The Airport Reference Point (ARP) is the approximate geometric center of all usable runways. The ARP position computation is somewhat similar to a center of mass computation, except that only two dimensions are considered The datums used in the computations are normally selected as the lowest absolute value latitude and longitude coordinates, respectively, of all runway ends used in the computation. This convention eliminates computing with negative moments.
ARPLAT = LATITUDE DATUM + (SUM OF RUNWAY MOMENTS ABOUT THE LATITUDE DATUM/SUM OF RUNWAY LENGTHS)

ARPLON = LONGITUDE DATUM + (SUM OF RUNWAY MOMENTS ABOUT THE LONGITUDE DATUM/SUM OF RUNWAY LENGTHS)

RUNWAY MOMENT ABOUT THE LATITUDE DATUM = RUNWAY GROUND LENGTH TIMES THE DISTANCE IN SECONDS BETWEEN THE APPROXIMATE RUNWAY CENTER POINT* AND THE LATITUDE DATUM

RUNWAY MOMENT ABOUT THE LONGITUDE DATUM = RUNWAY GROUND LENGTH TIMES THE DISTANCE IN SECONDS BETWEEN THE APPROXIMATE RUNWAY CENTER POINT* AND THE LONGITUDE DATUM

Runway coordinates must be entered as absolute values.
Runway Lengths must be entered as ground length, rounded to the nearest whole foot.

* The approximate runway center point is the mean of the latitudes and longitudes of a runway's ends.  This convention eliminates the need for complex geodetic formulas compute the precise runway center point, thus allowing simple and consistent ARP computations after only brief instructions.

A SAMPLE ARP COMPUTATION FOLLOWS [SEE FIGURE A2.1]:

APPROXIMATE RUNWAY CENTER PTS:

RWY 1/19 LAT = 39 24 57.7852 LON = 77 22 41.1951
RWY 5/23 LAT = 39 24 48.4806 LON = 77 22 34.9130
ARPLAT
 
 = 39 24 34.1979 + (4,000 FT(23.5873 SEC) + 3,799 FT(14.2827 SEC))/7,799 FT
= 39 24 34.1979 + 19.0549 SEC
= 39 24 53.3
ARPLON
 
 = 77 22 19.1959 + (4,000 FT(21.9992 SEC) + 3,799 FT(15.7171 SEC))/7,799 FT
= 77 22 19.1959 + 18.9391 SEC
= 77 22 38.1