1880. | Geography and Travels. 63 
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 plains on its 
western edge, nearly all this area is occupied by rugged moun- 
tains. Among them may be noted the Tetons, the Gros Ventre 
and the northern part of the Wind River ranges. 
The plan of the geographical work has been sketched in some 
detail in previous reports of this survey, and, therefore, nothing 
more than a brief epitome will here be attempted. 
The whole work is based upon a system of triangulation, car- 
ried on with an eight inch theodolite, reading to 10”. In the 
scheme there were two base-lines measured, one near Fort Steele, 
Wyoming, the other on Bear River, in South-eastern Idaho. Each 
error of closure of the triangles in the expansion was 5.3”. The 
sides ranged from twenty to seventy 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 esti- 
mated, and by perspective sketches, on which the topographical 
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 maps, in the office, the map-sketches are cor- 
rected by these locations. On these three sheets, about 3300 
points, including stations, were located, being one in every ten 
square miles. Altogether, between 600 and 700 stations were 
occupied, or about one in each fifty 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 circle of the theodolite. 
Camps, stations, and all salient points on the routes traversed, 
were measured by the former instrument. Aneroids were used- 
but little and the results accepted with great caution. The verti- 
cal circle was used in determining the relative heights of all points 
within range of the stations—all peaks, passes, gaps, heads of ae 
Spurs, etc., in short, everything that could be located, even ap- 
Proximately. Thus the heights of a great number of points were 
easily determined, and these, placed upon a perspective sketch, 
Which may be supposed to be reasonably accurate, indicate ap-— 
Proximately the heights of all portions of the sketch. — — —  — 
Difference of heights are expressed on these maps by contours. oe 
Space between two of these grade curves represents a differ- 
