text: Airspace Coordinating Measures
  decimal. To convert from decimal minutes to seconds multiply the decimal portion by 60. The product is the
  seconds (for example, .2 X 60= 12).
  AREA REFERENCE SYSTEMS
  B-11. Airspace element personnel use the Global Area Reference System and common geographic reference
  system as area reference systems.
  Global Area Reference System
  B-12. The Global Area Reference System (GARS) is the standardized area reference system across the
  Department of Defense. It is based on lines of longitude (long) and latitude (lat) 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 the components and simplifies communications. The point of origin for this system is 90 degrees
  south and 180 degrees east/west. The areas GARS describes coincide with even numbered WGS-84 degree
  and minute lines. GARS airspace is divided into cells, further divided into quadrants, and subdivided into
  keypads.
  Common Geographic Reference System
  B-13. The common geographic reference system (CGRS) is an early, theater specific system that may still
  be in use. CGRS uses a theater determined origin or starting point. CGRS airspace is divided into cells,
  further divided into nine keypads, and may be subdivided into quadrants.
  B-14. Since GARS and CGRS use common terms (such as keypads and quadrants), a risk of confusion exists
  since these common terms have different meanings in the individual reference systems. For example, a GARS
  cell (~30nm x 30nm) is divided first into four quadrants (~15nm x 15nm) then into nine keypads (~5nm x
  5nm), while a CGRS cell (~30nm x 30nm) is divided into nine keypads (~10nm x 10nm), which may be
  further subdivided into four quadrants (~5nm x 5nm). The risk in confusion is that a GARS keypad is ~5nm
  x 5nm while a CGRS keypad is ~10nm x 10nm.
  ALTITUDE MEASURING SYSTEMS
  B-15. Airspace element personnel use the mean sea level, above mean sea level, above ground level, height
  above ellipsoid, and flight level as altitude measuring systems.
  Mean Sea Level
  B-16. The MSL is determined by continuously measuring the rise and fall of the ocean at “tide gauge stations”
  on seacoasts for a period of about 19 years. This averages out the highs and lows of the tides caused by the
  changing effects of the gravitational forces from the sun and moon which produce the tides. The MSL then
  is defined as the zero elevation for a local or regional area. The MSL is elevation used for military maps,
  digital terrain elevation data (DTED) and by artillery airspace users. Some aircraft systems using Global
  Positioning System (GPS) data convert height above ellipsoid (see paragraph B-19) to MSL data before
  reporting.
  Above Mean Sea Level
  B-17. The above mean sea level (AMSL) refers to the elevation (on the ground) or altitude (in the air) of any
  object, relative to the average sea level datum. It is also used in aviation, where all heights are recorded and
  reported with respect to AMSL. Manned aircraft determine AMSL with a barometric altimeter corrected for
  local air pressure.
  Above Ground Level
  B-18. An altitude above ground level (AGL) is measured with respect to the underlying ground surface. AGL
  altitude varies constantly as the aircraft proceeds on course. Therefore, the only constant is the MSL. When