Datums, Ellipsoids, Grids, and Grid Reference Systems
TM 8358.1 Section 1.7 thru 3.5
1-7 REFERENCE SYSTEMS.
1-7.1 Rectangular grid reference systems are usually shown on military maps and chart at scales of 1:1,000,000 and larger. Maps and charts at all scales show the geographic graticule. Maps and aeronautical charts at 1:250,000 scale and smaller show the GEOREF.
1-7.2 The Military Grid Reference System is described in Chapter 3.
1-7.3 Grid reference systems used with operational nonstandard grids are described in Chapter 4.
1-7.4 The geographic coordinates are described in Chapter 5.
1-8 STANDARD AND NONSTANDARD GRIDS.
1-8.1 The standard grid for polar areas north of 84° north, and south of 80° south, is the Universal Polar Stereographic (UPS) grid.
1-8.2 Between 84°north and 80° south, the standard grid is the Universal Transverse Mercator (UTM) grid. Other grid systems are being phased out. The long term objective is to convert the mapping of all areas of the world to UTM and UPS grids.
1-8.3 Normally, grids are not portrayed on maps at scales smaller than 1:1,000,000.
1-9 MULTIPLE GRIDS.
The use of military grids presents complex conditions in junction areas, i.e., grid zone junctions within a grid system, grid junctions between various grid systems, datum junctions, and junctions between ellipsoids. Despite this complexity, these conditions lend themselves to a uniform graphical treatment of the grids with differences in grid orientation and grid color, labels, and values. The treatment of grids under various junction conditions is prescribed in later chapters of this manual.
1-10 OVERLAPPING GRIDS.
Maps at scales of 1:100,000 and larger, falling within approximately 40 kilometers of a grid junction, datum junctions or ellipsoid junction, usually show the adjacent (overlapping) grid by ticks and values around the neatline. In some instances, a coordinate conversion note may be used instead of an overlapping grid.
1-11 EXTENDED GRIDS.
An extended grid is form of overlapping grid used on city maps. It provides total coverage of a map on a single grid when a portion of the map falls on an adjacent grid. The major grid is extended to cover the adjacent area and is shown by full lines.
1-12 GRID AND DATUM RELATED MARGINAL NOTES.
Marginal notes on maps and charts should include projection, ellipsoid, grid zone or belt, horizontal datum, and magnetic declination data. Specific treatment of these items on each product is covered in the various product specifications.
This document supercedes DMA 8358.1, Preliminary Edition.
DATUMS, ELLIPSOIDS, PROJECTIONS
AND MILITARY GRIDS
2-1.1 The Earth is not a sphere, but an ellipsoid, flattened slightly at the poles and bulging somewhat at the Equator. The ellipsoid is used as a surface of reference for the mathematical reduction of geodetic and cartographic data.
2-1.2 A map projection is the systematic drawing of lines representing the meridians and parallels (the graticule) on a flat surface. Different projections have unique characteristics and serve differing purposes. They are depicted by projecting the graticule of the ellipsoid onto a plane; the intersections of the graticule are computed in terms of the ellipsoid.
2-1.3 U.S. Military maps use the sexagesimal system of angular measurement (the division of a full circle into 360°) for designating the values of the graticule. A degree is divided into 60 minute, and each minute into 60 seconds. Parallels are numbered north and south from O° at the equator to 90° at the poles. Meridians are numbered east and west from O° at the prime meridian to a common 180° meridian. The prime meridian used for U.S. Military mapping and charting is related to the Bureau International de I'Heure defined Zero Meridian, located near Greenwich, England. Some foreign produced maps may use the centesimal (decimal) system of angular measurement (the division of a full circle into 400 grads). A grad (or gon) is divided into 100 centigrade (grad minutes), and each centigrad into 100 deci-milligrads (grad seconds). Other prime meridians may be used in non-U.S. mapping.
2-1.4 Grids are applied to maps to provide a rectangular system for referencing and making measurements. There is a definite relationship between the grid and the graticule so that a corresponding geographic position can be determined for each grid position.
2-2 HORIZONTAL DATUMS
The identification, pertinent descriptive information, parameters, and attendant explanatory footnotes for some geodetic datums currently in use are contained in table 1. The datums preferred for use in the production of new and revised topographic maps, joint operations graphics, and selected large scale nautical charts are shown in Appendix D which also graphically depicts their areas of application.
