There are grid squares line (north/south and east/west) marked on each
map that create squares 1000 meters on a side (visualize also each
square as being subdivided into ten 100 meter increments per square by
invisible lines). These 1000 square meter grid squares are then grouped
into larger units designated by the first two alpha characters in each
grid coordinate (for example: XT, YD, AS and so on). I believe
these Alpha characters refer to an area that is one hundred kilometers
on a side (i.e. 100 squares on a side), and if that's correct, then the
YD grid area contains 1000 square kilometers and each square kilometer
therein contains 1,000,000 square meters (i.e. 1000 meters x 1000
meters).
Coordinates are normally include either four numeric or six numeric
characters (although they sometimes include eight digits). Although
they usually do not show a split, the numbers are actually split at the
center such that YD 3906 should be read YD 39-06, and YD 395068 should
be read YD 395-068 (at least in your mind).
In other words, to find grid 3906, start at the left hand side at the
bottom of the map and then move right until you find the vertical
gridline with the number 39 assigned to it. Now place your finger on
that #39 vertical grid line at the bottom of the map and then run it up
(i.e. north) along that 39 gridline until you come to the gridline
numbered 06 along the right or left vertical (east or west) edge of the
map.
Where
those two lines cross is the lower left hand corner of the grid square
designated by the coordinate YD 39-06. Somewhere in the 1,000,000
square meters inside that grid square is the site you're looking for.
(As a general rule, most bases and firebases were built on the highest
location possible, so it's reasonable to presume that the highest
terrain feature in a given grid square is the likely location of any
base or firebase identified as being in that square. The same is not
true for battles and other "event" locations. In
some cases, the bases are so big they might occupy several entire grid
squares; such as Long Binh Combat Base or Tan Son Nuht Airbase).
The more numeric characters each grid contains, the more refined and
exact the location it pinpoints. As we saw in the preceding example,
the grid YD 39-06 is only specific to the nearest 1,000,000 square
meters and is not very accurate for that reason. All that grid
coordinate tells you is that the place you're looking for is located
somewhere in the grid square to the right and above of the intersection
of those two grid lines. However, when a grid is expanded to include
six digits, it becomes much more accurate for pinpointing a specific
location.
Grid YD 395-068 is accurate to the nearest 10,000 square meters (i.e., a
square 100 meters on a side) because it further breaks down the grid
square 39-06 into sub-units 100 meters on a side. The digit 5 in "395"
means that the point to find relative to the 39 vertical gridline is 5
tenths (or 500 meters) to the right of the number 39 (or halfway between
gridline 39 and 40). The digit 8 in the second half of the grid number
("068") means that it is a point 8 tenths of a grid square (or 800
meters) above the horizontal gridline numbered 06. (An 8 digit grid
coordinate would be accurate to the nearest 100 square meters but such
accuracy is rarely available or was used in Vietnam.) When you find the
intersection of those two imaginary lines on your map, you're sitting on
the lower left corner of a 100 meter square box in which the event or
site was located (that's an area roughly 2 1/2 times the size of a
football field and definitely close enough for government work!))
General Instructions for Finding Map Coordinate Locations:
The guidelines for locating a specific grid coordinate are fairly
simple. First you must find the correct L-7014 Series, 1:50,000 scale
map for the grid coordinates involved (unfortunately, not always an easy
task). For information regarding identifying proper L-7014 series maps,
contact the Geography & Map Division, Library of Congress, Washington,
DC 20540, 202-707-6277, 202-707-8531 FAX. Ask them to fax or mail you
an L-7014 series Map index (they also carry the L-509 series 1:250,000
scale and other maps). You can then order black & white 1:1 scale copies
of all(?) L-7014 (and other) maps from the same source at a cost of
approximately $2.00 per lineal foot. As for locating the grid
coordinates provided in these lists on those maps, try the following
procedure:
ALL GRID COORDINATES ARE READ FIRST FROM LEFT TO THE RIGHT, THEN UP
("To
the right and up," was a catch phrase the Army drilled into the minds of
too few of its Basic Trainees.. Commit that phrase to memory and reading
maps is a cinch!).