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Theodolite Surveying

   Posted On :  21.08.2016 10:26 am

A theodolite is essentially a transit of high precision. Theodolites come in different sizes and weights and from different manufacturers.

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.

HORIZONTAL MOTION

 

 Located on the lower portion of the alidade, and adjacent to each other, are the horizontal motion clamp and tangent screw used for moving the theodolite in azimuth. Located on the horizontal circle casting is a horizontal circle clamp that fastens the circle to the alidade. When this horizontal (repeating) circle clamp is in the lever-down position, the horizontal circle turns with the telescope. With the circle clamp in the lever-up position, the circle is unclamped and the telescope turns independently. This combination permits use of the theodolite as a REPEATING INSTRUMENT. To use the theodolite as a DIRECTIONAL TYPE OF INSTRUMENT, you should use the circle clamp only to set the initial reading. You should set an initial reading of 0°30´ on the plates when a direct and reverse (D/R) pointing is required. This

will minimize the possibility of ending the D/R pointing with a negative value.

 

VERTICAL MOTION


.— Located on the standard opposite the vertical circle are the vertical motion clamp and tangent screw. The tangent screw is located on the lower left and at right angles to the clamp. The telescope can be rotated in the vertical plane completely around the axis (360°).

 

LEVELS.— The level vials on a theodolite are the circular, the plate, the vertical circle, and the telescope level. The CIRCULAR LEVEL is located on the tribrach of the instrument and is used to roughly level the instrument. The PLATE LEVEL, located between the two standards, is used for leveling the instrument in the horizontal plane. The VERTICAL CIRCLE LEVEL (vertical collimation) vial is often referred to as a split bubble. This level vial is completely built in, adjacent to the vertical circle, and viewed through a prism and 450

mirror system from the eyepiece end of the telescope. This results in the viewing of one-half of each end of the bubble at the same time. Leveling consists of bringing the two halves together into exact coincidence, as


Figure 11-12.—One-minute theodolite.




Figure 11-13.-Coincidence- type level.

 

shown in figure 11-13. The TELESCOPE LEVEL,  mounted below  the telescope, uses a

 

prism system and a 450 mirror for leveling operations. When the telescope is plunged to the reverse position, the level assembly is brought to the top.

 

TELESCOPE.— The telescope of a theodolite can be rotated around the horizontal axis for direct and reverse readings. It is a 28-power instrument with the shortest focusing distance of about 1.4 meters. The cross wires are focused by turning the eyepiece; the image, by turning the focusing ring. The reticle (fig. 11-14) has horizontal and vertical cross wires, a set of vertical and horizontal ticks (at a stadia ratio of 1:100), and a solar circle on the reticle for

 

making solar observations. This circle covers 31 min of arc and can be imposed on the sun’s image (32 min of arc) to make the pointing refer to the sun’scenter. One-half of the vertical

line is split for finer centering on small distant objects.




Figure 11-14.-Theodolite reticle.


The telescope of the theodolite is an inverted image type. Its cross wires can be illuminated by either sunlight reflected by mirrors or by battery source. The amount of illumination for the telescope can be adjusted by changing the position of the illumination mirror.

 

TRIBRACH.— The tribrach assembly (fig. 11-15), found on most makes and models, is a detachable part of the theodolite that contains the leveling screw, the circular level, and the optical plumbing device. A locking device holds the alidade and the tribrach together and permits interchanging of instruments without moving 

the tripod. In a "leapfrog" method, the instrument (alidade) is detached after observations are completed. It is then moved to the next station and another tribrach. This procedure reduces the amount of instrument setup time by half.

 

CIRCLES.— The theodolite circles are read through an optical microscope. The eyepiece is located to the right of the telescope in the direct position, and to the left, in the reverse. The microscope consists of a series of lenses and prisms that bring both the horizontal and the 



Figure 11-15.-Three-screw leveling head.

 

vertical circle images into a single field of view. In the DEGREE-GRADUATED SCALES (fig. 11-16), the images of both circles are shown as they would appear through the microscope of the 1-min theodolite. Both circles are graduated from 0° to 360° with an index graduation for each degree on the main scales. This scale’sgraduation appears to be superimposed over an auxiliary that is graduated in minutes to cover a span of 60 min (1°). The position of the degree mark on the auxiliary scale is used as an index to get a direct reading in degrees and minutes. If necessary, these scales can be interpolated to the nearest 0.2 min of arc.

