AMRC 2012 MODULE 2 Measurement of Distance and Elevation Equipment and Procedures CONTENTS Overview... 2-1 Objectives... 2-1 Procedures... 2-1 2.1 Introduction to Methods of Distance Measurement... 2-3 2.2 Types of Linear Measurement... 2-5 2.2.1 Pacing... 2-5 2.2.2 Odometer or Distance Measuring Wheel... 2-5 2.2.3 Electronic Distance Measuring Devices... 2-6 2.2.4 Stadia and Tacheometry... 2-6 2.2.5 Chaining or Taping... 2-6 2.3 Instrumentation... 2-7 2.3.1 Microwave... 2-7 2.3.2 Electro-optical... 2-7 2.4 Reflectors... 2-11 2.5 EDMs and the Total Station Concept... 2-13 2.6 Introduction to Levelling... 2-19
2.7 Types of Levels... 2-21 2.7.1 Dumpy Level... 2-21 2.7.2 Tilting Level... 2-22 2.7.3 Automatic Level... 2-23 2.7.4 Level Tripods... 2-24 2.8 Basic Level Operation... 2-27 2.8.1 Preparing and Moving the Level... 2-27 2.8.2 Setting up the Level... 2-27 2.8.3 Levelling the Instrument... 2-27 2.8.4 Focussing the Cross Hairs... 2-29 2.9 Levelling Rods... 2-31 2.9.1 Rod Levels... 2-32 2.9.2 Waving or Rocking the Levelling Rod... 2-33 2.9.3 Instrument Accuracy and Adjustment... 2-34 2.10 Common Sources of Levelling Error... 2-35 2.11 Self-Test... 2-37 ii
Module 2 Measurement of Distance and Elevation Equipment and Procedures Overview The measurement of distance, one of the fundamental components of field to survey work, can be performed in a variety of ways utilizing different equipment. This module introduces the types of equipment used in distance measurement and their levels of accuracy. The determination of elevation of existing features and the establishing of elevations for construction purposes is essential to development and construction surveying. In this module, the concept of levelling (or elevation determination) is introduced, as are the types of levelling instruments, their uses, and levels of accuracy. Procedures for reading survey rods are discussed, as are the related calculations. Objectives Upon completion of this module, the student will be able to: state and define the types of linear measurements made and the types of equipment used in the measurement of distance and length state and define the general principles of electronic distance measurement state and define the terms used in levelling, including horizontal plane, elevation, datum state and define the basic concepts of differential levelling state and define the basic types of equipment used in levelling and elevation measurement. Procedures Study the module materials and make notes as required. Perform the self-test on these principles and review the course materials in such a manner as to be able to successfully complete similar questions upon examination. AMRC 2012 2-1
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SECTION 2.1 Introduction to Methods of Distance Measurement Measurement of distances, also referred to as linear measurement, is one of the most important functions of surveying. It is necessary to know the type and the proper use of the equipment used for field measurements. The accuracy of any survey work is primarily dependent upon the accuracy of distance measurements carried out in the field. The office staff must have accurate and reliable field measurements in order to prepare the plans and to design the proposed facilities. Likewise, the field staff must have accurate and reliable design information in order to lay out the proposed facilities for construction. It is important to know that most property and legal plans, as well as construction and other development plans and documents, are based on horizontal distances and measurements. If a property plan, for example, shows a rectangular property with dimensions of 25 metres by 100 metres, these are the horizontal dimensions of the property, regardless of the topography and actual distances if measured on a slope. Field survey measurements may be taken on a slope, but must generally be corrected to horizontal distances. AMRC 2012 2-3
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SECTION 2.2 Types of Linear Measurement The methods utilized to measure distances will depend on the accuracy required and the type of equipment that is available. 2.2.1 Pacing Approximate distances can be determined in the field if you know what your pace length, or length of stride, is. The easiest way to determine your pace is to walk at a comfortable pace over a known distance of say 100 metres. Repeat this exercise several times and then divide your average number of paces per 100 m into 100. If, for example, your average number of paces was 106, then: 100 m 100 = = 0.94 m pace length average paces for 100 m 106 Knowing your pace length is particularly useful when trying to locate survey markers or doing a rough check on construction layout. Keep in mind when determining field measurements or distances by pacing, that the length of your pace may be affected by terrain slope or by difficult travel on muddy, overgrown, or snow-covered surfaces. If you are pacing out the boundaries of a sloping property from distances and dimensions shown on property plans, remember that the dimensions indicated on the plans are horizontal. Not only will your actual pace length be affected by trudging uphill or downhill, but the slope distance relative to the horizontal distance must be taken into account. The usefulness of pacing is; therefore, limited to rough measurements and field orientation. 2.2.2 Odometer or Distance Measuring Wheel An odometer consists of wheel of known circumference and a revolution counter or distance meter. The operator proceeds in the line of direction to be measured after recording the initial counter reading. The final reading less the initial reading equals the number of wheel revolutions. When multiplied by the known circumference of the wheel an approximate distance can be determined. Odometers are limited in use to relatively smooth, even surfaces and are very useful for determining approximate asphalt surface areas, curb lengths, etc. As with pacing, the odometer measures the actual distance on the terrain surface, and measured distances may need to AMRC 2012 2-5
