MIL-PRF-89201B
a.
The gisement of those charts oriented north-south shall be
determined for the mid-latitude of each of its bounding meridians.
b.
On skewed charts the point of determination shall be the
intersection of the parallel and meridian, to the nearest minute,
approximately at the physical center of the chart.
3.10.11.1 Computation of gisement. The computation of the gisement
shall be accurate to the nearest minute of arc, and shall be obtained by using
the following formula:
Grid Declination = DkSIN(v}
Departure from grid zone central meridian expressed in minutes,
multiplied by the sine of (latitude), gives the grid declination in minutes of
arc. The grid declination is east or west depending on whether the point is
to the east or west of the central meridian of the grid zone. When a zone
junction occurs within the chart limits, grid declination will be computed at
the zone junction, in addition to the grid declinations at the east and west
edges of the chart.
Computation of example grid declination
TOP LATITUDE
3141'00"
MINUS BOTTOM LAT
3119'00"
DIFFERENCE
22'
DIFFERENCE/2
11'
DIFFERENCE/2 +
BOTTOM LAT = LAT MIDPOINT
3130'00"
DEPARTURE
West Edge
East Edge
of Chart
of Chart
CENTRAL MERIDIAN
13500'
13500'
MINUS LON OF CHART EDGE
13340'
13420'
= DEPARTURE
120'
40'
DEPARTURE IN MINUTES
80'
40'
SIN(LAT.) MIDPOINT
x
DEPARTURE (IN MINUTES)
=
DECLINATION (MINUTES)
SIN of 3130'00" (Midpoint Lat.)
0.522498
0.522498
MULTIPLIED BY DEPARTURE
80'
40'
EAST OR WEST OF TRUE NORTH
(41.79)
(20.89)
NORTH (CENTRAL MERIDIAN)
42'W of TRUE N.
21'W of TRUE N.
The computation data page for each chart will be reviewed and inserted
as part of the chart record.
3.10.11.2 Gisement caution. The following caution will be shown in
place of the note specified in 3.10.10.1)(content of the note is variable).
The declination note is the same color as the grid.
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