78 A MANUAL OF TOPOGRAPHIC METHODS. 



the purpose may be adopted. Whenever elevations have been determined 

 within the area to be surveyed by the United States Coast and Geodetic 

 Survey or the United States Lake Survey, they may be accepted without 

 question. The work of these organizations has been sketched in the early 

 pail of this volume and is shown upon map No. 1. 



When these determinations are not available, initial bench marks 

 should, if possible, be obtained from the profiles of railroads traversing 

 the district. These have been adjusted and the results published in the 

 Dictionary of Altitudes (Bulletin No. 76, U. S. Geological Survey). In 

 case there are no railroads to furnish initial datum points, as may occur 

 in the sparsely settled regions of the West, or the profiles available are 

 regarded as untrustworthy, it may become necessaiy to use barometric 

 observations. Where a series of these, of a year or more in length is 

 available, the result may be regarded as sufficiently trustworthy for this 

 purpose. 



In regions where secondary triangulation is practicable the measure- 

 ment of heights may lie taken up with the plane table directly from datum 

 points, as above indicated, and carried throughout the work by means of 

 this instrument. Otherwise it becomes necessary to do more or less level- 

 ing in order to extend and multiply datum points to control the less 

 accurate work connected with the traversing. If practicable, the wye 

 level should be employed. 



The extent of the work of the wye level which may be required 

 depends mainly upon the contour interval of the map to be made. It may 

 be said in general, that a single line across a sheet will furnish a sufficient 

 number and a suitable distribution of points for the proper correction of 

 the subsequent work. Wherever practicable such lines should be run 

 along railroads, in order to obtain easy grades and thus lighten the work. 

 When railroads are not available, they should be run along wagon roads, 

 selecting, so far as they will suit the purpose, those having the easiest 

 grades and the straightest courses. 



Where the control of the map is effected by means of primary trav- 

 ersing, such traverse should be accompanied by a level line, unless that of 

 the railroad which the traverse follows appeal's to be of sufficient accuracy. 



