Office of Geomatics

Global Area Reference System (GARS)

GARS is the standardized battlespace area reference system. It is based on lines of longitude (LONG) and latitude (LAT), and is used to provide an integrated common frame of reference for joint force situational awareness to facilitate air-to-ground coordination, deconfliction, integration, and synchronization. This area reference system provides a common language between components and simplifies communications. It is important to note that GARS is primarily designed as a battlespace management tool and not to be used for navigation or targeting.

Design

GARS divides a surface of the earth into 30-minute by 30-minute cells. Each cell is identified by a five-character designation (ex. 006AG). The first three characters designate a 30-minute wide longitudinal band. Beginning with the 180° meridian and proceeding eastward, the bands are numbered from 001 to 720, so that 180° E to 179° 30'W is band 001; 179° 30'W to 179° 00'W is band 002; and so on. The fourth and fifth characters designate a 30-minute wide latitudinal band. Beginning at the south pole and proceeding northward, the bands are lettered from AA to QZ (omitting I and O) so that 90° 00'S to 89° 30'S is band AA; 89° 30'S to 89° 00'S is band AB; and so on.

• Each 30-minute cell is divided into four 15-minute by 15-minute quadrants:
  • The quadrants are numbered sequentially, from west to east, starting with the northernmost band. Specifically, the northwest quadrant is "1"; the northeast quadrant is "2"; the southwest quadrant is "3"; the southeast quadrant is "4".
  • Each quadrant is identified by a six-character designation (ex. 006AG3). The first five characters comprise the 30-minute cell designation. The sixth character is the quadrant number.
• Each 15-minute quadrant is divided into nine 5-minute by 5-minute areas:
  • The areas are numbered sequentially, from west to east, starting with the northernmost band. The graphical representation of a 15-minute quadrant with numbered 5-minute by 5-minute areas resembles a telephone keypad.
  • Each 5-minute by 5-minute area, or keypad "key" is identified by a seven-character designation. The first six characters comprise the 15-minute quadrant designation. The seventh character is the keypad "key" number (ex.006AG39).

MGRS is an alpha-numeric system for expressing UTM/UPS coordinates. A single alpha-numeric value references a position that is unique for the entire earth. Using "15SWC8081751205" as an example, the components of MGRS values are as follows:

• Leading zeros are included so that Zone 9 is "09".
• For polar areas outside the UTM area, these characters are omitted.

• Beginning at 80°S and preceding northward, the 20 bands are lettered C through X, omitting I and O.
• The bands are all 8° high except band X, which is 12° high.
• Outside the UTM area, and in leu of UTM zone numbers, A and B are used near the South Pole, Y and Z near the North Pole.
• The vertical UTM boundaries and horizontal latitude band boundaries form (generally) 6° X 8° Grid Zones. Hence, the first three letters of the MGRS value, e.g. "15S", are referred to as the Grid Zone Designator (GZD).

The fourth and fifth characters are a pair of letters identifying one of the 100,000-meter grid squares within the grid zone (or UPS area). See figure to the right.

• In the figure sample area, the Grid Zone Designators are shown in brown. The smaller gray letters are the 100,000-meter grid square identifiers. The example point "15SWC8081751205" is located in the square "WC" near the center of the figure.

The remaining characters within the MGRS coordinate correspond to the UTM values starting with the numbers in the ten-thousands place in the Easting and Northing, and including more numbers depending on desired precision level. See figure to right. MGRS coordinates may be rounded to reflect lesser refinement. For example:

• 15SWC8081751205 is at 1-meter refinement
• 15SWC80825121 is at 10-meter refinement
• 15SWC808512 is at 100-meter refinement
• 15SWC8151 is at 1000-meter refinement

There are two lettering schemes for the 100,000-meter grid square identifiers. Generally, one scheme is used for WGS-84, and the other is used for older ellipsoids associated with the local datums:

• Example: 15SWC8081751205 is on WGS-84. When converted to NAD-27 datum, Clarke 1866 ellipsoid, its value is: 15SWN8083350993
• The 100,000-meter grid square "WC" for WGS-84 generally coincides with the grid square "WN" for Clarke 1866.

The magenta arrows show how MGRS Easting and Northing values are determined from within the 100,000-meter grid square. The MGRS value of this position is 15SWC8081751205. See figure to the right.

UTM coordinates are based on a family of 120 Transverse Mercator map projections (two for each UTM zone, with one for each N/S hemisphere).

Numbering of zones begins at 180° and proceeds eastward:

• Zone 1 is from 180°W to 174°W
• Zone 2 is from 174°W to 168°W, and so on

Each zone also has a central meridian:

• Zone 1 central meridian is 177°W,
• Zone 2 central meridian is 171°W, and so on

Design

UTM divides the earth into 60 zones, each 6° wide in longitude (with the exception of a few non-standard-width zones for Svalbard and southwest Norway).

UTM is limited to the area between 84°N and 80°S. Beyond that, Universal Polar Stereographic (UPS) coordinates are used. See section on UPS.

• The X value, called the Easting, has a value of 500,000m at the central meridian of each zone. See figure on left.
• The Y value, called the Northing, has a value of 0m at the equator for the northern hemisphere and 10,000,000m at the equator for the southern hemisphere. See figure on right.

Positioning

UTM divides the earth into 60 zones, each 6° wide in longitude (with the exception of a few non-standard-width zones for Svalbard and southwest Norway).

• In the UTM system, positions are expressed as Easting / Northing, e.g. "580817mE, 4251205mN". In some cases, the letters are left off, e.g. "580817 4251205".
• If positions occur near UTM zone junctions, the UTM zone may also be specified, e.g. "580817mE, 4251205mN, Zone 15".
• Since the above expresses two possible positions on the earth, the hemisphere may also be specified, e.g. "580817mE, 4251205mN, Zone 15, Northern Hemisphere" (Figure 4).
• Many systems abbreviate the above, representing the hemisphere as a single letter, "N" for northern hemisphere, and "S" for southern hemisphere, e.g. "15N 580817 4251205".

Design

The Universal Polar Stereographic (UPS) Systems coordinates are based on a family of two Polar Stereographic map projections, one for each pole.

• The origin of the UPS coordinate system is at the poles, where X=2,000,000m and Y=2,000,000m.
• The X-axis lies along the meridians 90°E and 90°W:
  • Moving from the pole (north or south), X-values (Eastings) increase along the 90°E meridian
• The Y-axis lies along the meridians 0° and 180°:
  • Moving from the North Pole, Y-values (Northings) increase along the 180° meridian.
  • Moving from the South Pole, Y-values (Northings) increase along the 0° meridian.

UPS North, 100km MGRS Letters: top image
UPS South, 100km MGRS Letters: bottom image