US Army, Technical Manual, TM 5-6675-250-20P, SURVEY INSTRUMENT, AZIMUTH, GYRO, LIGHTWEIGHT, (LEAR-S

TM-5-6675-250-20P

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Alexa Actionable Analytics for the Web. AmazonGlobal Ship Orders Internationally. Amazon Inspire Digital Educational Resources. Amazon Rapids Fun stories for kids on the go. Since the control stations are usually distributed over com- paratively large areas, their relative positions are determined by topographic survey procedures. TM details the methods, techniques, and procedures used by the military topographic surveyor. The fundamental control sur- vey of the United States provides geographic posi- tions and plane coordinates of triangulation and traverse stations and the elevations of bench marks which are used as the basis for hydro- graphic surveys of the coastal waters, for the con- trol of the topographic survey of the United States, and for the control of many state, city and private surveys.

TM details the methods, techniques, and procedures used by the military geodetic surveyor in conducting basic control sur- veys. These surveys are con- ducted either with special equipment or for a spe- cial purpose. The celestial de- termination of! Separations are calculated by computing distances correspond- ing to measured angular displacements along the reference spheroid. TM details the methods, techniques, equipment, and procedures used for the establishment of astronomic latitude, longi- tude, and azimuths of second and higher order accuracy. These surveys are conducted by using an artificial earth satellite for long line surveys where the distance between sta- tions is from to miles.

They are con- ducted for worldwide surveys for interconti- nental, interdatum, and interisland geodetic ties. Astronomic, topographic, and basic control sur- veys are usually used in conjunction with satellite surveys. Special project instructions are written to detail methods, techniques, equipment and pro- cedures to be used in these surveys. A survey made in relation to any considerable body of water, such as a bay, harbor, lake, or river for the pur- poses of determination of channel depths for navi- gation, location of rocks, sand bars, lights, and bouys; and in the case of rivers, made for flood control, power development, navigation, water supply, and water storage.

TM details the methods, techniques, equipment, and procedures used for hydrographic surveys. Field inspection and identification of features which a map compiler is unable to delineate; iden- tification and delineation of political boundary lines, place names, road classifications, buildings hidden by trees, and so forth. Field classification may be included as part of the control survey effort and is normally completed prior to actual stereocompilation phase. Also, the process of com- paring aerial photographs with conditions as they exist on the ground, and of obtaining information to supplement or clarify that which is not readily discernible on the photographs themselves.

TM details the methods, techniques, and proce- dures used for field classification surveys. The collection of gravity survey data requires measurements of the intensity of the gravitation force at or near the earth's surface. While measurements on land are made at discrete points, those made in the air or over the ocean are recorded continuously during motion. The ultimate goal is to obtain a good density and distribution of gravity observations over the entire surface of earth.

Gravity observa- tions are divided, generally, into absolute and rel- ative measurements. The computation of relative gravity survey measurements is discussed in chapter 19, Change 1, TM The process of determin- ing boundaries and areas of tracts of land.

Construction > Topography

The term cadastral survey is sometimes used to desig- nate a land survey, but in this country its use should be restricted to the surveys of public lands of the United States ch. A survey exe- cuted for the purpose of obtaining information that is essential for planning an engineering pro- ject or development and estimating its cost. The information obtained may, in part, be recorded in the form of an engineering map. Geodetic and Plane Surveys The figure of the earth is not considered a sphere, but a spheroid an ellipsoid of revolution , flat- tened at the poles and bulging at the equator.

There are various determinations of the size and shape of the spheroid, which are normally differ- entiated by their degree of flattening. Different projections are based on these spheroids and each projection has unique characteristics and serve different purposes. The curvature of the earth's surface is very similar to that of a sphere.

