SUPPLIES AND EQUIPMENT - Well-numbering grid

                          GEOLOGICAL SURVEY
                      WATER RESOURCES DIVISION
                         WASHINGTON 25, D. C.

                                                 March 27, 1964
                                                 Code 4056 0001


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

               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)