be corrected to horizontal distance if it is necessary to eliminate slope factors. 2.2.3 Electronic Distance Measuring Devices Electronic Distance Measuring Devices (EDM) use light or microwave lengths which are emitted on a target and reflected back to the EDM. The time length for wave travel is determined and converted to distance travelled. 2.2.4 Stadia and Tacheometry Stadia and Tacheometry involve the use of survey instruments and level rods. Distances are determined using basic trigonometric functions and calculations. It is seldom used in modern surveys. 2.2.5 Chaining or Taping The surveyor s chain or steel tape is the fundamental instrument used in surveying and the importance of good chainage procedure cannot be overemphasized. The accuracy of any survey depends almost completely on the accuracy with which the chaining is carried out. The term chaining is used to designate the process of measuring distances with a variety of chains and tapes. The term originated when land surveyors used a chain composed of links of steel wire connected by small rings. For this course, only EDMs will be discussed to any length. 2-6 AMRC 2012
SECTION 2.3 Instrumentation The EDM instruments that are used in control surveys and mapping can be classified according to the type of electromagnetic radiation that carries the measuring (pattern or modulation) frequency. That is, they are classified according to the carrier frequency. There are two classes, microwave and electro-optical. 2.3.1 Microwave Instruments of this group radiate a microwave carrier, upon which are superimposed the pattern or modulation frequencies, from a master instrument to a remote. From the remote the signal is reradiated back to the master where phase comparison is made. 2.3.2 Electro-optical This group uses visible and near infrared radiation to carry the pattern frequency. They employ a visible helium-neon (He-Ne) laser of frequency λ = 0.63 m or gallium-arsenide (Ga-As) diodes that produce invisible radiation of wavelength 0.9 μm. One instrument employs a Xenon flash tube that produces a mixture of wavelengths with a mean wavelength of 0.43 μm. Older instruments used tungsten or mercury arc lamps and are now nearly obsolete having been replaced by laser and infrared instruments. Infrared instruments are characterized by weak radiation and thus are restricted to short range, maximum 1 to 3 kilometres, whereas lasers can be used on distances up to 30 kilometres. Whether microwaves or light carriers are used the fundamental principle is the same for both. The phase of the wave returned to the master is compared with the phase of the wave transmitted from the master and the phase difference, measured in each of several pattern or measuring frequencies, gives a measure of the distance. Generally, the shorter the carrier wavelength the more accurate the distance is determined. Microwave instruments are, therefore, in general, less accurate than electro-optical instruments but their range is greater. Some are capable of measuring up to 100 kilometres and can be used to measure through adverse atmospheric conditions such as a haze and fog. Electro-optical instruments can only be used in good visibility conditions. As previously mentioned, microwave systems use an instrument at each end of the line, that is, a master and remote instrument that are essentially the same. Each instrument has the dual capability of acting as either master or remote. In fact, measurements are quite often taken in one direction and then the roles of the instruments changed; the master becomes the remote and the remote becomes the AMRC 2012 2-7
master, and the distance is again measured. The two measurements are then averaged. Both machines require an operator. The EDMs using a light carrier measure from a master instrument to a passive reflector. This type of instrument has a greater application than the microwave type in engineering and construction surveys. A helium laser or a light-emitting diode is amplitude modulated by a precise crystal-controlled source. The modulated light beam is directed onto a retro-reflector positioned at the point to which the measurement is made. Figure 2.1 Phase Measurement Principle The reflected light beam from the retro-reflector returns to the instrument (Figure 2.1) where it is converted to an electric signal. The phase relationship between the transmitted and reflected wave beams of light are compared by the self-contained CPU in the unit and converted to a direct readout of the distance. The EDM measures the slope distance between the instrument centre and the prism centre. Provided they are both accurately centred and levelled over their respective survey points, and with proper allowance being made for any difference in tripod heights, the instrument will measure the slope distance between the two survey points. 2-8 AMRC 2012
Onboard CPUs convert the slope distance and vertical angle to horizontal distance and vertical distance. If the instrument height and prism height can be entered into the instrument, a direct elevation of the point can be determined in the field. If the coordinates of the instrument point and the orientation of the instrument are also known, then coordinates of each shot can be calculated and displayed by the instrument. These instruments are known as total stations. The instruments and prisms (reflectors) are so arranged that even when tilted at steep angles their optical centres are always maintained about their tilting axes. Unbalanced arrangements do sometimes occur and if this cannot be avoided a correction may have to be applied. AMRC 2012 2-9
