DESCRIPTION OF AUTHORITIES. 9 



RAILROADS. 



The elevations determined by railroad levels are credited to the 

 roads by which they were furnished. 



On pages 12-19 will be found a list of these railroads, with the 

 abbreviations used for their names arranged alphabetically. 



Unless otherwise stated, the elevation is that of the railroad track 

 at the station of the road given. In cases where two or more rail- 

 roads meet or cross at grade, the elevation furnished by one of them 

 only the best determined is given. 



The collection and adjustment of these railroad levels has formed 

 by far the greater part of the work of this compilation. There are 

 now in the possession of this office abstracts of the profiles of more 

 than nine-tenths of the railroad mileage of the country, and all this 

 material has been used in the preparation of the present volume. 

 For this material, which is of the greatest value in the preparation of 

 topographic maps, as well as for other purposes, this office is in the 

 main indebted directly to the chief engineers of the various railroad 

 companies. 



The interest taken in this compilation by railroad engineers, which 

 was noted in earlier editions of this work, has increased and spread, 

 until to-day there is scarcely a large railroad corporation which has 

 not in its possession a full and consistent profile of all its lines, reduced 

 to a common datum point, and these profiles have been courteously 

 furnished to this office. Many of them are wonderfully accurate, 

 some comparing favorably even with the precise leveling of the 

 United States Coast and Geodetic Survey and of the river commis- 

 sions. 



The errors due to careless connections between divisions and sec- 

 tions of roads, which were mentioned in former editions as a leading 

 source of inaccuracy, have been largely eliminated in later profiles. 



Acknowledgment in detail of indebtedness for this material would 

 render necessary a list of nearly ah 1 the leading railroad engineers of 

 the country, which want of space forbids. 



Besides the profiles received directly from railroad organizations, 

 numerous profiles have been obtained from other sources. First 

 among these should be mentioned those published by Mr. Warren 

 Upham, in Bulletin No. 72 of this Survey, entitled " Altitudes between 

 Lake Superior and the Rocky Mountains." While nearly all of these 

 profiles had previously been obtained directly from the railroads, 

 many of those presented by Mr. Upham have been used, in the belief 

 that his adjustment of them is the most consistent possible. 



The adjustment of railroad levels has proved a simpler task than 

 at the time of publishing the earlier editions. This is due in great 

 part to the fact that many more lines of precise levels have been run, 



