ACTIVITIES 33 



tions upon the ground, and also to carry on control by tri- 

 angulation or traverse with greater accuracy and to mark the 

 positions more permanently than had been possible without 

 statutory authority. Accordingly, on June n, 1896, an act 

 was approved which provided that "elevations above a base 

 level located in each area under survey shall be determined and 

 marked on the ground by iron or stone posts or bench marks," 

 etc. Thereafter it was possible to provide for the running of 

 careful spirit levels over all the areas under survey and to ex- 

 tend the amount of spirit leveling, thereby securing more ac- 

 curate location of contours. Since that act was passed the 

 topographic mapping has reached as high a degree of detail 

 and quality as seems desirable for the scales adopted. 



The selection of areas for survey is determined by a variety 

 of considerations. It is of course desired to extend the bene- 

 fits of the work as widely and equally over the country as a 

 whole as is economically practicable. Other things being 

 equal, however, it is more economical to extend surveys out- 

 ward continuously from areas already surveyed than to under- 

 take new work in areas entirely unconnected with them by tri- 

 angulation or otherwise. A special economic need or project 

 in an isolated area may nevertheless occasionally warrant the 

 initiation of a survey there. As between areas equally eco- 

 nomical of survey, preference is determined by a number of 

 factors, among which are the apparent local demand for sur- 

 veys, and the probable economic value of the resultant maps. 



The methods of field work followed by the Survey in mak- 

 ing topographic surveys are unique and have been developed 

 by the topographers of the Survey through nearly forty years 

 of field experience. *Wherever possible the results of triangu- 

 lation by other organizations are utilized. The degree of re- 

 finement in the detailed surveying depends upon the scale upon 

 which the resulting map is to be drawn. 



The results of the topographical surveys are published in 

 the form of Topographic Atlas Sheets, measuring 17 by 20 

 inches. These sheets are published on different scales, the 