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SECTION 2.4 Reflectors A plane mirror is not a suitable means of returning a light beam from the reflector station to the EDM instrument because of the difficulty of obtaining proper alignment. Today, reflectors are retrodirective prisms which have been cut from the corner of an optically flat, solid, glass cube (Figure 2.2). The corner prism presents three planes perpendicular to one another; the light striking one plane will be reflected from the other two planes before leaving the prism. Modern reflectors are assembled with the corner glass prisms individually cut, ground, polished, and measured. Figure 2.2 Reflector Formed from Corner of a Cube Reflecting prisms have also been manufactured from plastic. They can be used successfully at relatively short distances in conjunction with visible laser light sources but are not recommended for use with infrared light sources. AMRC 2012 2-11
Figure 2.3 Light Path in a Corner Reflector EDM instruments use the known speed of light in their computations. However, light travels more slowly through glass than through air and this fact must be taken into consideration. In addition, the prism mirror point, as mounted within the reflector housing, may not be directly on the plumb line so a further compensation is required. The result is that the reflector constant will vary with reflectors of different designs. Manufacturers usually furnish a constant which has a combined value for their own reflectors and instruments. To determine the prism constant for a particular EDM and prism combination, set three points on a straight line and label them A, B, and C. When measuring with an EDM, the prism constant is applied to all measurements. Measure the distance AC, AB, and BC. Since AB + BC = AC, the prism constant = AC AB BC. 2-12 AMRC 2012
SECTION 2.5 EDMs and the Total Station Concept Recent advances in the use of EDMs include automation of the EDM instruments in conjunction with theodolites used. With the two instruments combined into one unit and attached to a built-in or added-on computer data recorder, the need for booking of vertical and horizontal readings in field notes is eliminated. Figure 2.4(a) AMRC 2012 2-13
Figure 2.4(b) Figure 2.4(c) 2-14 AMRC 2012
The electronic field book or data recorder is actually a user programmable computer and using the appropriate interfaces field data can be transferred to the office computer and office computer calculations, i.e., initial coordinates of field stations, can be transmitted to the EDM via telephone interconnection. Many other data, computation, and programming capabilities exist depending on the equipment in use. Figures 2.5(a) (d) show typical data acquisition and data handling equipment. Figure 2.5(a) AMRC 2012 2-15
Figure 2.5(b) Figure 2.5(c) 2-16 AMRC 2012
Figure 2.5(d) With these advances in the uses of electronics it is now possible to proceed from initial field measurements for vertical and horizontal control to finished computer-plotted plans without any hard written or booked data or manual computations. This capability does not preclude the requirement for operator knowledge of instrument use, booking, computational, or plotting as this is essential in determining the reasonableness of result of the finished product. AMRC 2012 2-17
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SECTION 2.6 Introduction to Levelling The determination of existing elevations of the natural ground or of man-made features is an important part of the planning of any construction or development work. Likewise, the ability to establish design elevations in the field for construction purposes is essential to carrying out construction plans. This module covers the principles of levelling, the types and general operation of levelling instruments, levelling procedures, booking of level notes, and the field establishment of design elevations. Establishing the relative difference in height between two or more objects can be accomplished by the use of a survey levelling instrument, usually simply called a level, and a levelling rod. Maintaining the horizontal plane of equal elevation is the essential function of any survey level. The level, in its simplest form, is essentially a telescope with a bubble levelling system. The bubble ensures that the line of sight is maintained in a longitudinal plane with all points intersected by the horizontal cross hairs of the instrument falling at the same elevation, while the telescope permits accurate sighting of the rods at variable distances. The principle of differential levelling is based on the understanding that the instrument, once levelled, establishes a horizontal line of sight. When it is turned through a circle, it will describe a horizontal plane, for all intents and purposes perpendicular to the earth s radius. Therefore, all points intersected by this plane will have the same height. Thus, by employing a level rod and taking the readings on various points in turn, their heights or elevations relative to each other can be determined. By projecting a horizontal plane of sight, the relative differences in elevation are determined by rod readings. This is illustrated in Figure 2.6. A level set up at point A sites a rod positioned at a point of known elevation. This known elevation is termed a bench mark or datum elevation. For consistency of work at various locations, a fixed datum elevation of mean sea level is often used. If a small project or preliminary work is being carried out, then an assumed datum which is convenient is utilized. This would also apply where one structure must be built in relative elevation to another regardless of other fixed datum. In Figure 2.6 point B is known to be at an elevation of 100 metres. A backsight rod reading is always the first rod reading taken on a AMRC 2012 2-19
