A MANUAL OF TOPOGRAPHIC METHODS. 



By Henry Gannett. 



CHAPTER I. 



INTRODUCTION. 



The object of this manual is to present ;i description of the topographic 

 work, instruments, and methods used by the U. 8. Geological Survey, 

 primarily for the information of the men engaged upon this work. It is 

 nut intended as an elementary treatise upon surveying, as it presupposes a 

 knowledge of the application of mathematics to surveying equivalent to 

 that to be obtained in our professional schools. Neither is it intended as a 

 general treatise on topographic work, although it may, to a certain extent, 

 supply the existing need of such a work. 



The Geological Survey is engaged in making a topographic map of the 

 United States. Excepting for certain areas, lying mainly in the far West, 

 there existed, prior to the inception of this work, no maps upon a sufficiently 

 large scale and in suitable form for the use of the geologist. While the 

 primary object of the map is to meet the needs of the geologists of the 

 Survey, it has been thought economical to adjust the plans so that the result- 

 ing map may be adequate to serve all needs for which general topographic 

 maps are used. 



Certain areas, especially in the far West, have been surveyed and 

 mapped by other organizations, notably those of the general and state gov- 

 ernments, upon a sufficiently large scale, and with sufficient accuracy for 

 the use of the Geological Survey; much material also exists in the form of 



triangulation, of lines of levels, and of other partial surveys which can be 



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MON XXII -1 