2-3 TRANSFORMING COORDINATES FROM ONE HORIZONTAL DATUM TO ANOTHER HORIZONTAL DATUM.
2-3.1 Coordinates may be transformed from one geodetic datum to another geodetic datum by using the Abridged Molodenskiy Datum Transformation Formulas:
wherePhi = geodetic latitude. Lambda = geodetic longitude H = the distance of a point above or below the ellipsoid measured along the ellipsoid normal through the point.
Table 1 (page 1): Geodetic Datums Used in Map Production
Table 1 (page 2): Geodetic Datums Used in Map Production
Table 1: (page 3 - footnotes):
FOOTNOTES-GEODETIC DATUMS FOR MAP PRODUCTION
- The World Geodetic System (WGS) is not referenced to a single datum point. represents an ellipsoid whose placement, orientation, and dimensions best fit the Earth's equipotential surface which coincides with the geoid. The system was developed from a worldwide distribution of terrestrial gravity measurements and geodetic satellite observations. Several different ellipsoids have been used in conjunction with the various date WGS determinations. The dimensions of the WGS 72 Ellipsoid are:
a = 6,378,135 meters
f = 1/298.26
The dimensions of the WGS 84 Ellipsoid are:
a = 6,378,137 meters
f = 1/298.25722 3563
- This datum is not defined in terms of an origin. It results from a retriangulation of the area to a number of points whose latitude and longitude were known with respect to Greenwich.
- The dimensions of the Clarke 1880 Ellipsoid adopted by different countries vary in accordance with which of Clarke's original dimensions are used: (a, b) or (a, f), or which foot-meter relationship is used to convert the units from feet to meters. In the area referenced to Arc 1950 datum, the dimensions adopted are:
Semimajor axis: a = 6,378,249.145326 meters
Semiminor axis: b = 6,356,514.966721 meters
The above figures yield:
Flattening: f = 1/293.4663076
In the areas of Merchich and Voirol datum, the dimensions adopted are:
a = 6,378,249.2 meters
b = 6,356,515.0 meters
f = 1/293.46598
The values adopted by the Department of Defense are:
a = 6,378,249.145 meters
f = 1/293.465
The above figures yield:
b = 6,356,514.8696 meters
- Dimensions of the War Office Ellipsoid derived by G. T. McCaw (1924) are:
a = 6,378,300.58 meters
f = 1/296.
- Local Astro refers to several independently determined datum origins or to areas where maps are positioned by a network of astronomic positions that are not interconnected.
Table 1: Geodetic Datums Used in Map Production - End
2-3.2 Table 2 (Molodenskiy Transformation Constants to convert from local datum to WGS 84) lists the Delta X, Delta Y, Delta Z, Delta a, and Delta f to transform coordinates from the various datums shown in Appendix D to WGS 84. Values for a and f are listed with figure 1.
2-3.3 The direction of the transformation may be reversed by changing the signs of Delta X, Delta Y, Delta Z, Delta a, and Delta f. Note also that Rm and Rn must be computed with respect to the input ellipsoid.
2-3.4 Transformation procedures and constants are published in DMA TR 8350.2, Department of Defense World Geodetic System 1984.
2-4.1 Several ellipsoids are presently used in U.S. Military mapping. The goal is to eventually refer all positions to the World Geodetic System (WGS), which has a specific set of defining parameters, or to a WGS compatible ellipsoid. Ellipsoids may be defined by a combination of algebraically related dimensions such as the semi-major and semi- minor axes or the semi-major axis and the flattening. Figure 1 illustrates the defining elements and lists the dimensions of the ellipsoids used by the Defense Mapping Agency.
2-4.2 Appendix D (index of Preferred Grids, Datums, and Ellipsoids Specified for New Mapping) identifies the extent of currently effective ellipsoids.
2-5.1 The projections used as the framework of all U.S. Military maps and charts have a common characteristic in that they are conformal. Conformality indicates that small areas retain their true shape; angles closely approximate their true values; and, at any point, the scale is the same in all directions.
2-5.2 Certain projections are prescribed for U.S. Military topographic mapping and charting that shows military grids:
2-5.2.1 Maps at scales of 1:500,000 and larger for areas between 80° south and 84° north, and some hydrographic charts at 1:50,000 and larger, are based on the Transverse Mercator Projection.
