SUPPLIES AND EQUIPMENT - Well-numbering grid UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY WATER RESOURCES DIVISION WASHINGTON 25, D. C. March 27, 1964 Code 4056 0001 GROUND WATER BRANCH MEMORANDUM NO. 64.26 To: District Chiefs and Staff Officials (GW) From: Chief, Ground Water Branch Subject: SUPPLIES AND EQUIPMENT - Well-numbering grid Enclosed are copies of two interpolation grids that will be helpful in determining latitude-longitude well code numbers. These grids, designed by Messrs. E. J. McClelland and J. S. Bader, of the California district, are suitable for use on 7 1/2 and 15 minute quadrangle maps. A short paper entitled, "A Well-numbering Grid", by the same gentlemen describing the use of the grids is to appear in a forthcoming issue of the Water Resources Division Bulletin. The following material is quoted from the paper. The following description is directed to 7 1/2-minute quads, in west longitude and north latitude, but can be adapted to any other map where convenient latitude and longitude references are available. 1. Draw two lines of longitude, 2 1/2 minutes apart, which straddle the point to be located, and a line of latitude less than 2 1/2 minutes from the point. 2. Overlay the grid on the map with the parallel grid reference lines lying parallel to the line of latitude, and between the two selected lines of longitude. a. Slide the grid normal to the line of latitude until the 0- and 2 1/2-minute convergent longitude reference lines on the grid intersect in two common points with the line of latitude and lines of longitude previously selected. b. Draw a line on the grid corresponding to the selected latitude. This line is the latitude reference on the grid. 3. Slide the grid normal to lines of latitude until the latitude reference on the grid lies on the point to be located and fits between the longitudes selected in 1. above. 4. Count the minutes and second from the 0-minute longitude reference line on the grid to the point being located, along the latitude reference line and add to the easternmost of the two previously selected longitudes. 5. To obtain latitude, rotate the grid 90 degrees and follow a similar procedure using two latitudes 2 1/2 minutes apart and the adjacent longitude. For convenience the grid has two arrows at the approximate point of latitude measurement, but because paper maps are not scale-stable, the whole procedure should be followed. The procedure is easy to demonstrate but difficult to describe as is evident above. However, with a little practice, the procedure can be abbreviated and points located accurately. Interpolation should be accurate to + or - 1 second between 5-second lines on a 7 1/2-minute quad, and to + or - 2 seconds between 10-second lines on a 15-minute quad. It is recognized that the latitude lines on the grids, used with polyconic projection maps, should be curved, but as the grids are intended for use in arcs of 2 1/2- and 5-minutes the potential error is within the interpolation error and consequently this curvature has been ignored. The grids are in short supply. Only a few extras are available. If you will indicate your needs for additional copies by memorandum to this office, second orders for the grids may be issued depending on the number of requests received. (s) A. H. Lang for O. M. Hackett WRD Distribution: A, FO3, SL (with enclosure) AB (without enclosure)