Page 73 - Moving the Earth_ The Workbook of Excavation

SURVEYS AND MEASUREMENTS

SURVEYS AND MEASUREMENTS 2.23

Placing of contours therefore cannot be an entirely mechanical operation. In the first place, the
person making the field notes must understand topography sufficiently to take extra readings
where necessary to show up special forms. The valley at the top of this map would not have been
shown at all by readings on the grid points only.
The person drawing the contours should have a feeling for landscape forms, to avoid misinter-
preting the data from which she or he works.
Topographic Map. A topographic map is usually a contour map on which both natural and artifi-
cial features are indicated. The U.S. Geological Survey, Washington, D.C., has small-scale maps
covering most areas of the country that show contours, vegetation, roads, trails, lakes, streams,
and much other information. These maps are sold by mail and in bookstores at nominal prices.
They are handy for many purposes and essential for some jobs.
Profiles and cross sections may be taken from a contour map by laying a ruler across it, mea-
suring the interval between contour lines, and posting distances and elevations to cross-section
paper. The proposed new grades can then be drawn in, and the differences calculated.
The topographic map has a wide variety of uses, including locating highways, haul roads, borrow
pits, and dump areas; studies of drainage and stream flow; and estimation of quantities in cuts and fills.

INSTRUMENT ADJUSTMENTS

Surveying instruments are delicate and are easily put out of adjustment by failure of parts, care-
less handling, or accidents. It is often not possible to have them checked or repaired locally; the
return to the factory may mean loss of use for weeks or months.
It is therefore desirable that a person using an instrument be familiar with some adjustments
that can be made in the field, without special skill. These include setting of telescope spirit level,
and the horizontal and vertical crosshairs.
If these are properly set and the instrument is inaccurate, shop service is probably necessary.

Spirit Level. The telescope spirit level is usually fastened by a pair of vertical bolts. A single
nut holds it in a fixed position at one end, and a pair of nuts, one above and one below, permit
moving it up and down on the other end.
This level can be checked each time the instrument is set up. When the turntable is level, the
telescope should be able to swing in a full circle without changing the position of the bubble. If
no turntable screw adjustment will permit this, the level is presumed to be at fault.
To adjust, the turntable is leveled as accurately as possible and the bubble centered. The telescope
is swung a half circle, causing the bubble to shift. The bubble is brought one-quarter of the way back
to center by the adjusting nuts, and the rest of the way by using the turntable leveling screws.
The telescope is then swung to its original position, the bubble moved one-quarter of the way
to center by adjustment, and centered by the leveling screws. This process is repeated until swing-
ing the telescope does not affect the bubble.
Crosshair. If the horizontal crosshair is not exactly centered, all readings on the rod will be too
high or too low. Readings taken at about equal distances will agree. Greatest errors will be found
on long sights.
A reasonably accurate check and adjustment of this hair can be made with the help of a still
pond. Two stakes are driven flush with the water surface, about 100 feet apart. The instrument is
set in line with them, 10 feet beyond one.
A rod is set on the near stake and a reading taken. This is assumed to be accurate, because the
distance is too short for a perceptible error. The target is locked to the rod at this reading, or a note
made of it.
The rod is set on the far stake. If the hair is correctly adjusted, the reading should be the same.
If it is not, the hair should be raised or lowered until it agrees.