Be- cause of this curvature, surveys are divided basi- cally into geodetic and plane surveys. The basic difference between geodetic and plane surveys can best be expressed in terms of established points: Both technical classifica- tions involve the same types of measurements, made by the same basic methods, and there are few basic differences in the field operation. In geodetic surveys, the stations are normally long distances apart and more precise instruments and procedures are re- quired, than those of plane surveys. These dis- tances or areas measured on the surface of the earth are not along straight lines or planes but on a curved surface, and allowances must be made for this in the computations.

To accomplish this, the earth's major and minor diameters are com- puted accurately, and from these, a spheroid of reference. The position of each geodetic station is related to this spheroid. These positions are ex- pressed as latitudes angular distance north or south of the equator , and longitudes angular distance east or west of the prime meridian , or as northings and eastings on a rectangular grid system which is correlated with latitude and lon- gitude TM The computation methods used in geodetic surveys are described in TM When the extent of the sur- vey becomes small less than sq km in area and when only limited accuracy is required, the effect of curvature can be ignored.

These surveys are treated as if the measurements were made on a plane and are known as plane surveys. Highway and railroad surveys, which may extend for hundreds of kilometers miles , are usually in a narrow strip and are considered as plane surveys. However, a limited computation for earth's curva- ture is necessary in this case. The computation methods used in plane surveys are described in TM , TM , and appropriate chapters of this manual.

Survey Networks Horizontal and vertical survey control within a country is usually established by a network of control arcs, which are all referenced to a single datum and are therefore related in position and elevation to each other, regardless of their dis- tance apart. In the United States these networks are referred to as basic, supplementary, and aux- iliary networks.

These are all referenced to the North American Datum for horizontal con- trol points and the sea level datum of for vertical control points. Within the continental United States the definitions listed below are used. Each country uses similar networks tailored to meet that country's need and terrain. The supplementary arcs are used to fill in the areas between the basic control arcs. The ultimate objective is to place stations of either the basic or supplementary net- work at intervals of between 6 to 16 kilometer apart.

Digitized by Google TM 2 The supplementary vertical control is es- tablished by second order differential leveling, within the basic control arcs to provide a planned line spacing of about 10 kilometer. Along these lines bench marks are placed about every 2 kilo- meters.

Operations Field work in surveying consists of making and recording measurements. The operations are in general, as follows: Measuring differences in elevations and de- termining elevations — 1 To establish elevation reference points, called bench marks. While the horizontal control networks are being established, usually vertical control is also carried throughout these networks by trigonometric level- ing.

The elevations established by this method is referred to as "trig levels" and is used for artil- lery control, construction and engineering sur- veys, and mapping projects. Record field notes to provide a permanent record of the field work in the form of — 1 Planetable sheets. Factors Affecting Field Work The surveyor in the field must constantly be alert to the different conditions he encounters and the requirements of the survey.

The weather, terrain, personnel, equipment, purpose, accuracy of the survey, systematic procedures, and the expected rate of progress are some of the factors which will affect the work. Physical factors such as weather and terrain will affect each field sur- vey in varying degrees. Measurements using tele- scopes can be stopped by fog, mist, or smog. Swamps and flood plains under high water will impede taping surveys. Lengths measured over open water or fields of flat, unbroken terrain can create ambiguities in measurements when using microwave equipment.

The longest length that can be determined by lightwave distances measure- ments is reduced in bright sunlight. Generally, Digitized by boogie TM reconnaissance will predetermine the conditions and alert the survey party to the best time and method to use and the rate of progress to be ex- pected. Personnel The status of training of the per- sonnel is another factor that affects field work. Experience in handling the instruments being used for a survey can shorten survey time without introducing excessive errors which would require resurvey. The personnel factor is variable and will have an affect on the rate of progress.

In addition to the reasons cited in a and 6 above the equipment used in the survey will have an affect on the rate of progress. Therefore, reliability of equipment must be taken into con- sideration when setting completion dates. The purpose and type of the survey are primary factors in determining the accuracy requirements.

