BASIC SURVEYING INSTRUMENtsMost fieldwork done by an Engineering Aid(especially at the third- and second-class levels)is likely to consist of field measurements and/orcomputations that involve plane surveying ofordinary precision. This section describes the basicinstruments, tools, and other equipment used forthis type of surveying. Other instruments used formore precise surveys will also be described briefly.Surveying instruments come in various forms,yet their basic functions are similar; that is, they are all used for measuring unknown angles anddistances and/or for laying off known angles anddistances.

MAGNETIC COMPASS. A magnetic compass is a device consisting principally of a circular compass card, usually graduated in degrees, and a magnetic needle, mounted and free to rotate on a pivot located at the center of the card. The needle, when free from any local attraction (caused by metal), lines itself up with the local magnetic meridian as a result of the attraction of the earth’s magnetic North Pole.

The magnetic compass is the most commonlyused and simplest instrument for measuringdirections and angles in the field. This instrumenthas a variety of both civilian and militaryapplications. The LENSATIC COMPASS (available inyour Table of Allowance) is most commonly used forSEABEE compass courses, for map orientation, andfor angle direction during mortar and field artilleryfires.In addition to this type of compass, there areseveral others used exclusively for field surveys. The ENGINEER’S TRANSIT COMPASS, located betweenthe standards on the upper plate, is graduated from0° through 360° for measuring azimuths, and inquadrants of 90° for measuring bearings (fig. 11-4). Notice in figure 11-4 that the east and west markingsare reversed. This permits direct reading of themagnetic direction.The compass shown in figure 11-5 is commonlycalled the BRUNTON POCKET TRANSIT. Thisinstrument is a combination compass and clinometer.It can be mounted on a light tripod or staff, or it maybe cradled in the palm of the hand.Other types of compasses can also be found insome surveying instruments, such as the theodoliteand plane table.

Figure 11-4.-Engineer’s transit compass. (left)

Figure 11-5.-A Brunton pocket transit. (right)

ENGINEER’S TRANSIT A primary survey fieldwork consists of measuring horizontal and vertical angles or directions and extending straight lines. The instruments that can perform these functions have additional refinements (built-in) that can be used for other survey operations, such as leveling. Two types of instruments that fall into this category are the engineer’s transit and the theodolite. In recent years, manufacturing improvements have permitted con- struction of direct-reading theodolites that are soon to replace the vernier-reading transits. However, in most SEABEE construction, the engineer’s transit is still the major surveying instrument.

The transit (fig. left) is often called the universal survey instrument because of its uses. It may be used for measuring horizontal angles and directions, vertical angles, and differences in elevations; for prolonging straight lines; and for measuring distances by stadia. Although transits of various manufacturers differ in appearance, they are alike in their essential parts and operations. The engineer’s transit contains several hundred parts. For-descriptive purposes, these parts may be grouped into three assemblies: the leveling head assembly, the lower plate assembly, and the upper many plate or alidade assembly (fig. 11-7).

THEODOLITE A theodolite is essentially a transit of high precision. Theodolites come in different sizes and weights and from different manufacturers. Although theodolites may differ in appearance, they are basically alike in their essential parts and operation. Some of the models currently available for use in

the military are WILD (Herrbrugg), BRUNSON, K&E, (Keuffel & Esser), and PATH theodolites. To give you an idea of how a theodolite differs from a transit, we will discuss some of the most commonly used theodolites in the U.S. Armed Forces. One-Minute Theodolite The 1-min directional theodolite is essentially a directional type of instrument. This type of instrument can be used, however, to observe horizontal and vertical angles, as a transit does. The theodolite shown in figure 11-12 is a compact, lightweight, dustproof, optical reading instrument. The scales read directly to the nearest minute or 0.2 mil and are illuminated by either natural or artificial light. The main or essential parts of this type of theodolite are discussed in the next several paragraphs.

(fig left) one minute theodolite

SURVEYING TAPES Tapes are used in surveying to measure horizontal, vertical, and slope distances. They may be made of a ribbon or a band of steel, an alloy of steel, cloth reinforced with metal, or synthetic materials. Tapes are issued in various lengths and widths and graduated in a variety of ways. Metallic Tapes A metallic tape is made of high-grade synthetic material with strong metallic. strands (bronze- brass-copper wire) woven in the warped face of the tape and coated with a tough plastic for durability. Standard lengths are 50 and 100 ft. Some are graduated in feet and inches to the nearest one-fourth in. Others are graduated in feet and decimals of a foot to the nearest 0.05 ft. Metallic tapes are generally used for rough measurements, such as cross-sectional work, road-work slope staking, side shots in topographic surveys, and many others in the same category. Nonmetallic tapes woven from synthetic yarn, such as nylon, and coated with plastic are available; some surveyors prefer to use tapes of this type. Nonmetallic tapes are of special value to power and utility field personnel, especially when they are working in the vicinity of high- voltage circuits.

