Doesn't have to be North, you use at least 3 known points from a declination station.
DECLINATION CONSTANT
Since the magnetic needle of an aiming circle does not point to the grid north determined from a map, it is necessary to correct for this difference by using the declination constant. The declination constant of an instrument is the clockwise angle between grid north and magnetic north; that is, the grid azimuth of magnetic north. This constant differs slightly for different instruments and must be recorded on each instrument. The constant also varies for the same instrument in different localities.
To determine the declination constant, proceed as follows:
a. Declination Station. Declination stations are established by corps artillery, division artillery, and artillery battalion survey teams to determine the declination constants of instruments and to correct for local attractions, annual variations, and instrument errors. When a unit moves from one locality to another, a station should be established where all instruments are declinated. If the declination constants for all instruments of a unit are determined at the same station, grid azimuths measured with each instrument will agree with the map grid, and all instruments will agree with each other. The point chosen for the declination station must have a view of at least two distant, well-defined points with a known grid azimuth. Two additional points are desirable, one in each quadrant, as a check.
b. Procedure for Declinating Aiming Circle at a Declination Station. Where a declination station is available, the procedure for declinating the aiming circle is as follows:
STEP 1. Set up and fine-level the aiming circle directly over the declination station marker using the plumb bob. STEP 2. Place the grid azimuth of the first azimuth marker on the scales using the recording motion. Place the vertical cross line of the telescope on the azimuth marker using the nonrecording (orienting) motion. The aiming circle is now oriented on grid north.
STEP 3. With the recording motion, rotate the instrument to zero. Release the magnetic needle and look through the magnifier. Center the north- seeking needle using the recording motion, then relock the magnetic needle.
STEP 4. Notice the new azimuth on the scale, which is the declination constant--record it.
STEP 5. Recheck the aiming circle level and repeat steps 2 through 4 using the remaining azimuth markers until three readings have been taken. If there is only one marker, repeat the entire procedure twice using the same marker.
Not as hard as it sounds
This would be similar to what surveyors use, right?
So after all this, how does one obtain aiming coordinates?