Air Fronts: FM 21-26, Advanced Map and Aerial Photograph Reading - Section 6.

SECTION VI: GRIDS AND COORDINATES

27. MILITARY GRID. A military grid is a network of squares made up of north-south lines showing distance east of an arbitrary origin, and east-west lines showing distances north of the same reference point. The distance between grid lines is 1,000, 5,000, or 10,000 yards (or meters), depending on the scale of the map. This distance is called the grid interval. See FM 21-25 for a discussion and explanation of grid coordinates.

28. COORDINATE SCALE. a. General. The sides of grid squares may be divided with a reasonable accuracy by estimation, but it is easier and more accurate to use a coordinate scale. Two types are shown in figure 22 (1) and (2) Either may be made by laying off the length of one side of a grid square and dividing it into tenths.

b. To read coordinates. To locate RJ 37 in figure 23, place the coordinate scale as shown. The marked horizontal edge is along the bottom of the grid square in which the road junction is located and the marked vertical edge passes through RJ 37. The horizontal edge of the scale gives the 100-yard and 20-yard points to the right of grid line 1369. The vertical edge of the scale gives the distance from grid line 1792 to the road junction. The coordinates of RJ 37 are (69.72-92.25).

C. To plot a point. To locate the command post of the 1st Infantry Regiment at (74.60-92.47) place the coordinate scale as shown in figure 24 and mark the CP with a pencil.

(1) Coordinate scale, rectangular.      (2) Coordinate scale, L-shape.
Figure 22.

Figure 23. Coordinate scale used to read coordinates of a given position on map.

29. USING ENGINEER SCALE. The engineer scale may be used with the military grid to locate points. For example, to locate the headquarters of the 1st Infantry Regiment at (25.35-76.62), find the grid square (25-76)

Figure 24. Coordinate scale used to plot Position of known coordinates.

and place the scale in position figure 25, so that the "0" and "10" indicators are on successive vertical grid lines. The "0" of the scale is always placed on the lowest numbered grid line. Measure from the grid line 1325 along the scale to the thirty-fifth subdivision and make a mark on the map at J. Repeat this operation at position (2) and mark B on the map. The line drawn between points A and B is the coordinate line 25.35 or the RIGHT reading. To get the coordinate 76.62 place the scale in position (3) so that the "0" and "10" indicators are on successive horizontal grid lines with "0" always on the lowest numbered line. Measure along the scale to the sixty-second subdivision and make a mark on the map at C. Move the scale to position (4) and mark map at D. A line drawn between points C and D is the reading UP or coordinate 76.62. The command post is located where lines AB and CD intersect.

Figure 25. Engineer scale used to locate coordinates on gridded map.

30. POLAR COORDINATES. Polar coordinates consist of an angle from a known base direction and a distance from a known starting point. The base direction is normally expressed as azimuth, the distance in any convenient unit. Polar coordinates are especially useful in the field because magnetic azimuth can be determined from the compass and distance can be estimated. For example, in figure 26, a patrol leader at observation post (OP) A observes enemy guns in position at point B. He sends back information that an enemy artillery posi

Figure 26. Polar coordinates used to designate position on map.

tion is located 900 yards on a magnetic azimuth of 43° from OP A in Thorofare Village. On receiving the message the company commander converts magnetic azimuth 43° to grid azimuth. 35°. He plots the direction line from OP A and scales off 900 yards to the enemy position at point B.

31. RECTANGULAR COORDINATES. a. To plot map position by rectangular coordinates on ungridded maps. Rectangular coordinates consist of two distances measured at right angles from a base position. For example, the problem in figure 27 (1) is to locate point P, 1,500 yards east (magnetic) and 1,100 yards north (magnetic) of bench mark (BM) 38 at Accotink. From BM 38 draw an east line 1,500 yards long and measure off 1,100 yards to the north. This locates point P. The coordinate is written as BM 38, Accotink (village), 1,500 yards east (magnetic), 1,100 yards north (magnetic).

b. To find rectangular coordinates of a point with respect to a given base position and direction. From the base point, draw a guide line parallel to the base direction and a guide line perpendicular to the base direction. From the point whose rectangular coordinates are desired, drop perpendiculars to each of these lines. Scale the distance along the perpendiculars to the respective guide lines; these distances are the rectangular coordinates. For example, in figure 27 (2) the problem is to find the rectangular coordinates of point Z, given the base position CR 265 and base direction magnetic north. Draw the base direction line ab and at right angles to it draw line cd. From Z draw the perpendiculars ef and gh. Measure along the lines ef and gh to derive the rectangular coordinates of point Z: 1,200 yards east (magnetic) and 800 yards north (magnetic).