First order triangulation, traverse, or leveling, which becomes the basic or "control" of future surveys, must be made to the required accuracy standards. At the other extreme, cuts and fills for a highway survey requires accuracies of much lower standards. In some surveys inac- cessible distances must be computed. The distance is computed by trigonometry using angles and dis- tances which can be measured.

Therefore these measurements in many cases must be made to a high degree of precision in order to maintain the accuracy in the computed distance. As stated in d above, the purpose of the survey will determine the accuracy require- ments. The required accuracy, in turn, will influ- ence the selection of instruments and procedures. For instance, comparatively rough procedures can be used in measuring for earthmoving, but grade and alinement of a highway must be much more precise and require more accurate measurements.

Each increase in precision also increases the time required to make the measurement, since greater care and more observations must be made.

Each survey measure- ment will be in error to the extent that no meas- urement is ever exact. The errors classified as sys- tematic and accidental are discussed in appendix D. Besides errors, survey measurements are sus- ceptible to mistakes or blunders. These arise from misunderstanding of the problem, poor judgment, confusion, or simply from an oversight. By work- ing out a systematic procedure, the surveyor will often detect a mistake when some operation seems out of place. The system will be an advantage in setting up, in making observations, in recording field notes, and in making computations.

Survey speed is not the result of hurrying ; it is the result of saving time through the following: Field Notes The field notes of the surveyor must contain a complete record of all measurements made during the survey, when necessary sketches, diagrams, and narrations should be made to clarify the notes.

The best field survey is of little value if the notes are not complete and clear. The field notes are the only record that is left after the survey party departs the field survey site. All field notes must be lettered neat and legible. The lettering should be in free hand, Gothic style as illustrated in TM All notes should be in black or blue-black ink, suitable for photographic copying.

However, in special cases a 3H or 4H pencil may be used. Numerals and decimal points should be legible and permit only one interpretation. Notes must be kept on the standard survey forms and not on scraps of paper for later transcription. The survey notes are usually kept in a field notebook. These notebooks are of two types: The following information must appear in each book no matter which type is used.

On each page containing the measurement notes, the head- ing must be filled out to include, station name, date, instrument man, recorder, instrument used Digitized by boogie TM and where pertinent the weather.

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The body will contain all pertinent measurement notes. Each page must have the instrument man's initials on the bottom to indicate that he has checked the page for errors or omissions. Field note recording takes three general forms; tabulation, sketches, and descriptions. The numerical measurements are recorded in columns according to a prescribed plan depending upon the instrument used, order of accuracy of the survey, and type of measure- ment.

Sketches add much to clarify field notes and should be used liberally. They may be drawn to scale or approximate scale or exag- gerated for clarity. A planetable sheet is an exam- ple of a sketch drawn to scale. A control card sketch should be drawn to approximate scale, and exaggerated when necessary to show important details needed for clarity. The measurements should be added directly on the sketch or keyed in some way to the tabular data.

U.S. Army Technical Manuals TM Series 5

A very important requirement of a sketch is legibility. Tabulations with or with- out added sketches can also be supplemented with descriptions. The description may only be one or two words to clarify the recorded measurements. It may also be a lengthy narration if it is to be used at some future date, possibly years later, to locate a survey monument.

These abbrevi- ations, signs and symbols must be in accordance with current directives, such as AR , FM , and FM If there is any doubt as to the meaning or interpretation of a symbol or ab- breviation the words must be spelled out. Individual numbers recorded incorrectly will be lined out by a single diagonal line and the correct values added above. The circumstance of the correction of all original figures should be explained in the remarks column, except for obvious mathematical errors.

No position will be voided or rejected in the field notes, except in the case of bumping the instru- ment or observing the wrong target, and then a note must be made in the remarks column stating the reason for void. Pages that are voided or re- jected must be referenced to a substitute page. The procedure for corrections is mandatory since the field notes are considered legal evidence. This waterproofing can be accomplished by spraying a thin coat of acrylic clear plastic on the field record. This spray may be applied before the re- cording and it will make the paper waterproof, and it may still be written on with ordinary writ- ing instruments.

