading instruments, and topography was sketched and afterwards trans- 

 ferred to the platted framework. Heights were measured by barometer and 

 the vertical arc. 



The Geological and Geographical Survey of the Territories, under 

 Dr. F. V. Hayden, between 1873 and 1878, surveyed areas in Colorado, 

 New Mexico, Utah, Wyoming, Idaho, in all about 100,000 square miles. 

 The maps were published upon a scale of 4 miles to an inch, with a contour 

 interval of 200 feet. The base lines for the control of this work were 

 measured with steel tapes, under uniform tension, and with corrections for 

 temperature. Triangulation was carried on with 8-inch theodolites read- 

 ing to ten seconds, and was adjusted by a graphic method. Secondary 

 triangulation, the location of topographic details, and the measurement of 

 heights were effected by methods quite similar to those employed by the 

 Survey of the Fortieth Parallel. 