(1) Plotting position on map using rectangular coordinates on ungridded map.
(2) Determining of rectangular coordinates of a point with respect to a given base position and direction.
Figure 27.

32. THRUST LINE. In the thrust-line system, the base of all coordinates is a so-called thrust line designated by the commander. It is located on the map by two reference points or by a reference point and a direction. Somewhere on the thrust line is a base point from which all coordinates are measured. The commander names this base point at the time the thrust line is given. Points are located by giving the distance along the thrust line forward or back of the base point, F for forward and B for back; and, the distance at right angles from the thrust line, R for right and L for left. In determining whether a point is to the right or left of the thrust line, the map reader must assume that he is standing at the base point and facing forward along the line. The distance forward or back and right or left is given in miles, kilometers, yards, or meters; the scale of the map determines the accuracy of measurement. When the unit of measure is miles, the measurement is in tenths with the decimal point omitted. When yards or meters are used, the unit of measure is designated by the commander; no decimal point is involved in this case. See FM 21-25 for a detailed description of the thrust line.

33. MAP TEMPLATES M1 AND M2. a. General. A map template is a device for locating points by coordinates on gridded or ungridded maps and aerial photographs. The template is a transparent sheet about 8½ by 12 inches, having 24 large squares 1.8 inches on a side. Squares are lettered from "A" to "Y" omitting "O". Each lettered square of the M1 template is divided into 100 small squares, 0.18 inches on a side; each square is marked either by a numeral or punched hole as shown in figure 28. On the M2 template the lettered squares are not subdivided into smaller squares, but the holes are provided. In addition, both templates have nine larger holes marked by Roman numerals I to IX. These holes are located at each corner, at the middle of each side, and at the center of the template. They are origin points for orienting the template on the map.

b. Orientation of the template. To use the template system, the commander specifies an origin point on the template and a reference point on the map over which

(1)     Map template M 1.
        Figure 28.

the origin point is to be placed. He also specifies a template base line joining the template origin point and any other Roman-numeral point. He then designates a line on the map with which the template base line must coincide. The direction of the line on the map may be indicated by grid azimuth or as the line joining the map reference point with another map feature. The template is ready for use when the template origin point and base line are placed over the reference point and line on the map.

(2) Map template M2. Figure 28. - Continued.

The following message is a sample of a commander's instructions on orienting a template: "Reference special map A, 1:25,000. From 0900 to 1200, 24 Apr 1944, origin point I at BM 24. Line I-VII through schoolhouse NE of BM 24." As illustrated in figure 29, the map template is placed on the map with origin point I over BM 24 and pivoted until line I-VII coincides with a line drawn on the map from BM 24 through the schoolhouse symbol northeast of BM 24. The origin point and reference point may be ground features on the map given by map coordinates. Any Roman-numeral point can be used as an origin, and any line connecting the designated point with any other Roman-numeral point can be used as a base line.

c. Point designation. Once the template is oriented, a point is designated by the letter of the square in which it is located and by the number of the hole nearest it. For example, the building in the "U" square of figure 29 is in the sixth column from the left and in the third row from the bottom. Reading up the sixth column we count 51, 52, 53. The building would be designated as "U 53."

d. Security considerations. This method of using the map template provides, in certain situations, a convenient and rapid means of designating point locations. However, it should be emphasized that this method of use is not intended to provide security. Before the template can be used for purposes of secrecy instructions prepared by the Chief Signal Officer relative to the preparation and use of keys must be consulted.

34. GEOGRAPHIC COORDINATES. a. General. Unlike grid coordinates which are given in linear measure such as yards, geographic coordinates are stated in degrees (°), minutes ('), and seconds (''). To understand these coordinates, one must be familiar with the line of latitude and longitude on the earth.

Figure 29. Use of map template M1.

(1) The equator is an imaginary line round the earth halfway between the North and South Poles (fig. 30).

Crossing the equator at regular intervals are north and south lines which pass through the poles. These are known as meridians of longitude or simply meridians. One is marked zero and is called the prime meridian. From this prime meridian, longitude is measured both east and west around the world. Lines east of the prime meridian are numbered from 0° to 180°, and are called east longitude. Lines west of the prime meridian are numbered from 0° to 180° and are called west longitude. The prime meridian on American and British maps is a line through Greenwich, England.