Table 2 (page 1): Molodenskiy Transformation Constants to Convert from Local Datum to WGS 84
Table 2 (page 2): Molodenskiy Transformation Constants to Convert from Local Datum to WGS 84
Table 2 (page 3): Molodenskiy Transformation Constants to Convert from Local Datum to WGS 84
Figure 1. Defining Parameters of Ellipsoids
2-5.2.2 Maps at 1:1,000,000 scale between 80° south and 84° north, some hydrographic charts, and aeronautical charts at 1:500,000 between 80° south and 80° north, are based on the Lambert Conformal Conic Projection.
2-5.2.3 Maps at 1:1,000,000 scale and larger of the polar regions (south of 80° south and north of 84° north), some hydrographic charts smaller than 1:50,000 and at latitude between 70° and the poles, and aeronautical charts at 1:500,000 north of 80° north or south of 80° south, are base, on the Polar Stereographic Projection.
2-5.2.4 Coastal charts at 1:75,000 scale and smaller are based on the Mercator Projection.
2-5.2.5 General maps at scales smaller than 1:1,000,000 are based on projections individually selected to conform with the intended use of the map. Because of their variety, complexity, and limited use, such projections are not described in this manual.
2-5.2.6 Maps produced by coproducing nations in non-U.S. areas of responsibility may be based on other projections such as the Transverse Mercator Projection, the Lambert Conical Orthomorphic Projection (Lambert Conformal Conic Projection), Laborde Projection, New Zealand Map Grid Projection, the Rectified Skew Orthomorphic Projection, etc.
2-5.3 The following paragraphs contain concepts of some of the prescribed projections; in practice, however, the projections are reduced to a plane surface by use of mathematical formulas. (See Chapter 1 for references to mathematical tables.) Figures 2, 3, 4, 5, and 6 are provided as an aid in the, understanding of these concepts.
2-5.4 The Mercator Projection is not normally used for military topographic maps; however, it is used extensively for naval ocean navigation and bathymetric charts. Its description also serves as a basis for understanding the Transverse Mercator Projection. The Mercator Projection can be visualized as an ellipsoid projected onto a cylinder with tangency established at the Equator and with the polar axis of the ellipsoid in coincidence with the cylinder axis as shown in figure 2. The origins of the projection lines vary and are about three-quarters of the way back along the diameters in the equatorial plane. When the cylinder is opened and flattened, a distortion appears in the polar regions, in as much as the line representing the Equator is the true distance and each parallel is represented by a line as long as the Equator. The poles are infinitely distant from the Equator and can not be shown on the projection. Distortion becomes more pronounced as the distance north and south of the Equator increases. For example, the map scale at 60° north and 60° south is approximately twice that at the Equator.
2-5.5 A Transverse Mercator Projection is a Mercator Projection where the cylinder has been rotated or transversed 90°. The ellipsoid and cylinder are thus tangent along a meridian. By projecting the surface of the ellipsoid onto the cylinder, as shown in figure 3, in the same manner as for the Mercator Projection, the Transverse Mercator Projection is developed on the surface of the cylinder, which is then opened and flattened.
2-5.5.1 Distortion - The east and west extremities appear distorted at the outer edges when projected onto a cylinder. The two shaded areas of figure 3 show the varying distortion of two equivalent geographic areas on the some projection. Note that both areas extend 15°in longitude within the 30° to 45° south latitude bond. The area bounded by the 30° and 45° east meridians is greatly magnified in comparison to the area bounded by the 90° and 105° east meridians. When a meridian is tangent to the cylinder of projection, there is no distortion along that meridian. Distances along the tangent meridians are true distances, and all distances within 3° of the meridians are relatively accurate. Therefore, to minimize distortion, the Transverse Mercator Projection, for military purposes, uses 60 longitudinal zones, each zone 6° wide. For example, a zone centered on 3° (central meridian) is bounded by the O° and 6° meridians, and a zone centered on 9° is bounded by the 6° and 12° meridians.