Steel Tapes For direct linear measurements of ordinary or more accurate precision, a steel tape is required. The most commonly used length is 100 ft, but tapes are also available in 50-, 200-, 300-, and 500-ft lengths. All tapes except the 500-ft

one are band-types, the common band widths being 1/4 and 5/16 in. The 500-ft tape is usually a flat-wire type. Most steel tapes are graduated in feet and decimals of feet, but some are graduated in feet and inches, meters, Gunter’s links, and chains or other linear units. From now on, when we discuss a tape, we will be talking about one that is graduated in feet and decimals of a foot unless we state otherwise. Some tapes called engineer’s or direct reading tapes are graduated throughout in subdivisions of each foot. The tape most commonly used, however, is the so-called chain tape, on which only the first foot at the zero end of the tape is graduated in subdivisions, the main body of the tape being graduated only at every 1-ft mark. A steel tape is sometimes

equipped with a reel on which the tape can be wound. A tape can be, and often is, detached from the reel, however, for more convenient use in taping. Various types of surveying tapes are shown in figure 11-36. View A shows a metallic tape; view B, a steel tape on an open reel; view C, a steel tape or, a closed reel. View D shows a special type of low-expansion steel tape used in high-order work; it is generally called an Invar tape or Lovar tape.

Invar Tapes Nickel-steel alloy tapes, known as Invar, Nilvar, or Lovar, have a coefficient of thermal Figure 11-36.-Surveying tapes. expansion of about one-tenth to one-thirtieth (as low as 0.0000002 per 10F) that of steel. These tapes are used primarily in high-precision taping. These tapes must be handled in exactly the same manner as other precise surveying instruments. The alloy metal is relatively soft and can be easily broken or kinked if mishandled. Ordinarily, Invar tapes should not be used when a steel tape can give the desired accuracy under the same operating conditions. Invar tapes are used for very precise measurements, such as those for base lines and in city work. When not in use, the tape should be stored in a reel, as shown in figure 11-36, view D. Except for special locations where the ground surface is hard and flat, such as roadways or railroads beds, the Invar tape is used over special supports or stools and is not permitted to touch the ground.

PLUMB BOBA plumb bob is a pointed, tapered brass or bronze weight that is suspended from a cord for the general purpose of determining the plumb line from a point on the ground. Common weights for plumb bobs are 6, 8, 10, 12, 14, 16, 18, and 24 OZ; the 12- and the 16-oz are the most popular. Typical plumb bobs are shown in figure 11-40 . A plumb bob is a precision instrument and must be cared for as such. If the tip becomes bent, the cord from which the bob is suspended will not occupy the true plumb line over the point indicated by the tip. A plumb bob usually has a detachable tip, as shown in figure 11-40, so if the tip becomes damaged, it can be renewed without replacing the entire instrument. Each survey party member should be equipped with a leather sheath, and the bob should be placed in the sheath whenever it is not in use. The cord from a plumb bob can be made more conspicuous for observation purposes by the attachment of an oval form aluminum target (fig. 11-41, view A). The oval target has reinforced edges, and the face is enameled in quadrants alternately with red and white. Also, a flat rectangular plastic target may be used (fig. 11-41, view B). It has rounded corners with alternate red and white quadrants on its face. These plumb bob string targets are pocket size with approximate dimensions of 2 by 4 in.

Top- Plumb bob, cord, and target.Bottom- types

LEVEL RODA leveling rod, in essence, is a tape supported vertically and is used to measure the vertical distance (difference in elevation) between a line of sight and a required point above or below it. This point may be a permanent elevation (bench mark), or it may be some natural or constructed surface.The leveling rod may be read directly by the instrument man sighting through the telescope, or it may be target-read. Conditions that hinder direct reading, such as poor visibility, long sights, and partially obstructed sights, as through brush or leaves, sometimes make it necessary to use targets. The target is also used to mark a rod reading when numerous points are set to the same elevation from one instrument setup

PANTOGRAPH An instrument for copying plans, maps,

and other drawings, on the same, or on a reduced or an enlarged, scale.

For the accurate transferring, reducing and enlarging of drawings, plans, patterns, templates, pictures, trade signs etc. Bar 500 mm long, made of anodized light alloy, complete in case with extensive accessories.

MARKING PINMade of 3/16-inch spring steel, the 14-inch pins are powder-painted red and white, alternating at each inch. There are eleven pins to a set.Taping pins. A taping pin is a steel pin pointed on one end with a ring at the other. Use taping pins for marking measured tape lengths on the ground. This will help determine the number of tape lengths measured since the last station. The ring and the upper part of the pin are painted red. The rest of the pin is white. Taping pins are issued in sets of 11 pins each. Properly used, taping pins will prevent a "missed" or "dropped" tape length, a common mistake in distance taping.

SEXTANTA sextant is an instrument used to measure the angle between any two visible objects. Its primary use is to determine the angle between a celestial object and the horizon which is known as the altitude. Making this measurement is known as sighting the object, shooting the object, or taking a sight. The angle, and the time when it was measured, can be used to calculate a position line on a nautical or aeronautical chart. A common use of the sextant is to sight the sun at noon to find one's latitude. See celestial navigation for more discussion. Since the sextant can be used to measure the angle between any two objects, it can be held horizontally to measure the angle between any two landmarks which will allow for calculation of a position on a chart. A sextant can also be used to measure the Lunar distance between the moon and another celestial object (e.g., star, planet) in order to determine Greenwich time.

PLANIMETERA planimeter is a measuring instrument used to determine the area of an arbitrary two-dimensional shape.