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)