SKETCH OF THE METHODS OF SURVEY IN THE GEOGRAPH- 

 ICAL DEPARTMENT. 



By James T. Gardner, Geographer. 



"Washington, July 18, 1874. 

 Sir : I have the honor to inclose a sketch of the methods upon which 

 the work is based in the geographical department of this survey, and 

 also the results of my investigations to determine the true elevation of 

 Denver; regretting that the latter has occupied so much time that full 

 rei)orts of other branches of my work must be delayed till next year. 

 Very respectfully, 



JAS. T. GAEDNEE, 



Geogra])lier. 

 F. Y. Hayden, Geologist-in- Charge, 



U. S. Geological and Geogra-pMcal Survey of the Territories. 



The horizontal measurements of this survey are made by a connected 

 system of large and small triangles, developed from a measured base near 

 Denver. I have endeavored to bring the trigonometric work to such a 

 grade of accuracy that its errors will not be appreciable within the limits 

 of the Territory on maps of a scale of four miles to one inch. With this 

 purpose, the primary triangulation was expanded by me from the Denver 

 base, four miles of which are on the track of the K. P. E. E. and two 

 more are over very level ground. The termini are, however, so much 

 higher than the central part that the line of sight between them is never 

 less than 20 feet above the surface and generally 40 feet. Its length 

 was twice measured with a steel tape, compared before and after meas- 

 urement with the United States Coast-Survey standard. The tape was 

 under 20 pounds strain, and temperature-observations were taken every 

 five minutes. The profile was determined by leveling, and results of the 

 measurements corrected for level and temperature. Triangular pyra- 

 mids 30 feet high were erected at the stations on the plains, by which 

 the triangles were expanded to the mountains, where roughly-built stone 

 monuments were used to sight at, except in the very long lines, where 

 they became invisible, and the exact summit of the peak was taken. 

 The angles were measured with an 8-inch circle graduated to 10", and 

 reading to 5", constructed by Wm. Wiirdemaun for the United States 

 Coast-Survey. The measurements were generally repeated six times on 

 different parts of the circle. The plan of the triangulation is shown on 

 the accompanying map. 



Azimuths were observed at five of the principal trigonometric stations 

 by repeated observations between Polaris and an illuminated signal on 

 one of the lines of the triangulation. 