The field notes can be sprayed again after the recording and the plastic then fixes the writing and prevents water damage to the rec- ords. One such spray is "Krylon, Workable Fixa- tif "; however, there are many other sprays on the market and any of them may be used. Hand and Voice Signals Members of a survey party will find themselves at some distance apart at times and have to commu- nicate with each other. During taping operations, this distance would be a tape length where voice communications are adequate. When making angle measurements, this distance may be several kilometers and require radio communication.

Be- tween these two extremes, many operations are at distances far enough apart where voice signals could not be heard and hand signals must be used. The hand signals have been standardized and are recommended for surveyors. The most common hand signals used are illus- trated in figure There are many other signals that may be used, but they must be agreed upon and understood by all members of the party.

Each signal is given while facing the person being sig- naled. A radio is used for voice communications on long line surveys and must be operated according to current directives. Only frequencies which are obtained through the local signal officer may be used. Operations Office work in surveying consists of converting the field measurements into a more usable form.

The conversion or computation may be required immediately to continue the work or it may be held until a series of field measurements is com- pleted before it can be compiled and adjusted. This is called office work even though some of the operations may be performed in the field during lapses between measurements to save time. Some office work requires special equipment such as slide rules, conversion tables, computing ma- chines, or drafting equipment. This equipment may not be available in the field and the work must be done in the office.

Working Up Field Notes. During survey op- erations, many field measurements require some form of arithmetical computation. It may be a simple addition of several full lengths and a par- tial tape length to record a total distance between two points. It may be adding or subtracting dif- ferences in elevation to determine height of in- strument or elevation during leveling, or it may be checking angles to see that the allowable error is not exceeded.

Since the process differs for each type of surveying operation, the method will be discussed under recording for each operation. Office computing converts dis- tances, angles, and rod readings into a more usa- ble form. The measurements may end up as a computed volume of dirt to be moved for a high- way cut or fill, an area of land needed for a con- struction project, or an adjusted position of some point or mark from which other measurements can be made.

For higher accuracy, corrections must be applied for temperature, tension, and sag of the tape. The desired result is the horizontal distance between points. In electronic distance measuring, the distance is almost always on a slope and has to be reduced to the equivalent horizontal distance. In many opera- tions, the measured angles are converted into directions of a line away from a north or south line. These directions are called bearings or azi- muths of the line.

If the direction is given as a bearing or azimuth, a trigonometric formula using sine or cosine of the angle, multiplied by the distance, will result in a coordinate difference between the two points. The computa- tions determine the grade and position of a road or railroad, and the volume of earth to be moved to prepare the bed.

Airfield and airstrip surveys are quite similar to road surveys in alining and grading the runways. Area computations are required at times after the positions of points and distances between them have been determined. This is per- formed in the office either by drafting the survey to scale and measuring graphically or by comput- ing the area by trigonometry or by arithmetic. Some survey techniques are not complete until one more procedure in computing, known as adjusting, is performed.

Adjusting is the determina- tion and application of corrections to the data being adjusted thereby making the data consistent within themselves and providing the most proba- ble value for the determination of points to some reference point. Small errors which are not ap- parent during individual measurements can accu- mulate to a sizeable amount. For example, assume that measurements were made to the nearest unit for the accuracy required. This requires esti- mating the nearest one-half unit during measure- ment.

Adjusting this means each measure- ment is reduced 0. Since the measure- ments were only read to the nearest unit, this adjustment would not be measurable at any point and the adjusted result would be correct. Some of the more precise surveys require least square ad- justments which are discussed in TM Traverse is the measurement of the lengths and directions of a series of lines between points on the earth, and is used to deter- mine the positions of the points. When finally computed, the accumulated closing error shows up as a position displacement and is distributed among the traverse points.