(2) Crossing the meridians in an east-west direction between the equator and both poles are the parallels of

Figure 30. Geographic coordinates. Point A is latitude 40°00'00" N, longitude 15°00'00'' E; point B is latitude 15°00'00" S, longitude 30°00'000'' E, point C is latitude 15°00'00" N, longitude 15°00'00" W.

latitude, called simply parallels. Starting at the equator, these are numbered from 0° to 90°. Thus, the parallel halfway between the equator and the North Pole is 45° north latitude.

Figure 31. Lines of geographic coordinate grid plotted by connecting ticks and crosses.

b. Placing geographic coordinate grid on a rnap (1) On most maps in common use, lines of latitude and longitude curve so little they appear to be straight lines. American military maps usually locate lines of latitude and longitude by ticks at the margins and crosses within the map. For example, in grid square (67-99) on figure 31 there is a marginal tick marked 47° 04'. Across the map horizontally to the left are three crosses. A line drawn through the tick and the crosses is parallel of latitude 47° 04' N. Similar parallels north of this one are indicated by ticks at 1' intervals. These ticks and crosses connected form the east-west lines of a geographic grid system.

(2) Lines of longitude are indicated on the top margin of figure 31. In the grid square (67-05) there is a tick numbered 122° 36' W. Vertically below this tick are four crosses and another tick at the bottom margin. The vertical line through these is a meridian of longitude 122° 36' W. Similar meridians west of this one are indicated by ticks at 1' intervals. If these ticks and crosses are connected, the lines formed will complete the north-south geographic grid lines.

(3) On British maps the term "graticule" is used instead of "geographic grid."

c. Writing and reading geographic coordinates. Some American and British maps have parallels and meridians printed as full lines. These are readily disttinguished from grid lines by their numbering. Lines of latitude and longitude are always numbered degrees and minutes, and, if necessary, in seconds. In locating a point by latitude and longitude, N or S represents latitude, and E or W represents longitude, for example 47° 04' N, 122° 36' W. The general world location of the area covered by a map is indicated by the numbers on latitude and longitude lines. If the latitude numbers increase from bottom to top on a map, the area is north of the equator; if they decrease, it is south. If the longitude numbers increase from left to right, it is east of the prime meridian; if they decrease, it is west. Applying these rules to figure 31, the area covered by the map is north of the equator and west of the prime meridian. In writing geographic coordinates, latitude is given first and parentheses are omitted. To illustrate, refer to RJ "A", grid coordinates (65.75-99.45), figure 31. The geographic coordinates of this point are: latitude 47° 04' 05" N, longitude 122° 37'00" W.

d. To find geographic coordinates of a point. Problem: to find the geographic coordinates of point A on figure 32. The geographic grid interval is 1', and since there are 60" to a minute the sides of the grid quadrangle must be divided into 60 equal parts. To find the latitude of point A, the engineer scale is placed across the latitude lines so that 60 convenient divisions span the distance, position (1). Each of the 60 divisions represents 1" of latitude. Longitude is found with the engineer scale in position (2). The complete geographic coordinates of point A are latitude 38° 42' 20" N., longitude 77° 13'30" W. If the geographic grid interval is 5', the sides of the quadrangle must be divided into 300 equal parts.

35. GEOGRAPHIC COORDINATES ON FOREIGN MAPS a. Prime meridian. Many foreign maps do not use the Greenwich meridian as a prime meridian. For example, French maps use the Paris meridian, and Italian maps the Rome meridian. Marginal data of foreign maps must be examined to determine the prime meridian used. Longitude measured from a foreign prime meridian may be converted to longitude with

Figure 32. Engineer scale used to read geographic coordinates of Position on map.

respect to Greenwich meridian by adding or subtracting the Greenwich longitude of the foreign prime meridian based on the Greenwich meridian. The following table gives the Greenwich longitude of some principal foreign prime meridians:

b. Grad. On most maps latitude and longitude are measured in degrees, minutes, and seconds, but the unit of measure on French maps is the grad. There are 400 grads in a circle as compared to 360 degrees in a 360 circle so a grad is  or .9 of a degree. The grad is 400 based on a decimal system, that is:

1 grad = 100 minutes

1 minute = 100 seconds

The angle 9.628 grads would be written 9^G62'80''; symbols for minutes and seconds, in this system, incline from left to right.

c. Converting French geographic coordinates to standard coordinates. Using a French map the map reader locates a point, for example, latitude 39^G 71' N, longitude 03^G 36' W. To convert to coordinates based on the Greenwich meridian, proceed as follows:

First, grads are converted to degrees:

39^G 71' = 39.71^G = (39.71 x .9) degrees =  35.739°

03^G G 36' = 03.36^G = (03.36 x .9) degrees = 3.024°

Second, decimals of degrees are converted to minutes and seconds:

35.739° = 35° 44' 20.4", found as follows: .739° x 60 = 44.34'

.34' x 60 = 20.4"

Similarly, 3.024° = 3° 01' 26.4'

Based on Paris meridian, the coordinates of the point are latitude 35° 44' 20.4" N, longitude 3° 01' 26.4" W. In this case the Paris meridian of 2° 20' 14" E must be subtracted from 3° 01' 26.4", to obtain longitude based on Greenwich. Complete Greenwich coordinates are written latitude 35° 44' 20.4" N, longitude 0° 41' 12.4" W.

36. U. S. DOMESTIC GRID SYSTEM. a. General. (1) The grid system covering the United States is called the "U. S. Domestic Grid." The country is divided into seven lettered zones, J through G (fig. 33); zones H and J were added when the system was expanded to form the World Polyconic Grid. Each zone extends through 9° of longitude which includes a 1° overlap of adjacent zones (½ on each side); the net width of a zone is therefore 8°.

Figure 33. U. S. Domestic Grid System, extended by Zones J and H.

(2) Each zone has a different origin of coordinates.

For example in figure 33, zone A has its origin at the intersection of central meridian 73° W with parallel 40° 30' N while zone B has its origin at the intersection of central meridian 81° W and parallel 40° 30'N. At the origin of each zone the north-south or vertical grid line (frequently called "Northing") is given an arbitrary value (frequently called "false coordinate") of 1,000,000 yards, and the east-west or horizontal grid line (frequently called "Easting") a value of 2,000,000 yards. This is done to avoid negative coordinates. A similar grid system covers Hawaii, Panama and the Philippines.

(3) The complete numerical value of the horizontal and vertical grid lines whose ends are nearest the southwest corner of the map is written in the margin on any map using this system. The last three ciphers are commonly omitted from other grid-line values shown in the margin, but occasionally where the 100- or 500-yard interval is used complete numbers are shown on all grid lines.

b. Identification. The U. S. Domestic Grid is identified by a grid note printed in the map margin. Its form is THOUSAND YARD GRID COMPUTED FROM "Grid System for Progressive Maps in the U. S."  ZONE   U. S. C. & G. S. SPECIAL PUBLICATION NO. 59.

Grid lines for this system are printed in black. Other American systems printed in black cover Hawaii, Panama, and the Philippines.

c. Map of area in overlap of two zones. On maps of scale 1:125,000 and larger that lie in two zones, the grid of one zone is shown in solid black lines, and the grid of the other zone is indicated by ticks 0.3 inch long along the border of the map. To use the grid of the second zone on these maps, draw in the grid lines by connecting the ticks with dotted lines as shown in figure 34.

Figure 34. Grid lines on a map where two zones overlap (ticks connected by dotted lines).

d. Use. United States grid systems are used in the same general way as explained in paragraph 34. Careful attention to marginal notes on each map is essential.

37. WORLD POLYCONIC GRID. The World Polyconic Grid is an extension of the U. S. Domestic Grid System to cover parts of the world not gridded by the British and areas not previously gridded by the United States. The world is divided into five north-south divisions called "bands." Each band is made up of nine zones. The seven zones A to G in the U. S. Domestic Grid System and the extension zones H and J comprise the total width of band I, from longitude 68° 30' W to 141° 30' W. Band I, however, also takes in all the area between latitude 72° N and 72° S. Each band is divided at the equator and designated, for example, as band I-N for the north half and band I-S for the south half. Band II is west of band I and takes in the area from longitude 140° 30' W to 146° 30' E. Bands III, IV and V are the next succeeding bands in a westerly direction. Each band is divided into nine standard size zones lettered from A to J in the same way as band I. Figure 35 shows an outline of the bands throughout the world. The blank portions are sections of the world covered by some other type of grid and for most of which gridded maps are available. World polyconic grid lines appear in purple on maps of scales smaller than 1:125,000; numbers and other pertinent detail are purple at all scales.

