6 A MANUAL OF TOPOGRAPHIC METHODS. 



In addition to the material enumerated above, numerous astronomic 

 determinations of position have been made by governmental organizations 

 and by private parties. These positions, scattered over the interior, will, as 

 far as they go, relieve the Geological Survey from carrying on this expen- 

 sive work. * 



In addition to all this material, the railroads of the country furnish, in 

 their profiles, a vast body of measurements of height. These differ greatly 

 in value, those of certain railroads, ami generally those of the great systems, 

 being of a high degree of accuracy, while others are worthless. The errors 

 in these profiles are seldom in the leveling itself, but are due to the fact 

 that a road is leveled in sections, the profile of each section being based 

 upon an arbitrary datum point. Mistakes often occur in joining the profiles 

 of the several sections, and in correcting them for differences in their datum 

 points. 



PLAN OF THE MAP OF THE UNITED STATES. 



The field upon which the Geological Survey is at work is diversified. 

 It comprises broad plains, some of which are densely covered with forests, 

 while upon others trees are entirely absent. It contains high and rugged 

 mountains, plateaus, and low, rolling hills. In some regions its topographic 

 forms are upon a grand scale, while in others the entire surface is made up 

 of an infinity of minute detail. Some parts of the country are densely 

 populated, as much so as almost any region upon the surface of the globe, 

 while great areas in other parts of the country are almost without settle- 

 ment. Geologically, portions of the country are extremely complex, re- 

 quiring, for the elucidation of geologic problems, maps in great detail, while 

 other areas are simple in the extreme. 



It is obvious that from this diversity of conditions, both natural and 

 material, maps of different areas should differ in scale, and that with the 

 difference in natural conditions ami the difference in scale there must come 

 differences in the methods of work employed. The system which is found 

 to work to advantage in the high mountain regions of the west is more or 

 less inapplicable to the low forested plains of the Mississippi valley and the 

 Atlantic plain. , 