These sides Digitized by Google TM become the known distance values of adjoining triangles. Computing continues through a net until a new measured or previously determined base line is reached. The closure difference be- tween the measured base line distance and the distance computed through the triangles must be within prescribed limits. This amount of closure is then adjusted back through the net affecting the positions of the stations. Normally, elevations on a level line are computed in the field as the measure- ments are made. When the line is closed or tied to a known elevation, the difference in elevation be- tween the measured and known elevations is ad- justed over all the stations in the line.

Office computations reduce the field notes to tabular or graphic form for a per- manent record or for continuation of field work. Standard forms, printed on letter size sheets are available for many survey computations. The computer is not limited to the use of these forms and can improvise new ones when needed.

All computations pertaining to one survey are bound into a single file and referenced to the book and page number of the field book. Each page of the file also contains the names of the computer and the person who checked the computations. Usually, enough of the field notes are transcribed to the computation file so that further reference to field notebooks is unnecessary. Most mathematical problems can be solved by more than one method. In checking a set of computations, a method which differs from the original computation is used, if possible. An inverse solution, starting with the computed value and solving for the field data, is one possibility.

The slide rule, the planimeter, and the protractor are also used for approximate checking. A graphi- cal solution can be used, when feasible, especially if it takes less time than an arithmetical or loga- rithmic solution. Each step that cannot be checked by any other means must be recomputed. When an error or mistake is found, the computation should be rechecked before the correction is accepted.

Significant Figures The term significant figures refers to those digits in a number which have meaning, that is, whose values are definitely known to be correct. In a measured quantity, the number of significant figures is determined by the accuracy of the measurement. For example, a roughly measured distance of meters has three significant figures. More carefully measured, the same distance, If measured still more accurately, In surveying, the significant fig- ures should reflect the allowable error or tolerance in the measurements. For example, suppose a measurement of We can say that The number could vary from The fifth digit in this measurement is meaningless.

The number has only four significant figures and should be written as such. The significant figures in a number ending in one or more zeros are unknown unless more information is given. The zeros may have been added to show the location of the decimal point. If the number is written Using decimals, the significant figure is not always the number of digits.

A zero may or may not be significant depending on its position with respect to the decimal and the dig- its. As mentioned in c above, zeros may have been added to show the position of the decimal point. Some examples are given below. The values are car- ried out to one more digit than required in the result. The number is rounded off to the required number of digits as a final step. Rounding off Numbers Rounding off is the process of dropping one or more digits and replacing them with zeros, if nec- essary, to explain the correct number of signifi- cant figures.

Numbers are rounded off according to the following rules: Digitized by boogie TM a. When the digit to be dropped is less than 5, the number is written without the digit or any others that follow it.

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When the digit is equal to 5, the nearest even number is substituted for the preceding digit. When the digit to be dropped is greater than 5, the preceding digit is increased by one. Dropped digits to the left of the decimal points are replaced by zeros. Dropped digits to the right of the decimal point are never replaced. Introduction Surveying instruments are the devices with which measurements are made. Many of these devices have similar features. Other features which are unique to only one or two instruments will be discussed in fol- lowing chapters that deal with the individual in- strument.

NOTE Where the word "crosshair" or "cross- wire" is used in reference to the reticle of a telescope, it must be understood that these may be actual spider hairs, or they may be lines engraved on glass. The make and type of instrument is the con- trolling factor on what is used. There- fore these words will be used inter- changeably within this manual when re- ferring to the reticle of the different tel- escopes.

The appropriate maintenance manual should be consulted for exact no- menclature. Tripod The tripod is the base or foundation which sup- ports the survey instrument and keeps it stable during observations. It consists of a tripod head to which the instrument is attached, three wooden or metal legs which are hinged at the head, and metal pointed shoes on each leg to press or anchor into the ground to achieve a firm setup. The tripod head may have screw threads on which the instrument is mounted directly ; it may have a screw projecting upward through the plate ; or it may have a hole or slot through which a special bolt is inserted to attach the instrument.

