LETTER TO THE SECEETARY. XXI 



The three regular topographical atlas sheets are on the same scale (4 miles to the 

 inch) as those in the Colorado Atlas. These sheets illustrate j)ortion8 of Wyoming, 

 Idaho, and Utah. Each of them covers 2^° of longitude and 1J° of latitude, and in- 

 cludes an area of about 11,000 square miles. 



The southeastern of these sheets covers the country from longitude 107° to 109° 30' 

 and from latitude 41° 45' to 43°. It includes the barren i3lateaus of the continental 

 divide, north of the Union Pacific Railroad, the valleys of the Sweetwater and Wind 

 Rivers, and a part of the Wind River range. 



The southwestern sheet lies directly west of the latter, extending to longitude 112°. 

 It embraces the Green River Basin, and, farther westward, a succession of parallel 

 ranges of no great height, alternating with broad valleys drained by Bear River and 

 branches of the Snake. 



The third sheet lies north of the last, extending to latitude 44° 15'. Besides a small 

 portion of the Snake River j)lains on its western edge, nearly all this area is occuijied 

 by rugged mountains. Among them may be noted the Tetons, the Gros Ventre and 

 the northern part of the Wind River ranges. 



The j)lan of the geographical work has been sketched in some detail in previous re- 

 ports of this Survey, and, therefore, nothing more than a brief epitome will here be 

 attemi^ted. 



The whole work is based upon a system of triangulation, carried on with an eight- 

 jnch theodolite, reading to 10". In the scheme there were two base-lines measured, 

 one near Fort Steele, Wyoming, the other on Bear River, in Southeastern Idaho. Each 

 of these was between 5 and 6 miles iu length. The mean error of closure of the tri- 

 angles in the expansion was 3". 5. The sides ranged from 20 to 70 miles in length. 

 Altogether, forty-nine points were located by the primary triangulation. 



Secondary triangulation was carried on by a theodolite reading to minutes. The 

 mean error of closure of secondary triangles is 3'. 



The topography was secured from elevated points by map sketches made on an 

 assumed scale, with distances and angles estimated, and by perspective sketches, on 

 which the tox^ographical features were represented as they appeared to the observer 

 at his station. All salient points in the landscape, peaks, angles of plateau, minor 

 summits and hills, and junctions of streams, were located by intersections of sight 

 lines from two or more stations ; and, in plotting the ma^^s in the office, the map- 

 sketches are corrected by these locations. On these three sheets, about 3,300 poiuts, 

 including stations, were located, being one in every ten square miles. Altogether, 

 between 600 and 700 stations were occupied, or about one in each 50 square miles. 



A few words as to the measurements of heights, and the method of construction of 

 contour lines. Elevations were measured by means of the barometer, and the vertical 

 circleof the theodolite. Camps, stations, and all salient points on the routes traversed 

 Avere measured by the former instrument. Aneroids were used but little and the re- 

 sults accepted with great caution. The vertical circle was used in determining the 

 relative heights of all poiuts within range of the stations — all peaks, passes, gaps, 

 heads of spurs, &c., in short, everything that could be located, even approximately. 

 Thus the heights of a great number of points were easily determined, and these, 

 placed upon a x^erspective sketch, which may be supposed to be reasonably accurate, 

 indicate approximately the heights of all portions of the sketch. 



Difference of heights are expressed on these maps by contours. The s^jace between 

 two of these grade curves rexiresents a difference of elevation of 200 feet. Where the 

 slopes arc gentle the curves are far apart, while among the cliffs of the mountains and 

 plateaus tbey are crowded together, iu many places being almost run into a single 

 line. These curves are not " run," nor are they accurately located, as would be done 

 in a minute survey. They do, however, express the orography, and, approximately, 

 the elevation, over the whole map. They are constructed mainly from the persi)cctive 

 sketches, aided and directed by the measured heights. As an example of tlie method 

 of their construction, take a mountain spur, starting from the peak and extending to 

 the valley below. Its summit and base, and each poiut of change of slope, are located, 



