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Figure 4-3.Relationship between grid zone and 100,000-meter squares
Figure 4-5.Portion of a naval shore bombardment chart of Vieques Island

Aerographers Mate, Module 01-Surface Weather Observations
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This  pattern  of  subdividing  the  squares  and identifying   the   point   with   successively   smaller columns,   and   then   row   numbers   may   continue indefinitely, but is normally only carried out to the nearest  lo-meter  square  for  a  precision  targeted position. A 10-meter position would have eight digits following  the  letters,  such  as  34PDM  76673168,  as shown in figure 4-3, view D. To read this position, mentally  divide  the  number  into  two  groups  of  four digits (7667 / 3168). Think of it as moving eastward from the west side of the DM 100,000-meter grid boundary  by  (7)  10,000-meter-square  subgrids,  then  (6) 1,000-meter-square grids, then (6) 100-meter-square grids, then (7) lo-meter-square grids. Then, move northward from the southern boundary of the DM grid (3) 10,000-meter grid squares, then (1) 1,000-meter grid square, then (6) 100-meter grid squares, and then (8) 10- meter grid squares. When military units pass positions in message traffic  within  a  region,  frequently  the  grid  zone designation is left out of the position, such as DM 40132879. This practice is common within NATO military  forces  in  Europe. Now let’s look for a moment at figure 4-4.  Since the east and west boundaries of each grid zone are specified longitude  lines  and  the  100,000-meter  squares  (fig.  4-3, view A) are actual distances on earth, the 100,000- meter squares along the edges of each grid zone overlap as  the  longitude  lines  converge  near  the  poles. Overlapping  portions  are  truncated  at  the  grid  zone boundaries, and the charted grid zones appear to have some partial 100,000-meter squares. When identifying locations,  remember  that  the  lettering  or  numbering system assumes that the missing or partial portions of the 100,000-meter squares are intact. Notice that the 100,000-meter grid lines are only parallel to the longitude lines near the center of each grid zone. This means the grid lines are oriented slightly westward of true north in the east half of a grid zone, and eastward of true north in the west half of a grid zone. The grid column lines are used as a reference called "grid north" (GN). The UTM grid lines drawn on a chart depend on the scale of the chart. Figure 4-5 shows a portion of a coastal chart with grid lines drawn every 200 meters. On this chart, the grid location may be easily estimated to the nearest 10 meters. Based on the latitude and longitude, the grid zone is 20Q and the 100,000-meter square is PC. The numbers indicate the 10,000-meter grid coordinates.    The lower-left comer of the box marked M is 20QPC 40401040. While the grid zone designation and the letters of the 100,000-meter square may be used to approximate the general area of a UTM location in geographical coordinates,   precise   conversions   of   UTM   grid coordinates to geographical coordinates must be done by referencing the UTM position on a gridded chart, and Figure 4-4.—Partial 100,000-meter squares at edges of grid zones. 4-5







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