Two types of tripods are furnished to surveyors — the fixed leg tripod and the extension leg tripod. A leg of the fixed leg tripod consists of two lengths of wood as a unit or one solid piece of wood, attached to a tripod head with a hinge and fitted with metal tipped shoes. At points along the length, the two piece legs are perpendicularly braced to give great stability. A leg of the exten- sion leg tripod is made of two sections which slide up and down. The lower section slides into or out of the upper section, enabling the surveyor to ad- just the height of tripod as desired.

On rough terrain, the legs can be adjusted to different lengths to establish a horizontal tripod head or to set the instrument at the most comfortable work- ing height. The fixed legs must be swung in or out in varying amounts to level the head. Therefore the instruments height is not as easily controlled with the fixed legs as with the extension legs, and the observer must learn the correct spread of the fixed legs to get the desired observing height. It is recommended that the extension leg tripod be used when the surveying is to be done over rough terrain.

When mounting the survey instrument on the tripod it must be gripped firmly to avoid dropping. The transit is held by the standard opposite the vertical circle while it is being mounted. The engineer level is held at the center of the telescope and usually theodolites and precise levels are gripped near the base of the instrument.

The instruments must be screwed down to a firm bearing, but not so tightly that they will bind or that the screw threads will strip. Setting the Tripod The tripod legs must be properly placed to achieve a stable setup. The legs must also be anchored by firmly embedding, tying together, or plastering down. Digitized by Google TM a. Loosen the restraining strap from around the legs and secure it around one leg. An effective way to set the tripod down is to grip it with two of the legs close to the body while standing over the point where the setup is re- quired.

Lower the tripod until this third leg is on the ground. Place one hand on each of the first two legs and spread them while taking a short backward step, using the third leg as a support point. When the two legs look about as far away from the mark as the third one, lower the two legs and press them into the ground, if possible. Any slight adjustment to level the head further is made by moving the third leg in or out before embedding.

On exten- sion leg tripods, this final adjustment can be made by sliding the proper leg up or down. On smooth or slippery paved or rock surfaces, the tripod leg hinges should be tightened while setting up to prevent the legs from spreading and causing the tripod to fall. Use should be made of holes or cracks in the ground to brace the tripod. As a safety factor in some cases, the three legs should be tied together at the proper distances, braced with rocks or cemented in place with plaster of paris, after they are set to keep them in the proper position.

When setting up on steeply sloping surfaces, the third leg is placed uphill and at a greater distance from the mark. The other two legs are set downhill as a above. Before re- leasing the downhill legs they should be checked for stability to see that the weight of the instru- ment on the tripod head will not overbalance and cause the tripod to slip or fall. Care of Tripod When setting the legs in the ground, care must be taken to apply pressure longitudinally.

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Pressure across the leg can crack the wooden pieces. The hinged joint should be adjusted para and not overtightened to cause strain on the joint, or to strip or lock the metal threads. The machined tripod head must be kept covered when not in use, and should not be scratched or burred by mishan- dling. The head cover should be attached to the tripod leg when not in use.

Any damage to it can be transferred to the tripod head. Mud, clay, or sand adhering to the tripod must be removed and the tripod wiped with a damp cloth and dried. The metal parts should be coated with a light film of oil and wiped with a clean cloth. Foreign matter can get into the hinged joints or on the machined surfaces and cause wear.

Stability is the tripod's greatest asset. Instability, wear, or damaged bear- ing surfaces on the tripod can evolve into unex- plainable errors in the final survey results. Level Vial Many surveying instruments are equipped with one or more level vials to indicate the horizontal plane, and to assist in keeping parts of the instru- ment horizontal and others vertical while the in- strument is in use. This is very important since it defines the vertical axis and horizontal plane at the instrument from which measurements are made.

A level vial is a glass tube with the inside surface along the long axis ground as an arc of a circle. The vial is filled almost com- pletely with liquid and sealed at both ends. The liquid is normally either alcohol or ether which do not freeze at low temperatures. The level vial is sometimes referred to as a spirit level. When the vial is held horizontally, the remaining unfilled space takes the form of an air bubble and seeks the highest part of the arc. The vial is usually marked with graduations in both directions from the center or from one end to the other.

