Page 61 - Moving the Earth_ The Workbook of Excavation

SURVEYS AND MEASUREMENTS

SURVEYS AND MEASUREMENTS 2.11

FIGURE 2.11 Rod readings.

reading is taken the first time the instrument is used, and again each time it is set up. These read-
ings will probably all be different, but in each case the elevation of the instrument may be found
by adding the rod reading when the rod is sitting on the benchmark to the benchmark elevation.
Since the benchmark is the most permanent point observed, it is good practice to assign an ele-
vation to it, and to calculate all other elevations from that. An assigned number should be large
enough that no elevation less than zero will be found on the job, as working with minus figures
may cause confusion and error.
If levels have been taken previously in the area, engineers’ benchmarks may be found, in which
case it is wise to use them. If possible, the elevation assigned to them in the previous survey
should be used to facilitate comparison between the two sets of levels.
Even if engineers’ benchmarks cannot be used directly in surveying the job, it may be advan-
tageous to run a level to one, and note its elevation in relation to the contractor’s own benchmark,
so that the two systems can be compared if necessary.
If the job is a type that will involve frequent checks of levels, as on a road where stakes may
be knocked out by machinery, it is a good plan to set up benchmarks so that one will be visible
from each point where the instrument will be used. This saves time in taking grades on a few
stakes and eliminates common errors in moving the instrument, or in taking an elevation from the
wrong line stake, or a stake which has been disturbed.
All benchmarks should be figured very carefully and rechecked at least once.
Recording Readings. Another requirement in recording observations is to identify the spot at
which each reading is taken. This is usually done by taking readings at set intervals, such as 10,
or 50, or 100 feet. These distances should be marked by stakes, pegs, small rock cairns, or in other
ways. The first stake or mark of the series is called the zero stake, and the others are identified by
their distances from it. It is customary to give distances in units of hundreds, followed by a plus
sign and the other figures of the distance. The zero is written 0 0, the 50-foot mark 0 50, and
the 100-foot mark as 1 0. If any points on the line are needed which are not in the series, the
distance is measured and entered in the notes with the reading, as 0 35.
Important ground features along the centerline, such as crests of rises, bottoms of dips, or
beginnings of rock outcrops, should be taken in addition to the stake readings.
Elevations may be taken from the ground, from the top of the stake, or more rarely, from a
mark on a stake. If taken from the ground, it should be stamped or cut flat. Such readings are not
as accurate as those taken from the top of a stake, and may be very difficult to check back, but
they can be used directly in preparing profiles and figuring cut and fill. If the top of the stake is
used, it is necessary to measure the stake height.

Tapes. Measuring is usually done with a steel tape, often called a chain by surveyors. Fifty-foot
and 100-foot lengths are standard, and will suffice for most purposes. They should have a non-
rusting finish, as it is often difficult to dry and oil them immediately after wet work. Care should
be taken not to kink a tape, or to bend it sharply, as such abuse may break it.