Figure 2. Mercator Projection
Figure 3. Transverse Mercator Projection
Figure 4. Secant Condition of Transverse Mercator Projection; Typical 6-degree Projection Zone
2-5.5.2 Secant condition - The cylinder of projection is modified by reducing its elliptical dimensions and making it secant to the ellipsoid, intersecting the ellipsoid along lines parallel to the central meridian (fig. 4). For the Universal Transverse Mercator grid this condition establishes, in one 6° zone, two lines of secancy approximately 180,000 meters east and west of the central meridian. These lines of seconcy, in effect, allow a more congruous relationship between ellipsoid and map distances than that of the central meridian tangency. Since the central meridian of all zones is given a false easting value of 500,000 meters east (mE), the secant lines have coordinates of approximately 320,000 mE and 680,000 mE respectively. Figure 4 also gives a schematic representation of the scale distortion in any 6° zone. Note that the scale of the projection at the lines of secancy is exact.
2-5.5.3 Scale factor - For Most military operations, map and ground distances are assumed to be equivalent. However, in certain geodetic and artillery operations, where long distances are involved and accuracy of results is essential, it is necessary to correct for the difference between distances on the map and distances on the ground. This is done by the use of scale factors from prepared tables or by formula. For the Transverse Mercator Projection, the scale factor is 1.00000 (unity) at the lines of secancy, decreasing inwardly to 0.9996 at the central meridian, and increasing outwardly to about 1.0010 near the zone boundaries at The equator.
2-5.6 The Polar Stereographic Projection, a conformal azimuthal projection, is similar in both the northern and southern polar regions. The projection is developed on a plane tangent at a pole with the projection lines originating from the opposite pole. The plane is perpendicular to the minor axis, as shown in figure 5. For use with the Universal Polar Stereographic grid, a scale factor of 0.994 is applied at the origin (pole) to lower the plane of projection to intersect the sphere at approximately 81°07' latitude. This arbitrary geometry is applied to reduce the maximum scale distortion of the tangent projection. As shown in figure 5, the scale is exact (unity scale factor) at approximately 81'07' latitude. The scale factor decreases to 0.994 at the pole, increases to 1.0016076 at 80°00' and attains its maximum value of 1.0023916 at 79°30'. The scale factor is constant along any given parallel.
2-5.7 The Lambert Conformal Conic Projection can be visualized as the projection of the ellipsoid onto a cone whose axis coincides with the polar axis of the ellipsoid as in figure 6. Usually, the cone is secant to the ellipsoid, intersecting along two parallels of latitude. These two parallels are called standard parallels. Meridians appear as straight lines radiating from a point beyond the mapped areas. Parallels appear as arcs of concentric circles which are centered at the point from which the meridians radiate. None of the parallels appear in exactly the projected positions; they are mathematically adjusted to produce the property of conformality. This adjustment is slight if the standard parallels are sufficiently close together.
2-5.8 The characteristics of prescribed projections are tabulated in table 3.
Figure 5. Polar Stereographic Projection
Figure 6. Lambert Conformal Conic Projection
Table 3. Characteristics of Projections
2-6 MILITARY GRIDS
2-6.1 Military grids consist of parallel lines intersecting at right angles and forming a regular series of squares. The north-south lines are called eastings and the east-west lines northings. Each grid line is one of an even-interval selection of measurement units. The interval is selected in accordance with the map scale. The unit intervals shown on military map scales are:
MAP SCALES UNIT INTERVALS 1:12,500 1,000 1:25,000 1,000 1:50,000 1,000 1:100,000 1,000 or 10,000 1:250,000 10,000 1:500,000 10,000 1:1,000,000 100,000 with ticks at 10,000
Table 4. Grid Unit Intervals for Various Scale Topographic Maps.
2-6.2 The grids preferred for military maps are:
2-6.2.1 Universal Transverse Mercator (UTM) grid for areas between 80° south and 84° north.
2-6.2.2 Universal Polar Stereographic (UPS) grid for the polar regions south of 80° south and north of 84° north. 2-6.2.3 Other grids for certain parts of the world as shown in Appendix D. These grids are being progressively replaced by the UTM grid, with the intent to eventually cover all military mapping of the world with a universal metric grid system.
2-6.2.4 Area of application for the various other grids are given in Appendix D. A general description of the grids and numbering systems is given in Chapter 4.
2-6.3 Specifications for the Universal Grid Systems follow:
2-6.3.1 Universal Transverse Mercator (UTM) Grid.