The bub- ble is centered in the middle of the vial of a well adjusted and leveled instrument fig. The vial is usually mounted in a protective metal hous- ing that is attached to the instrument with screws to permit its removal. The level vial is adjusted by Bubble off-center Bubble centered Figure The sensitivity of a bubble is a measure of the amount it moves for a given incli- nation, and depends on the construction of the vial.

The longer the radius of the arc in the vial, the flatter the curve and the greater the sensitiv- ity, since a smaller vertical movement of the end of the level vial will start the bubble moving. The markings on the vial are not standardized on all instruments and sensitivity is described by the radius of the bubble arc.

This radius is about 6 meters for a transit plate level, about 8 meters for the alidade levels, about 21 meters for the engi- neer level, and about meters for precise levels. The high sensitivity vials are used on the instru- ments for precise work. High sensitivity vials on less precise instruments wastes time in centering and the quality of the work is not necessarily improved. The exact amount of bubble movement measured in divisions of the vial to unit of arc is determined by calibration.

The level vials on very precise instruments are extremely sensitive and bringing the bubble into exact center would be time con- suming. The vial, therefore, is graduated so that the inclination may be read and a correction com- puted. This level correction is determined by cali- bration of the level vial in seconds of arc per graduation. There are many methods used in cali- brating level vials and these methods and compu- tations are covered thoroughly in chapter 3, TM When a bubble vial must be re- placed, the new vial can be replaced as follows: The bubble may not be cen- tered because the surface which was selected is out of level by the amount that the bubble is off center.

Circular Level Some instruments have a circular level vial in ad- dition to the plate level. The sensitivity is usually lower than the long vial bubbles, but it is suffi- cient for quick or approximate leveling. Leveling Heads Many surveying instruments have an assembly called a leveling head which supports the instru- ment on the tripod and is used to level the instru- ments. Two types of heads are in use; the four- screw and three-screw type.

This refers to the number of leveling screws used to level the instru- ment. A cross section is shown in figure A ball-and-socket joint, consisting of a hem- ispherical piece C and a socket in the footplate D , forms a flexible connection. The leveling screws E bear on the footplate and are threaded into arms which are part of the socket. Thus, by manipulating the screws in pairs, the axis of the spindle can be made vertical as indicated by the level vial F. The lower end of the screw may bear on the footplate as in the four-screw type, be held against the foot- plate by a starplate, or have a tribrach arrange- ment fig.

The tribrach is an assembly which consists of the leveling screws, the optical plumbing assembly and circular level, and is de- tachable from the instrument head. This latter combination permits interchangeability of instru- ments without moving the tripod. After observa- tions are completed, the instrument is detached at its base and moved to the next location to another tribrach, while the tripod and tribrach remain at their set position.

The next instrument or target is then snapped into place, checked for level, and is ready for its operation. Leveling the Instrument The instruments are leveled in a prescribed se- quence. This depends on whether there is a four Figure t After the instrument is firmly attached to the tri- pod, it is leveled as follows: Four Leveling-Screw and One Level. With these screws, tilt the instrument until the level bubble is approximately centered in the vial.

Note that the level bubble moves in the same direction as the left thumb. The two screws are turned in opposite directions at the same rate, keeping a slight pressure between the leveling screw feet and the footplate. Once again, bring the level bub- ble to the approximate center.

If the bubble remains at center, the instrument is leveled along this axil. Manipulating the leveling screws. Estimated Delivery within business days. Estimated delivery dates - opens in a new window or tab include seller's handling time, origin Postal Code, destination Postal Code and time of acceptance and will depend on postage service selected and receipt of cleared payment - opens in a new window or tab. Delivery times may vary, especially during peak periods. International shipping and import charges paid to Pitney Bowes Inc.

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