MAP READING 101
By : Mike M-60 Kelley - D/1/502d Inf 101st Div 69-70
[modestly amended and corrected 05/02/2023]
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. As for locating the grid coordinates provided in these lists on those maps, try the following procedure:
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).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 or in the CC's, they normally have 8 characters ... two letters which identify the group area, and 6 numbers which pinpoint the grid and section of that grid. 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 395065 should be read YD 395-065 (at least in your mind).
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!).
In other words, to find grid 395065, 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 or top of the map and then run it up or down (i.e. north or south) 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. Move approx 1/3 of the grid further north and 1/2 of the grid further west an you will pinpoint the position.
(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 Nhut 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 395-065 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 5 in the second half of the grid number ("065") means that it is a point 5 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!)