f l GEOID CELESTIAL SPHERE Figure 2-20  Astronomic Coordinates c.  Astronomic coordinates are computed independent of each other.  They can be connected by geodetic methods and adjusted to a geodetic network. 2-21  Prime Meridian a.  General.  The Prime Meridian is the meridian of longitude referenced as 0° for a particular geographic system.  In most cases, the term Prime Meridian is meant to be considered the Greenwich Meridian; however, Figure 2-21 lists several systems using other meridians of longitude as the Prime Meridian for that system. b.  Converting to Greenwich.  In any case where survey data is provided in a system not referencing the Greenwich Meridian as 0° longitude, a simple conversion can be made by applying the longitude offset to the survey data longitude.  See Figure 2-21.   2-22  Angular Measurements a.  General.  Care must be taken to ensure that if survey data is provided covering other nations, including mapping products, that the data is depicted or measured in the correct angular system.  Two angular systems are used to depict coordinate systems on maps and to coordinate survey points: centesimal and sexagesimal.  Table B-4 in Appendix B lists these and other angular conversions. b.  Centesimal System.  The unit usually associated with a centesimal system is the grad (used extensively in Europe and North Africa).     1.  A grad is defined as the hundredth part (1/100th) of a right angle.  In this system, one grad equals one hundred minutes and one minute equals one hundred seconds.     2.  Grads are notated by a superscript g (i.e.. ); 12^g centesimal minutes by a superscript c (i.e.. ); and 48^c centesimal second by a superscript cc (i.e.. ).  The 27^cc entire number is notated together as in the following example:   .          12^g48^c 27^cc c.  Sexagesimal System.  The unit usually associated with a sexagesimal system is the degree.     1.  A degree can be defined as the ninetieth part (1/90th) of a right angle.  In this system, one degree equals sixty minutes and one minute equals sixty seconds.     2.  Degrees are notated by the symbol ° (i.e.. 24°); sexagesimal minutes by a ' (i.e.. 38'); and sexagesimal seconds by a " (i.e.. 02").  The entire number is notated together as in the following example:  24° 38' 02". 2-23  Deflection of the Vertical a.  The deflection of the vertical at a point is the angular difference between the vertical (plumbline), which is perpendicular to the geoid, and a line through the point which is perpendicular to the reference ellipsoid.  See Figure 2-22.  This term can be more accurately referred to as the Astro-geodetic Deflection of the Vertical. b.  Due to the deflection of the vertical in the plane of the prime vertical (a circle in the east-west direction of the observer's horizon), there is a difference between astronomic and geodetic longitude and between astronomic and geodetic azimuths.  This is called the Laplace condition and is expressed by the Laplace Equation.  The laplace equation yields a correction which when subtracted from an astronomic azimuth will produce a geodetic azimuth.        DRAFT 2-12