Page 74 - Moving the Earth_ The Workbook of Excavation

SURVEYS AND MEASUREMENTS

2.24 THE WORK

This is done by turning setscrews at the top and bottom of the frame which hold the hair. The
screws are unlocked by twisting one or the other a quarter or half turn, after which both are turned
in the same direction.
If no pond is available, or a more accurate adjustment is required, two stakes should be driven
firmly into the ground, 200 to 400 feet apart, and at almost the same level. The instrument is set
up halfway between them, as in Fig. 2.21(A), and leveled carefully.
A leveling rod is held on each of the outer stakes, and an exact reading taken according to the
crosshair. These readings will be accurate with reference to each other, as any error in crosshair
height is canceled in observations taken at equal distance.
The stake standing on lower ground is assigned an elevation of zero, and has the higher rod
reading. The elevation of stake (b) is the difference between the two readings.
The instrument is now set up in line with the two stakes as in (B), about 10 feet beyond stake
(b). A reading is taken of (b). Then a reading is taken at (a), which should equal the elevation of
stake (b) plus the reading there.
If it does not, adjustment is made in the manner described above.
Vertical Hair. The vertical cross or direction hair may be checked by driving three stakes exactly
in line at 200-foot intervals. The line may be determined by sighting with the vertical hair, and the
distances measured by stadia or tape. The stakes should be about on the same level.

FIGURE 2.21 Checking the crosshair.