PUBLIC LAND SUKVEYS. 101 



difference D and divide the product by 900. This correction is of the same 

 sign as (t -f- 1' — 64). By applying it, we obtain a second approximate dif- 

 ference of level, D'. 



In Table III, with D' and the mean latitude of the stations, find the 

 correction for variation of gravity in latitude, and add it to D', paying due 

 attention to the sign. 



In Table IV with D', and in Table V with D' and the height of the 

 barometer at the lower station, take the corrections for the decrease of 

 gravity on a vertical, and add them to the approximate difference of level. 



The sum thus found is the true difference of level between the two 

 stations, or Z ; by adding the elevation of the lower station above the level 

 of the sea, when known, we obtain the altitude of the upper station. 



UTILIZATION OF THE WORK OF THE PUBLIC LAND SURVEYS. 



In all the states and territories except the original thirteen, together with 

 Vermont, Kentucky, Tennessee, Texas, and Alaska, the public-land sur- 

 veys have been carried on, and many of these states have been entirely 

 covered by these surveys. 



These surveys were made for the purpose of dividing the land into 

 parcels suitable for sale or other disposition, and with little reference to 

 map purposes. The work differs widely in quality in different parts of the 

 country, in some regions 1 icing very bad, in others of high quality. Gener- 

 ally speaking, the later work is much the better. 



This work is extensively used by the Geological Survey as an aid in 

 the preparation of its maps. The extent to which it is utilized, and the 

 methods employed in using it, will be detailed in this chapter. Before 

 proceeding with this, however, it is desirable to describe the methods 

 by which this work has been and is carried on. 



The system of subdivision is an extremely simple one. It consists, first, 

 in the division of the land into large blocks, the division of these blocks into 

 townships, approximately 6 miles on a side, and the subdivision of these 

 townships into sections, each containing about 1 square mile. Further 

 subdivision of these sections into quarter sections, or even smaller areas, has 

 been done by private surveyors. 



