Page 91 - Moving the Earth_ The Workbook of Excavation

SURVEYS AND MEASUREMENTS

SURVEYS AND MEASUREMENTS 2.41

FIGURE 2.37 Using a string level.

FIGURE 2.38 Figuring string heights on banked curve. (Courtesy of CMI Corp.)

Carpenters’ Level. A carpenters’ level may be used to level a string, although not as conve-
niently. A string leveled by a carpenters’ level may be used for direct adjustment of a string level.
Altimeter. Altimeters or barometers which are small enough to be carried, and sensitive enough
to react to small changes in elevation, may be used for taking preliminary levels.
The most convenient altimeters are mountaineers’ pocket models, the size of a large watch.
Engineers’ models, which are rather rare, have a glass for magnifying the scale, for fine reading.
An altimeter is set for correct or assumed altitude when work with it is started. As it is carried
up- or downhill, the hands will point to higher or lower altitudes on the scale, and notes may be
made of the reading whenever desired. It is advisable to tap the instrument before each reading.
Altimeters provide the quickest and easiest means of finding heights and depths in rugged and
overgrown country. They are not accurate enough to be used in setting grades, except in very
experienced hands.

STRINGLINE

A stringline is a string or wire that indicates location, grade, or both location and grade. Its use in
construction is widespread and very old. A familiar example is string stretched between batter
boards for reference in laying foundations and pipes.
A stringline may serve as a visual guide for a machine operator to prevent him or her from running
off-line between widely spaced stakes. If the machine is equipped with a pointer or a suspended plumb
bob on the frame of its grading blade, ditching wheel, or other grade-making tool, it can be adjusted so
that following the string accurately with it ensures working to proper alignment and grade.
The principal and increasing use of stringlines has been in automatic control of grading, exca-
vating, and paving machines. These controls, which are described in Chap. 19, are usually made
up of a tracer or wand that stays in contact with the string, and a sensor that reacts to any change
in tracer position by actuating hydraulic valves to restore proper relationship with the string.