Page 87 - Moving the Earth_ The Workbook of Excavation

SURVEYS AND MEASUREMENTS

SURVEYS AND MEASUREMENTS 2.37

positions for grading. The grader operator uses the control box to enter the desired elevation and
slope that TS5 will maintain.
When the machine begins grading, information from each of the four sensors is sent to the con-
trol box where comparisons are made to the desired grade entered by the operator. If necessary,
the control box sends out correction signals to the hydraulic valves to adjust the blade to the
required grade. The TS5 actually performs the measurements and corrections more than 30 times
a second so that the grader can move at a relatively high speed.
The Leica Geosystems GPS base station receives time and place information from the GPS
satellites in the sky. The base station, which is standard, modem survey equipment, communicates
via radio modem to all machines. The precise position is computed on the machine relative to the
base station and is then combined with slope and other sensors to determine the exact position and
heading of the grader’s blade.
By comparing these values to the project data stored in the computer on board the grader, cor-
rections for height and cross slope are transmitted to the SonicMaster blade controller, also on
board, freeing the operator to concentrate on steering. Only one or two passes are required to get
to final grade. And the result provides accuracy of +/− 0.10 feet (+/− 3 cm) in height for rough
grading. A more refined Grade Star TPS solution provides accuracy to +/− 0.02 feet (+/− 5 mm)
in height and +/− 0.04 feet (+/− 10mm) for position.
Distance from the roving grader is constrained by the strength of the radio link, from 1,000 feet
up to six miles.

DISTANCE METERS

Several battery-powered instruments are available that can read the distance to a target by mea-
suring reflection time of multiple-phase signals.
The short-range Cubitape uses a modulated infrared light source, for distances up to 6,000 feet,
with differences in range depending on the type of target and atmospheric conditions. The target
is located by means of a built-in telescope.
The distance is automatically displayed in digits for feet (or meters) and their decimal frac-
tions. See Fig. 2.34A.
The Electrotape (Fig. 2.34B) uses a microwave beam for distances of 100 feet to 30 miles. Two
identical instruments are used, with built-in short wave communication between them for the
operators. The readings are taken first by one unit, then by the other, as an accuracy check.
These instruments measure direct line-of-sight (slope) distances. If there is a difference in ele-
vation between the points, the horizontal distance must be calculated.
Correction for the curve of the earth’s surface is needed if elevations are required. An approx-
imate formula, good up to distances of a few hundred miles, is
(distance in miles) 2
Correction
8,000
Figure 2.35 is a table for various distances. The correction is subtracted from the apparent ele-
vation indicated by the instrument. It will be noted that the curvature is very slight within ordi-
nary survey distances, but must be considered in long sights.

MINOR INSTRUMENTS

Hand Level. Rough levels may also be run with hand levels, such as the one shown in Fig.
2.36A. This consists of a sighting tube, in the top of which is a small spirit level parallel with the
line of sight. A slanted mirror reflects the spirit level so that it is seen vertically beside the field
of view. The object glass is marked with a centerline, and may have two or more stadia lines.