CONTROL OF THE MAPS. 11 



the map itself, but serve only to correct the sketch, while the sketch sup- 

 plies all the material of the map. The correctness of the map depends 

 upon four elements: first, the accuracy of location; second, the number of 

 locations per square inch of the map; third, their distribution; and, fourth, 

 the quality of the sketching. It is in connection with the first of these 

 elements that it seems desirable to define what constitutes accuracy. The 

 greatest accuracy attainable is not always desirable, because it is not 

 economic. The highest economy is in properly subordinating means to 

 ends and it is not economic to execute triangulation of geodetic refinement 

 for the control of maps upon small scales. The quality of the work should 

 be such as to insure against errors of sufficient magnitude to appear upon 

 the scale of publication. While the tendency of errors in triangulation is 

 to balance one another, still they are liable to accumulate, and this liability 

 must be guarded against by maintaining a somewhat higher degree of 

 accuracy than would be required for the location of any one point. It is 

 not difficult to meet this first condition of accuracy of the maps The 

 maximum allowable error of location may be set at one-hundredth of an 

 inch upon the scale of publication. This admits of an error upon the ground 

 not greater, on a scale of 1:62500, than 50 feet. 



The second condition of correctness, that is, the number of locations 

 necessary for the proper control of the work, is not easily defined. The 

 requirements differ with the character of the country. A region of greai 

 detail and of abrupt features requires more control than one of great uni- 

 formity and gentle slopes and of large features, so that no general rule can 

 be laid down. Furthermore, it depends upon the quality of the sketching; 

 with indifferent sketching a greater number of locations is required in order 

 to bring the map up to the requisite quality. 



The following table presents a summary of the amount of control 

 obtained during the field season of 1891 in the different fields of work in 

 this survey. It is presented not as a type of what should be, but to show 

 what has been done and also to illustrate the wide range in the amount of 

 control brought about by the differences in the character of the country 

 and methods of work. 



