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 earth’s 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: the Universal 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. 4-2