3-14  Grid Systems a.  General.  A grid system is a two-dimensional plane-rectangular coordinate system which is usually based on, and mathematically adjusted to, a map projection.  This allows for the transformation from geodetic positions (latitude and longitude) to plane coordinates (easting and northing) and for the computations relating to those coordinates to be made by ordinary methods of plane surveying. b.  Local and Universal Grids.  Many grid systems are currently in use.  The majority of the "local" systems will eventually be converted to one of the universal grid systems; however, many areas will continue to be mapped in a local system such as the British National Grid (BNG), the Irish Transverse Mercator Grid (ITMG), the Madagascar Grid (MG), etc.  Many local grid systems are discussed in detail in DMA TM 8358.1.  Two universal grids are used by the US military and its allies: Universal Transverse Mercator (UTM) and Universal Polar Stereographic (UPS).    c.  Grid Lines.  Grids consist of a system of evenly spaced parallel lines lying perpendicular to another system of evenly spaced parallel lines forming squares. The ground distance between the lines is dependent on the scale of the map and the type of grid system.  Most systems use meters as a basis for grid line spacing; some systems; however, use yards or feet.  Standard scale military maps generally adhere to the following: grid lines on large scale maps are spaced at 1000 meters, grid lines on medium scale maps at 1:250,000 are spaced at 10,000 meters.  For scales smaller than 1:250,000, the grid lines may or may not be depicted, dependent on the purpose of the map. d.  Grid Line Values.  The north-south lines in a grid system are called Eastings and increase in value from west to east.  The east-west lines in a grid system are called Northings and their value increases from south to north.  (These rules do not apply to grid systems which cover the polar regions such as the UPS.)  The numeric value of an easting and northing are referenced to a specific origin.  In other words, a false value is applied to the easting or northing grid line which falls at a particular reference line or point. Usually, that line or point is a meridian of longitude (i.e. central meridian of a zone) or a parallel of latitude (i.e. equator); however, it can be other references such as a Datum Point or the meridian or parallel at a mountain peak or river junction.  The origin for the false easting and false northing are normally different lines or points. 3-15  Grid Convergence     Grid convergence is the angular difference between true north and grid north.  The direction (+,-) and the value of the angle is computed differently depending on the grid system.  In some grid systems, grid convergence can be considered the same as convergence of the meridians (paragraph 2-18c). 3-16  Universal Transverse Mercator Grid a.  The Universal Transverse Mercator (UTM) Grid System is referenced to the Transverse Mercator Projection.  The ellipsoid is divided into 60 grid zones, each 6° wide, extending from 84° N latitude to 80° S latitude.  The zones are numbered from 1 to 60, with zone 1 starting at 180°- 174° W longitude, zone 2 at 174° W - 168° W longitude, continuing east to zone 60 at 174° E - 180° longitude.  The Prime Meridian (0° longitude) separates zones 30 and 31.  See Figure 3-17. b.  The location of any point in the UTM grid system can be designated by coordinates by giving its distance east-west (easting) and its distance north-south (northing) from the origin of the grid zone.  This origin, for each UTM grid zone, is the intersection of the equator and the central meridian of the zone.  Each UTM zone has a central meridian corresponding to the central meridian of each Transverse Mercator Projection zone.  The grid is oriented by placing the east-west axis of the grid in coincidence with the equator and the north-south axis of the grid in coincidence with the central meridian of the zone. c.  Once the grid is oriented, the origin for easting and northing are assigned false values.  The central meridian (origin for easting) of each zone is assigned an easting value of 500,000 meters.  The easting increases east of the central meridian and decreases west.  The equator (origin for northing) is assigned two false values; if you are operating in the northern hemisphere the northing of the equator is 0 meters and DRAFT 3-15 Section II GRID SYSTEMS