38. BRITISH GRID SYSTEM. a. General. Troops operating in Europe, Africa, Australia, India, China, and many islands of the South Pacific will use maps bearing British military grids. British grid systems subdivide the world into zones or belts identified by name and color, as: Northern European Zone III (Blue), Egypt Purple Belt, Egypt Red Belt, etc. Each of these grids has a specific origin and the grid lines are usually printed in the same color as the color designation of the area on maps of scales 1:250,000 and 1:500,000; thus, grid lines on maps covering the Northern European Zone III (Blue) appear in blue. The significance of color, however, no longer applies always to grid designation, but rather to grid differentiation on sheets carrying more than one system.

b. Grid squares. (1) A grid zone is ordinarily divided into squares 500,000 meters on a side. This basic square is assigned a letter, the letters being alphabetical and reading from left to right and down within a zone (fig. 36). Each 500,000-meter square is further divided

Figure 35. World Polyconic Grid System. Blank areas are covered by other grid systems.

into 100,000-meter squares each of which is also designated by a letter. Thus a 100,000-meter square of a zone may be identified by two letters. (See fig. 36.) However, some zones are so long that more than one 500,000-meter square is assigned the same letter, while in a few zones no letters are used.

(2) On maps of scales of 1:250,000 to 1:500,000, the letter indentifying the 500,000-meter square and the 100,000-meter square letter are both shown on the face of the map. Ordinarily on maps of scales larger than 1:250,000, only the 100,000-meter square letters are shown, although the letter identifying the 500,000-meter square may be indicated by a grid index diagram in the margin.

(3) The frequency of the grid lines is controlled by the scale of the map as follows:

c. Reading grid coordinates. Point identification by grid reference indicates, in order, the 500,000-meter square, the 100,000-meter square, the abbreviated east-west (easting) coordinate, and the abbreviated north-south (northing) coordinate. The procedure is as follows:

(1) Indicate the letter identifying the 500,000-meter square, as shown directly on the face of the map or as indicated by the grid index diagram.

(2) Indicate the 100,000-meter square as shown on the face of the map, normally by a large open block letter printed in the same color as the grid lines.

Figure 36. British Military Grid System.

(3) Write the east-west coordinate in the same manner as with the United States military grid, omitting the small figure or figures which precede the enlarged grid number. The grid value is carried out by estimation or measurement to the minimum value desired. The hyphen or dash between "right" and "up" coordinates, always used in the United States domestic grid references, is omitted in British grid references.

(4) Write the north-south value in the same manner. The small numbers which precede the large figure at the end of the grid lines represent the total distance from the false origin of the grid coordinates and are always omitted in point designation. See figure 37 for illustration of point designation. Coordinates of Point I are written RY4010.

d. Marginal data. British maps and American reproductions of British maps employing the British grid systems always have full instructions for the expression of grid references somewhere in the margin. In a few British grid zones, the yard is used as a unit instead of

Figure 37. British military Grid. Coordinates of point I are RY 4010.

the meter. The entire procedure in grid reference, however, is identical in either case.

39. AIR DEFENSE GRID. The Air Defense Grid is designed to satisfy the requirements of world-wide air defense and to permit easy transmission of accurate positional information. The Air Defense Grid is based entirely on lines of longitude and latitude, and divides and subdivides all the earth's surface from latitude 80° N and 80° S in such a way as to keep the grid divisions and subdivisions approximately square. This is accomplished by changing the size of the grids in degrees and minutes of longitude several times between the equator and the poles. The only large variations in shape occur between latitudes 72° and 80° N and S where the top of the grid lines converge considerably toward the poles. Figure 38 illustrates the converging meridians and successive changes in grid lengths. Complete information on the Air Defense Grid can be found in TM 44-225.

40. FIRE-CONTROL GRID. A fire-control grid is a rectangular grid superimposed on a military map; on large-scale maps, the military grid serves as the firecontrol grid. Grid interval is normally 1,000 yards. Fire-control grids must be accurate as to scale and azimuth. Fire-control maps are 1:25,000, but maps of 1:50,000 or larger are considered usable substitutes. Photographs or photomaps used for fire control require the same scales as for maps. The battle maps of the United States conform to the standard military grid system of the United States. Other military maps may be provided with a standard grid for the particular theater of operations or with an arbitrary grid used for a single map or for a limited area. When an arbitrary

Figure 38. Air Defense Grid covers the world. Spherical Presentation of first and second divisions.

grid is used, it is generally established on the spot by the firing unit on the basis of the best data available, whether that be by sound or visual survey. For example, the grid may originate as a division grid and be extended for corps control. When data become available, however, the standard military grid is substituted for the arbitrary division or corps grid. Use of the firecontrol grid is covered in TM 44-225.

41. POINT-DESIGNATION GRID. The point-designation grid is an arbitrary grid printed on aerial photographs. It is explained in paragraph 69.