Projection: Transverse Mercator (Gauss-Kruger type) in zones 6° wide.Ellipsoid: International Bessel 1841 World Geodetic System 1984 Geodetic Reference System (GRS 1980)
Longitude of Origin: Central meridian (CM) of each projection zone (3°, 9°, 15°, 21°, 27°, 33°, 39°, 45°, 51°, 57°, 63°, 69°, 75°, 81°, 87°, 93°, 99°, 105°, 111°, 117°, 123°, 129°, 135°, 141°, 147°, 153°, 159°, 165°, 171°, 177°, E and W).
Latitude of Origin: O° (the Equator).
False Northing: 0 meters at the Equator for the Northern Hemisphere; 10,000,000 meters at the Equator for the Southern Hemisphere.
False Easting: 500,000 meters at the CM of each zone.
Scale Factor at the Central Meridian: 0.9996.
Grid Zone Designations: See Chapter 3 and Appendix B.
Latitude Limits of System: From 80°S to 84°N.
Limits of Projection Zones: The zones are bounded by meridians, the longitudes of which are multiples of 6° east and west of the prime meridian.
Overlap: On large-scale maps and trig lists, the data for each zone, datum, or ellipsoid overlaps the adjacent zone, datum, or ellipsoid a minimum of 40 kilometers. The UTM grid extends to 80°30'S and 84°30'N, providing a 30-minute overlap with the UPS grid.
2-6.3.2 Universal Polar Stereographic (UPS) Grid.
Projection: Polar Stereographic.
Ellipsoid: World Geodetic System 1984
Longitude of Origin: O° and 180°E-W.
Latitude of Origin: 90°N and 90°S.
False Northing: 2,000,000 meters.
False Easting: 2,000,000 meters.
Scale Factor at the Origin: 0.994.
Grid Zone Designations: See Chapter 3 and Appendix B.Limits of System: North Zone: Polar area north of 84°N. South Zone: Polar area south of 8O°S.
Overlap: The UPS grid extends to 83°30'N and 79°30'S, providing a 30-minute overlap with the UTM grid.
2-6.4 Formulas for constructing UTM and UPS grids are contained in DMA TM 8358.2.
2-7 TRANSFORMING COORDINATES FROM ONE GRID SYSTEM TO ANOTHER GRID SYSTEM.
Coordinates may be transformed from one grid system to another grid system, for instance, between a Lambert grid and a UTM grid or between different grid zones. The preferred procedure is to transform the grid coordinates from the first grid system to geographic positions. Then transform the geographic positions to grid coordinates of the second grid system. Note: This procedure does not change the datum. See paragraph 2-3 for the procedure to use when changing from one datum to another datum.
CHAPTER 3 THE U.S. MILITARY GRID REFERENCE SYSTEM (MGRS)
3-1 GENERAL DESCRIPTION
3-1.1 The U.S. Military Grid Reference System (MGRS) is designed for use with the UTM and UPS grids.
3-1.2 For convenience, the world is generally divided into 6° by 8° geographic areas, each of which is given a unique identification, called the Grid Zone Designation (fig. 7). These areas are covered by a pattern of 100,000-meter squares. Each square is identified by two letters called tie 100,000-meter square identification. This identification is unique within the area covered by the Grid Zone Designation. Exceptions to this general rule have been made in the post to preserve the 100,000-meter identifications on mapping that already exists. Appendix B shows the method for finding the 100,000-meter square identifications.
3-1.3 A reference keyed to a gridded map of any scale is made by giving the 100,000- meter square identification together with the numerical location. Numerical references within the 100,000-mater square are given to the desired accuracy in terms of the easting (E) and northing (N) grid coordinates for the point. The Grid Zone Designation usually is prefixed to the identification when references are made in more than one grid zone designation area.
3-2 THE GRID NORTH DESIGNATION.
3.2.1 An MGRS position location uses the standard military practice of reading "right (easting) and up (northing)". In each portion of a military grid reference (grid zone designation, 100,000-meter square identification, and grid coordinates), the first part provides the easting component and the second part provides the northing component.