 

The vertical circle reads 0° when the theodolite’stelescope is pointed at the zenith, and 180°

 

when it is pointed straight down. A level line reads 90° in the direct position and 2700 in the reverse. The values read from the vertical circle are referred to as ZENITH DISTANCES and not vertical angles. Figure 11-17 shows how these zenith distances can be converted into vertical angles.



Figure 11-16.-Degree-graduated scales.





Figure 11-17.-Converting zenith distances into vertical angles (degrees).

 

In the MIL-GRADUATED SCALES (fig. 11-18), the images of both circles are shown as they would appear through the reading micro-scope of the 0.2-mil theodolite. Both circles are graduated from 0 to 6,400 mils. The main scales are marked and numbered every 10 mils, with the


Figure 11-18.-Mil-graduated scales.



Figure 11-19.-Vertical angles from zenith distances (mils).

 

last zero dropped. The auxiliary scales are graduated from 0 to 10 roils in 0.2-mil increments. Readings on the auxiliary scale can be interpolated to 0.1 mil. The vertical circle reads 0 mil when the telescope is pointed at the zenith, and 3,200 mils when it is pointed straight down. A level line reads 1,600 roils in the direct position and 4,800 roils in the reverse. The values read are zenith distances. These zenith distances can be converted into vertical angles as shown in figure 11-1

The excavation of material in underwater areas is called dredging, and a dredge is an excavator afloat on a barge. A dredge may get itself into position by cross bearings, taken from the dredge on objects of known location on the beach, or by some other piloting method. Many times, however, dredges are positioned by survey triangulation. The method of determining direction angles from base line control points is the same as that just described.

 

 

 

 

LAND SURVEYING

 

Land surveying includes surveys for locating and monumenting the boundaries of a property; preparation of a legal description of the limits of a property and of the area included; preparation of a property map; resurveys to recover and remonument property corners; and surveys to subdivide property. It is sometimes necessary to retrace surveys of property lines, to reestablish lost or obliterated corners, and to make ties to property lines and corners; for example, a retracement survey of property lines may be required to assure that the military operation of quarry excavation does not encroach on adjacent property where excavation rights have not been obtained. Similarly, an access road from a public highway to the quarry site, if it crosses privately owned property, should be tied to the property lines that are crossed so that correctly executed easements can be obtained to cross the tracts of private property.

 

EAs may be required to accomplish property surveys at naval activities outside the continental limits of the United States for the construction of naval bases and the restoration of such properties to property owners. The essentials of land surveying as practiced in various countries are similar in principle. Although the principles pertaining to the surveys of public and private lands within the United States are not necessarily directly applicable to foreign countries, a knowledge of these principles will enable the EA to conduct the survey in a manner required by the property laws of the nation concerned.

 

In the United States, land surveying is a survey conducted for the purpose of ascertaining the correct boundaries of real estate property for legal purposes. In accordance with federal and states laws, the right and/or title to landed property in the United States can be transferred from one person to another only by means of a written document, commonly called a deed. To constitute a valid transfer, a deed must meet a considerable number of legal requirements, some of which vary in different states. In all the states, however, a deed must contain an accurate description of the boundaries of the property.

 

A right in real property need not be complete, outright ownership (called fee simple). There are numerous lesser rights, such as leasehold (right to occupancy and use for a specified term) or easement (right to make certain specified use of property belonging to someone else). But in any case, a valid transfer of any type of right in real property usually involves an accurate description of the boundaries of the property.

 

As mentioned previously, the EA may be required to perform various land surveys. As a survey team or crew leader, you should have a knowledge of the principles of land surveys in order to plan your work accordingly.

 

PROPERTY BOUNDARY DESCRIPTION

 

A parcel of land may be described by metes and bounds, by giving the coordinates of the property corners with reference to the plane coordinates system, by a deed reference to a description in a previously recorded deed, or by References to block and individual property numbers appearing on a recorded map.

 

By Metes and Bounds

 

When a tract of land is defined by giving the bearings and lengths of all boundaries, it is said to be described by metes and bounds. This is an age-old method of describing land that still forms the basis for the majority of deed descriptions in the eastern states of the United States and in many foreign lands. A good metes-and-bounds description starts at a point of beginning that should be monumented and referenced by ties or distances from well-established monuments or other reference points. The bearing and length of each side is given, in turn, around the tract to close back on the point of beginning. Bearing may be true or magnetic grid, preferably the former. When magnetic bearings are read, the declination of the needle and the date of the survey should be stated. The stakes or monuments placed at each corner should be described to aid in their recovery in the future. Ties from corner monuments to witness points (trees, poles, boulders, ledges, or other semipermanent or permanent objects) are always helpful in relocating corners, particularly where the corner markers themselves lack permanence. In timbered country, blazes on trees on or adjacent to a boundary line are most useful in reestablishing the line at a future date. It is also advisable to state the names of abutting property owners along the several sides of the tract being described. Many metes-and-bounds descriptions fail to include all of these particulars and are frequently very difficult to retrace or locate in relation to adjoining ownerships.

