prime meridian, and may be drawn on a chart at any
convenient interval, such as every 5, 10, 15, or 30
degrees. The opposite of the prime meridian, either
180°E or 180°W longitude, is the international date line.
Parallels are also measured in degrees of arc, but
referenced as either north or south of the equator, which
is 0° latitude. The highest latitude measurements are
90° north (the North Pole), and 90° south (the South
Pole). Parallels are normally drawn on charts every 5,
10, or 15 degrees.
With the system of parallels and meridians, any
point on the earths surface can be accurately located.
Each degree (°) may be subdivided into 60 minutes ('),
and each minute into 60 seconds ("). In standard use, a
location is referenced by latitude, and then longitude,
such as 43°21'13"N 073°54'03"W. A nautical mile is
defined as 1 minute of arc on a great circle chart. The
frequently used approximation that 1° of latitude (on
any chart) is 60 nautical miles was derived from this
definition. This relationship does not hold true for
degrees of longitude, because the meridian lines
converge toward the poles.
Locations used to plot positions on meteorological
charts need only be accurate to within a few nautical
miles. Most meteorological positions are converted to
degrees and tenths of a degree during encoding to
simplify coding. To convert to tenths of a degree, divide
the minutes of both latitude and longitude by 6, and
discard the remainder. For example, 43°20'13"N
results in 43.3°N (20/6 = 3 remainder 2).
Military Grid Reference System
The Military Grid Reference System is used
extensively by all military forces for target information
and to locate positions ashore. In a warfare situation,
naval guns are aimed by using the grid system, and
weather observers may be tasked with providing
environmental information for shore targets referenced
by the grid system. In NBC warfare situations and in
warfare situations requiring electro-optical or
electromagnetic support, targets areas are commonly
referenced in the grid system.
The Military Grid Reference System uses two
separate grids to locate positions:
Transverse Mercator (UTM) grid and the Universal
Polar Stereographic (UPS) grid system. Nearly every
military topographic chart at scales 1:50,000 and
smaller already contain the military grid and
geographical coordinate systems.
UNIVERSAL TRANSVERSE MERCATOR
GRID.The UTM grid system is a series of grid zone
rectangles measuring 8° latitude by 6° longitude that
covers earth from 80°S latitude to 84°N latitude (fig.
4-2). Columns of grid zones are numbered sequentially
beginning at 180° and progressing eastward. Rows of
grid zones are lettered beginning with C at 80°S
extending northward. The letters I and O are omitted to
avoid confusion with the numbers I and O. The
northern-most row of grid zones, identified as row X,
extends from 72°N to 84°N and is the only row that is
not equal to 8° latitude in height. A grid zone is
identified by the column number followed by the row
letter, such as 34P, which is shown shaded in the figure.
Next, each grid zone is subdivided into 100,000
meter (100 kilometer) squares, called 100,000-meter
squares (fig. 4-3, view A). The 100,000-meter squares
are identified by two letters. Again, the letters I and 0
are omitted to avoid confusion. The rest of the letters, A
through V, are used to identify columns of the 100,000-
meter squares, starting at 180° and extending eastward.
These 20 letters are repeated every 18° longitude (every
three grid zones west to east). The horizontal rows are
identified from the equator northward with the letters A
through V, and from the equator southward in reverse
alphabetical order by the letters V through A. In the
north-south orientation, there are nine 100,000-meter
grid square rows in each 8° grid-zone row. In figure 4-3,
view A, the shaded 100,000-meter square is identified
by the grid zone (34P), then the column-letter (D),
followed by the row-letter (M)34PDM.
Next, note that each 100,000-meter grid square is
divided into 10 rows and 10 columns, resulting in
10,000-meter squares, as shown in figure 4-3, view B.
These squares are identified by the numbers 0 to 9 from
the western-most line eastward, and then from the
southern-most line northward. Point A would be
referenced by the grid zone (34P), the 100,000-meter
designation (DM), plus the column number (7), and row
number (3)34PDM 73.
Then, each 10,000-meter square is divided into 10
rows and 10 columns to form 1,000-meter squares, as
shown in figure 4-3, view C. Again, the columns are
identified from the western-most line eastward with the
numbers 0 to 9, and the rows are identified from the
southern-most line northward with the numbers 0 to 9.
Point B is identified by the grid zone (34P), the 100,000-
meter-square letters (DM), the 10,000-meter-square
column number (7), followed by the 1,000-meter-
square column number (6), then the 10,000-meter-
square row number (3), followed by the 1,000-meter-
square row number (l)34PDM 7631.