3.2.2 The MGRS is on alphanumeric version of a numerical UTM or UPS grid coordinate.
3-2.2.1 For that portion of the world where the UTM grid is specified (80° south to 84° north), the UTM grid zone number is the first element of a Military Grid reference. This number sets the zone longitude limits. Zone 32 has been widened to 9° (at the expense of zone 31) between latitudes 56° and 64° to accommodate southwest Norway. Similarly, between 72° and 84°, zones 33 and 35 have been widened to 12° to accommodate Svalbard. To compensate for these 12° wide zones, zones 31 and 37 are widened to 9° and zones 32, 34, and 36 are eliminated.
3-2.2.2 The next element is a letter which designates a latitude bond. Beginning at 80° south and proceeding northward, twenty bands are lettered C through X, omitting I and O. The bands are all 8° wide except for bond X which Is 12° wide. Thus, in the UTM portion of the MGRS, the first three characters designate one of the 1197 areas with the dimensions as shown in Table 5.
3-2.2.3 In the Polar regions, there is no zone number. A single letter designates the semicircular area and hemisphere. Since the letters A, B, Y, and Z are used only in the Polar regions, their presence in an MGRS, with the omission of a zone number, designates that the coordinates are UPS.
Figure 7. Grid Zone Designations of the Military Grid Reference System
3-2.3 The grid zones are divided into a pattern of 100,000-meter grid squares forming a matrix of rows and columns. Each row and each column is sequentially lettered such that two letters provide, a unique identification, within approximately 9°, for each 100,000- meter grid square. Appendix B provides the location and identification of the grid zones and 100,000-meter grid squares.
Latitude Longitude Number 8° 6° 1138 8° 9° 1 8° 3° 1 12° 6° 53 12° 9° 2 12° 12° 2
Table 5. Dimensions of Grid Zone Designation Areas.
3-2.3.1 For many years efforts hove been made to reduce the complexity of grid reference systems by standardization to a single world-wide grid reference system. This effort is continuing and will generate additional changes to Appendixes B and D.
3-2.3.2 The remainder of this chapter describes the determination of the 100,000-meter square identification, and the military grid reference.
3-3 100,000-METER SQUARE IDENTIFICATION.
3-3.1 The 100,000-meter columns, including partial columns along zone, datum, and ellipsoid junctions, are lettered alphabetically, A through Z (with I and O omitted), north and south of the Equator, starting at the 180° meridian and proceeding easterly for 18°. The alphabetical sequence repeats at 18° intervals.
3-3.2 To prevent ambiguity of identifications along ellipsoid junctions changes in the order of the row letters are necessary. The row alphabet (second letter) is shifted ten letters. This decreased the maximum distance in which the 100,000-meter square identification is repeated.
3-3.3 The 100,000-meter row lettering is based on a 20-letter alphabetical sequence (A through V with I and O omitted). This alphabetical sequence is read from south to north, and repeated at 2,000,000-meter intervals from the Equator.
3-3.3.1 The row letters in each odd numbered 6° grid zone are read in an A through V sequence from south to north.
3-3.3.2 In each even-numbered 6° grid zone, the some lettering sequence is advanced five letters to F, continued sequentially through V and followed by A through V.
3-3.3.3 The advancement or staggering of row letters for the even-numbered zones lengthens the distance between 100,000-meter squares of the same identification.
3-3.4 Users are cautioned that deviations from the preceding rules were mode in the past. These deviations were an attempt to provide unique grid references within a complicated and disparate world-wide mapping system.
3-3.5 Determination of 100,000-meter grid square identification is further complicated by the use of different ellipsoids. Figure 8 shows the basic lettering system. Appendix B provides detailed guidance for finding the correct identification in each ellipsoid area.
Figure 8. Basic Plan of the 100,000-meter Square Identifications of the U.S. Military Grid Reference System, Between 84°N and 80°S
3-4 THE MILITARY GRID REFERENCE.
3-4.1 The MGRS coordinate for a position consists of a group of letters and numbers which include the following elements:
3-4.1.1 The Grid Zone Designation.
3-4.1.2 The 100,000-meter square letter identification.
3-4.1.3 The grid coordinates (also referred to as rectangular coordinates); the numerical portion of the reference expressed to a desired refinement.
3-4.2 A reference is written as an entity without spaces, parentheses, dashes, or decimal points.
