Figure 6-3.—Two-peg test method.
the line of sight is not parallel to the axis of the level
tube, then you must adjust the line of sight. The method
used for adjustment is known as the two-peg test (fig.
6-3). This method requires you to do the following steps:
1. Setup and level the instrument (first setup, fig.
6-3). Drive stake (peg) A about 150 feet away, then drive
stake B at the same distance in the opposite direction.
2. Take a rod reading a on stake A and a rod reading
b on stake B. With the instrument exactly halfway
between the two stakes, b-a is the true difference in
elevation between the stakes.
3. Move the instrument close to stake A (second
setup, fig. 6-3) so that the eyepiece is within a half inch
from the rod. Then, by sighting through the
objective-lens end of the telescope, take a rod reading c
on stake A. Next, take a rod reading d on stake B in the
normal manner. If the instrument is in adjustment, d-c
will equal b-a.
4. If the instrument is out of adjustment, calculate
what the correct rod reading e should be on the farther
rod B (e = c + b - a). Set the rod reading e with a target
for accurate reading. Move the horizontal cross hair to
the correct reading (on target) by loosening the correct
vertical screw and tightening the opposite screw.
5. Check the horizontal cross hair adjustment
again. The ring may have rotated during this adjustment.
6. Rerun the peg test to check the adjustment.
NOTE: The compensation for the above maladjust-
ment is careful balancing of your backlights and
foresights.
Transit Adjustments
You must be capable of performing six commonly
performed tests and adjustments of the transit. All tests
and adjustments of the transit are made with the
instrument mounted on its tripod and setup in the shade.
You must make these tests periodically and in the
sequence in which they are discussed in the following
paragraphs. When one of the tests indicates that an
adjustment is necessary, you must make this adjustment
and then you must repeat all previous tests before
proceeding with the next test.
6-5
