HOW TO USE MAPS AND PROFILES 



A. Commonly the information we are asked to give the public is the probable depth of a 

 well at a selected spot and reaching a specified aquifer. 



First it is necessary to determine the location of the proposed well, its latitude and longi- 

 tude, and the surface elevation at the proposed site. These data most easily can be obtained by 

 using the topographic maps of the United States Geological Survey or the maps recently 

 published by the United States Army Engineers, but, particularly in the City of New York 

 and other built-up areas, special detailed street maps may have to be used first in order to 

 locate the well on the topographic maps. 



Lists and key maps of the above topographic sheets are appended. 



The site of the proposed well is plotted on the appropriate topographic map by reference 

 to streets, roads and other objects. The elevation at the site is determined and the latitude and 

 longitude. 



The site of the well is then plotted on the corresponding maps in this report. As the well 

 must be located on several maps, three point dividers will be found very convenient. Finally, 

 the elevation of the top of each formation and aquifer under this site is obtained by interpo- 

 lation between the contours on the various maps. 



From this information, a prospective log of the well can be plotted. This may be checked 

 by locating nearby wells and looking up the data on them, either from our publications or in 

 our Long Island office. Estimates of water levels, quality and probable yield can be obtained 

 in the same manner. 



B. From studies of water levels and the fluctuating drawndown, interference between wells 

 and other characteristics, the profiles will be most useful. If the published profiles are too 

 remote or inconveniently oriented, special profiles readily can be prepared from the contour 

 maps. 



C. We depend on the logs furnished us by the drillers to correct these maps and profiles 

 and keep them up to date. Eventually the driller will help himself as well as others by giving 

 us the most accurate information possible. This is more particularly important in those cases 

 where our maps are found to be greatly in error, as may too often happen. 



D. Drillers and others doing much of this work and needing the most accurate information 

 will probably prefer to use our Geological Atlas (Bulletin GW-19) or separate sheets from it. 

 This atlas contains the same maps as are contained in this report but on a larger scale and 

 are constantly subject to revision to keep them as nearly up to date as possible. This atlas also 

 contains profiles on all fifteen minute meridians and parallels of latitude and longitude, only 

 a few of which are reproduced in this report. This atlas, or individual sheets from it, may be 

 dbtained from the Commission at cost. 



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