Examples 18S (Locating a point within the Grid Zone Designation) 18SUU (Locating a point within a 100,000-meter square) 18SUU80 (Locating a point within a 10,000-meter square) 18SUU8401 (Locating a point within a 1,000-meter square) 18SUU836014 (Locating a point within a 100-meter square)
3-4.3 To satisfy special needs, a reference can be given to a 10-meter square and a 1-meter square as:
18SUU83630143 (Locating a point within a 10-meter square) 18SUU8362601432 (Locating a point within a 1-meter square)
3-5 MGRS APPLICATION.
3-5.1 All elements of a grid reference need not be used. Their use depends upon the size of the area of Activities, the type of military operations, and the scale of the map to which the reference is keyed. The military area commander usually designates the elements of the grid references to be used. The following paragraphs provide guidance for the use of Grid Zone Designations and 100,000-meter square identifications.
3-5.1.1 For military operations spanning large geographical areas, the Grid Zone Designation is usually given (such as IBS). This designation will alleviate ambiguity between identical references that may occur when reporting to a station outside the area. The Grid Zone Designation is always used in giving references on 1:1,000,000 scale and 1:500,000 scale maps.
3-5.1.2 For operational areas of lesser extent, but exceeding 100,000 meters, the 100,000-meter square identification is used (such as UU80). The 100,000-meter square identification is uses in reporting references on the 1:250,000 and larger scale maps to avoid ambiguity between identical references which occur every 100,000 motors, and near grid zone junctions and ellipsoid junctions.
Figure 9. Method of Reading a U.S. Military Grid Reference from a 1:250,000 Scale Map
3-5.1.3 For small and localized operational areas, the Grid Zone Designations and 100,000-meter square identifications are not used, unless reporting falls within the parameters explained in preceding paragraphs. In the instance of local report only the numerical part of the grid reference is used (such as 836014). This condition applies to 1:100,000 scale maps and larger.
3-5.1.4 Topographic maps at scales 1:500,000 and larger provide a grid reference box with the elements and instructions for making a complete grid reference.
Figure 10. Method of Reading a U.S. Military Grid Reference from a Large Scale Map
3.5.2 The numerical part of a grid reference always contains an even number of digits. The first half of the total number of digits represents the easting, and second half the northing. The standard military practice of reading "right (easting) and up (northing)" is employed.
3-5.2.1 To read the easting coordinate, locate the first easting (vertical) grid line to the left of the point of reference and read the large digit (or digits), the principal digit labeling the line either in the top or bottom margin or on the line itself. Smaller digits shown as part of a grid number are ignored. Estimate, or scale to the closest tenth of the grid interval, the distance between the easting grid line to the left of the point and the point itself.
3-5.2.2 The reading of the northing coordinate is made in a similar manner. Locate the first northing (horizontal) grid line below the point of reference and read the principal digits labeling the line located in the left or right margin or on the line itself. Then estimate, or scale to the closest tenth of the grid interval, the distance between the northing grid line below the point and the point itself.
3-5.2.3 The numerical part of a point reference taken from a 100,000-meter grid (on maps of 1:1,000,000 scale) is a two-digit number; for example: 80. Reading from left to right, the 8 represents the 10,000 digit of the first easting grid line (or grid tick) to the left of the point; the 0 represents the 10,000 digit of the first northing grid line (or grid tick) below the point.
3-5.2.4 The numerical part of a point reference taken from a 10,000-meter grid (on maps smaller than 1:100,000 scale and larger than 1:1,000,000 scale) is a four-digit number; for example: 8401. Reading from left to right, the 8 represents the 10,000 digit of the first easting grid line to the left of the point, the 4 represents the estimated tenths (nearest 1,000 meters) from the easting grid line to the point, the 0 represents the 10,000 digit of the first northing grid line below the point, and the 1 represents the estimated tenths (nearest 1,000 meters) from the northing grid line to the point. See figure 9.
3-5.2.5 Normally, the numerical part of a point reference taken from a 1,000-meter grid (on maps at scales of 1:100,000 and larger) is a six-digit number; for example: 836014. Reading from left to right, the 83 represents the 10,000 and 1,000 digits of the first easting grid line to the left of the point, the 6 represents the estimated or scaled tenths (nearest 100 meters) from the easting line to the point, the 01 represents the 10,000 and 1,000 digits of the first northing grid line below the point, and the 4 represents the estimated or scaled tenths (nearest 100 meters) from the northing grid line to the point. See figure 10.
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