 

One of the reasons why the determination of boundaries in the United States is often difficult is that early surveyors often confined themselves to minimal description; that is, to a bare statement of the metes metesToday, good practice requires that a land surveyor include all relevant information in his description.

 

In preparing the description of a property, the surveyor should bear in mind that the description must clearly identify the location of the property and must give all necessary data from which the boundaries can be reestablished at any future date. The written description contains the greater part of the information shown on the plan. Usually both a description and a plan are prepared and, when the property is transferred, are recorded according to the laws of the county concerned. The metes-and-bounds description of the property shown in figure 10-34 is given below.

 

"All that certain tract or parcel of land and premises, hereinafter particularly described, situate, lying and being in the Township of Maplewood in the County of Essex and State of New Jersey and constituting lot 2 shown on the revised map of the Taylor property in said township as filed in the Essex County Hall of Records on March 18, 1944.

 

"Beginning at an iron pipe in the northwesterly line of Maplewood Avenue therein distant along same line four hundred and thirty-one feet and seventy- one-hundredths of a foot north-easterly from a stone monument at the northerly corner of Beach Place and Maplewood Avenue; thence running (1) North forty-four degrees thirty-one and one-half minutes West along land of. . ."

 

 

Another form of a lot description maybe presented as follows:

 

"Beginning at the northeasterly corner of the tract herein described; said corner being the intersection of the southerly line of Trenton Street and the westerly line of Ives Street; thence running S6°29´54´´E bounded easterly by said Ives Street, a distance of two hundred and twenty-seven one hundredths (200.27) feet to the northerly line of Wickenden Street; thence turning an interior angle of 89°59´16´´ and run-ning S83°39´50´´W bonded southerly by said Wickenden Street, a distance of one hundred and no one-hundredths (100.00) feet to a corner; thence turn-ing an interior angle of. . . ."

 

You will notice that in the above example, interior angles were added to the bearings of the boundary lines. This will be another help in retracing lines




Figure 10-34.—Lot plan by metes and bounds.

 

INTRO TO ANTIQUE SURVEY INSTRUMENTS

 

First, some basics about their composition and finish... most instruments were made of wood, brass, or aluminum, although you will find whole instruments or instrument parts made of iron, steel, ebony, ivory, celluloid, and plastic. It is important to remember that many surveying instruments were "needle" instruments and their magnetic needles would not seek north properly if there were local sources of interference, such as iron. The United States General Land Office issued instructions requiring brass Gunters chains to be used in close proximity to the magnetic needle. (They soon changed that requirement to steel brazed link chains; the brass chain could not stand up to the type of wear and tear a chain received.

 

In American surveying instruments, wood was common until about 1800; brass instruments were made approximately 1775 to 1975, and aluminum instruments from 1885 to the present.

 

The finish of instruments has changed. Early wooden instruments were generally unfinished and were usually made of tight grained woods which resisted water well. Early brass instruments were usually unfinished or polished and lacquered to retain the shine. In the mid-1800s American instrument makers began finishing brass instruments with dark finishes for two reasons: first, that the dark finish reduced glare and as a result reduced eyestrain, and secondly, that the dark finish helped to even out the heating of an instrument in the sunlight and as a result reduced collimation problems caused by the heating. Beware of being taken in by polished and lacquered brass instruments; prior to 1900 that may have been the original finish for the instrument, but after 1900 , bright brass finishes are usually not original finishes.

 

 

There are three kinds of surveying instruments that are rather unique to North American surveying. They are the compass, the chain and the transit. In addition, the engineer's or surveyor's level contributed very strongly to making the United States the leading industrial nation in the world by virtue of the highly efficient railroad systems it helped design in the mid 1800's. I take a great deal of satisfaction in pointing out that in this country it was the compass and chain that won the west, not the six-shooter!

 

The following is a list of antique surveying instruments and tools with a brief and basic description of how they were used.

 

ABNEY HAND LEVEL - Measures vertical angles.

