An antique surveyor's transit
A transit or theodolite is an instrument used to measure distances and angles in surveying. Measurement of horizontal direction -- also known as azimuth or bearing -- can be done with respect to either magnetic north, true north or a local north direction. True north is the direction of the spin axis of the earth; magnetic north is the direction that the north needle of a compass points and local north is a north direction defined on a map or other reference. Does this Spark an idea?
Instructions
Setup
1. Set up and level the transit.
2. Determine the amount and direction of declination of magnetic north at your location. Declination varies by 20 degrees or more across the United States alone, so it's critical to know the magnetic declination in your vicinity.
3. Decide if the declination needs to be added to or subtracted from the true north bearing. West declinations should be subtracted from 360 degrees, and east declinations should be added to zero degrees (the azimuth of true north).
Transit with Magnetic Compass
4. Set the azimuth to the value you determined in Section 1.
5. Loosen the base of the transit and unlock the compass.
6. Use the coarse adjustment screw to orient the zero degree mark of the base so it is pointing nearly north according to the compass.
7. Use the fine adjustment screw to orient the zero degree mark of the base so it reads exactly zero degrees.
8. Lock the compass and tighten the base of the theodolite.
Transit Without Magnetic Compass
9. If the transit is not equipped with a magnetic compass, you'll have to use a handheld magnetic compass to determine the bearing to a distant object. Correct it for magnetic declination, and then align the transit base.
10. Locate a suitable reference point. The more distant it is from the transit base, the better. A tree on a distant hill or a far-away landmark (like a radio tower) are good choices.
11. Use a compass to determine the magnetic bearing to the reference object.
12. Add the east declination to or subtract the west declination from the magnetic bearing to the reference object to obtain the corrected bearing direction.
13. Loosen the transit base. Rotate the transit base until the azimuth ring indicates the corrected bearing direction when the telescope is centered on the reference object.
14. Tighten the transit base. The true north direction will then be at the zero degree mark on the azimuth ring.
Tags: north direction, transit base, degree mark, magnetic north, reference object
