284 REPORT UNITED STATES GEOLOGICAL SURVEY. 



convenient form, as they are taken into account in all subsequent tri- 

 angles in which these sides enter. In this manner the work is carried 

 on from station to station until all are located. 



The method of plotting the results as calculated from the uncorrectisd 

 foresights I consider a very good check on the previous work, as any 

 erroneous location in the previous work must appear at the new point. 



Figure 4, Plate XVII, represents the foresight intersections as plotted 

 from the preliminary calculations for the location of Mount Rito Alto. 

 All of these sights come within a circle of about 5 feet diameter, the 

 centre of which is assumed to be the most probable position, and the 

 sights were accordingly swung to that point. 



Figure 5 represents the condition of the foresight intersections on Sum- 

 mit Peak. Here we have one of the widest ranges occurring in the calcu- 

 lations of the work of 1876, and this is probably due to the fact that this 

 point presents a broad top as seen from the northeast, and all of the sights 

 from tba<^ direction are taken from a long distance; but as four out of 

 the eight sights used meet very nearly, those triangles sum up very 

 close to ISOo, while the triangles containing the sights from Mount Eito 

 Alto, Hunt's, Stations 24 and 28, sum up too large. I assumed the 

 error to be mostly on those sights, and was convinced that the intersec- 

 tions of the sigh1:s from Blanca and South Eiver Peaks are correct, and 

 accordingly swung the other sights to that point. 



Figure 6 represents the sights as plotted on Rio Grande Pyramid, and 

 the small triangle, the point chosen as the station. Figure 3 shows the 

 condition of sights on West Elk Peak. 



Figure 2 gives the intersections on Mount Wilson. The peculiar posi- 

 tion of this point makes it one of the best i)roofs of the accuracy of the 

 previous work that occurs in the whole system, although it was onl.y oc- 

 cupied as a secondary station. But it was sighted from every direction, 

 and the various points from which it was sighted were located more or less 

 by ditierent series of triangles. The arrow-point shows from which 

 direction the sight was taken, and the name of the point from which it was 

 taken is placed on the other end of the line, or, in other words, nearest 

 the station. All of these sights meet within a small area except one, 

 and that being so much out as compared with the others, it is probable 

 that some error was made in sighting from that station ; therefore it was 

 given no weight in the final location of the point. Many more exam- 

 ples might be given, but these few will be sufficient to give an idea of 

 the character of the work, and will also serve to illustrate the general 

 method of adjustment. 



I consider the foregoing method of adjustment very simple, and, at 

 the same time, sufficiently accurate for the class of work to which it is 

 applied. 



The primary object of this triangulation is to locate points at short 

 intervals, upon which the topographical work could be based, and that 

 these points should be located with such a degree of accuracy that the 

 errors would not be appreciable within the limits of the territory on our 

 maps, the scale being four miles to one inch, and I believe that this has 

 been fully accomplished. 



The accompanying map shows a general plot of the triangulation ; 

 all of the occupied stations are given, and a few of the located points. 

 I did not consider it necessary to give the numerous points that have 

 been located by foresight intersections, as it would only serve to make 

 the plot more confusing. 



The latitudes and longitudes are based on the stations located for us 

 by the kind co-operation of the United States Coast Survey at Denver, 