 

ALIDADE - Used on a Plane Table to measure vertical and horizontal angles & distances.

 

ALTAAZIMUTH INSTRUMENT - Measures horizontal and vertical angles; for position "fixing".

 

ASTRONOMIC TRANSITS - Measures vertical angles of heavenly bodies; for determining geographic position.

 

BAROMETER, ANEROID - Measures elevations; used to determine vertical distance.

 

BASE-LINE BAR - Measures horizontal distances in triangulation and trilateration surveys.

 

BOX SEXTANT - Measures vertical angles to heavenly bodies.

 

CHRONOGRAPH - Measures time.

 

CHRONOMETER - Measures time.

 

CIRCUMFERENTER - Measures horizontal directions and angles.

 

CLINOMETER - Measures vertical angles.

 

COLLIMATOR - For adjusting and calibrating instruments.

 

COMPASSES - Determines magnetic directions; there are many kinds, including plane, vernier, solar, telescopic, box, trough, wet, dry, mariners, prismatic, pocket, etc.

 

CROSS, SURVEYORS - For laying out 90 and 45 degree angles.

 

CURRENT METER - Measures rate of water flow in streams and rivers.

 

DIAL, MINER'S - A theodolite adapted for underground surveying; measures directions as well as horizontal and vertical angles.

 

GONIOMETER - Measures horizontal and vertical angles.

 

GRADIOMETER - Also known as Gradiometer level, it measures slight inclines and level lines-of-sight.

 

HELIOGRAPH - Signalling device used in triangulation surveys.

 

HELIOSTAT - Also known as a heliotrope, it was used to make survey points visible at long distances, particularly in triangulation surveys.

 

HORIZON, ARTIFICIAL - Assists in establishing a level line of sight, or "horizon".

 

HYPSOMETER - Used to estimate elevations in mountainous areas by measuring the boiling points of liquids. This name was also given to an instrument which determined the heights of trees.

 

INCLINOMETER - Measures slopes and/or vertical angles.

 

LEVEL - Measures vertical distances (elevations). There are many kinds, including Cooke's, Cushing's, Gravatt. dumpy, hand or pocket, wye, architect's, builder's, combination, water, engineer's, etc.

 

LEVELLING ROD - A tool used in conjunction with a levelling instrument.

 

LEVELLING STAVES - Used in measuring vertical distances.

 

MINER'S COMPASS - Determines magnetic direction; also locates ore.

 

MINER'S PLUMMET - A "lighted" plumb bob, used in underground surveying.

 

MINING SURVEY LAMP - Used in underground surveying for vertical and horizontal alignment.

 

OCTANT - For measuring the angular relationship between two objects.

 

PEDOMETER - Measures paces for estimating distances.

 

PERAMBULATOR - A wheel for measuring horizontal distances.

 

PHOTO-THEODOLITE - Determines horizontal and vertical positions through the use of "controlled" photographs.

 

PLANE TABLE - A survey drafting board for map-making with an alidade.

 

PLUMB BOB - For alignment; hundreds of varieties and sizes.

 

PLUMMETS - Same as plumb bob.

 

QUADRANT - For measuring the angular relationship between two objects.

 

RANGE POLES - For vertical alignment and extending straight lines.

 

SEMICIRCUMFERENTER - Measures magnetic directions and horizontal angles.

 

SEXTANTS - Measures vertical angles; there are many kinds, including box, continuous arc, sounding, surveying, etc.

 

SIGNAL MIRRORS - For communicating over long distances; used in triangulation surveys.

 

STADIA BOARDS - For measuring distances; also known as stadia rods.

 

STADIMETER or STADIOMETER - For measuring distances.

 

TACHEOMETER - A form of theodolite that measures horizontal and vertical angles, as well as distances.

 

TAPES - For measuring distances; made of many materials, including steel, invar, linen, etc. Also made in many styles, varieties, lengths, and increments.

 

THEODOLITE - Measures horizontal and vertical angles. Its name is one of the most misused in surveying instrument nomenclature, and is used on instruments that not only measure angles, but also directions and distances. There are many kinds, including transit, direction, optical, solar, astronomic, etc.

 

TRANSIT - For measuring straight lines. Like the theodolite, the transit's name is often misused in defining surveying instruments. Most transits were made to measure horizontal and vertical angles and magnetic and true directions. There are many kinds, including astronomic, solar, optical, vernier, compass, etc.

 

WAYWISER - A wheel for measuring distances



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