point that has a known elevation in this example, point B. The level reads a backsight of 2.6 m; therefore, the horizontal plane of the line of sight (LOS) of the instrument is established as 102.6 m. A foresight rod reading is always taken on a point that has an unknown elevation in this example, point C. If the rod is moved to point C and the level sighted on the rod, maintaining its horizontal plane elevation of 102.6 metres, the foresight rod reading when subtracted from horizontal plane elevation, will determine the elevation of point C. In this case a foresight rod reading of 0.5 metres at C subtracted from the horizontal plane elevation of 102.6 metres results in determination of point C s elevation as 102.1 m, or 2.1 metres above point B. Figure 2.6 Differential Levelling 2-20 AMRC 2012
SECTION 2.7 Types of Levels Levels vary widely in their design and construction, depending on their purpose and the precision required, but they can generally be divided into the following classifications: dumpy level tilting level automatic level hand levels. Most instruments are manufactured in Europe and Asia with Germany and Japan as the principal suppliers. The dumpy, tilting, and automatic levels are of the type generally used on construction surveys and are the only ones to which the following portion of the text pertains. Tripods for level instruments are also discussed in this section. 2.7.1 Dumpy Level The dumpy level was very popular at one time, but is not used too often these days. This type was designed for accuracy, reliability, permanence of adjustments, and low cost. It also contained a minimum of parts and was used for grade levelling when fast operation was not a controlling factor. It consisted of a telescope, with adjustable cross hairs, mounted on a horizontal bar by means of two vertical supports. The bar rotated in a horizontal plane on a vertical spindle connected to a levelling head and base plate. The rotary motion of the bar was controlled by a clamp and tangent screw. A spirit level was fastened to the horizontal bar and could be adjusted in a vertical plane. Figure 2.7 Dumpy Level AMRC 2012 2-21
2.7.2 Tilting Level The tilting level (see Figures 2.8 and 2.9) is noted for its light weight and compactness, and for the speed and accuracy that can be attained with it. Its numerous parts and higher cost are its major disadvantages. It is less rigidly constructed than the dumpy level and has to be handled more carefully. The telescope on this type of level has non-adjustable cross hairs. The telescope hinges vertically on a centre pivot connected to a horizontal bar. This bar has a circular level attached to it, and the bar turns in a horizontal plane on a vertical spindle which is connected to a three levelling screw base. The horizontal motion is controlled by a clamp and tangent screw. Figure 2.8 Tilting Level (side view) Figure 2.9 Tilting Level (end view) 2-22 AMRC 2012
A level vial is connected to the side of the telescope and has adjusting screws for both horizontal and vertical adjustment. By means of a prismatic reading device the level bubble is made to appear to be split longitudinally and transversely. When the two halves coincide, see Figure 2.10, the bubble is centred in the vial. The bubble is observed through a viewing tube placed on the left side of the telescope. The bubble is centred by raising or lowering one end of the telescope. This accomplishment is made by turning a micrometer or tilting screw placed on the horizontal bar below the eyepiece of the telescope. This type of level varies in construction, with some having four screw levelling bases and others a mirror device for observing the telescope level bubble. Figure 2.10 Split Bubble Images 2.7.3 Automatic Level The automatic level has a built-in gravity-referenced prism compensation which automatically maintains the horizontal line of sight of the instrument when the instrument has been levelled. Automatic levels are equipped with a circular spirit or bull s-eye level and after the bubble has been centred by use of the levelling foot screws, the compensator takes over to maintain the horizontal line of sight in any direction, even if the telescope is slightly tilted. AMRC 2012 2-23
Figure 2.11 Automatic Level Many of today s modern levels have either internal or external horizontal circle reading capability with which approximate angles can be estimated. However, the user must be careful to check the equipment and the horizontal circle graduations carefully some levels have the horizontal circle divided into 400 gons, rather than 360. In order to function properly, the compensation mechanism must be able to move freely. As some of the components are suspended by fine wires, the operation can be checked by tapping the end of the telescope while looking through the instrument. If the compensator is functioning, the line of sight will veer from the horizontal when tapped and the cross hairs will appear to deflect momentarily before returning to the original rod reading. The automatic compensator should be checked constantly during operation to avoid errors in the levelling procedure. 2.7.4 Level Tripods The tripods used to support the level are produced by various instrument manufacturers and some tripods are interchangeable between level types depending on the tripod plate and type of plate clamp screw. See Figures 2.12(a) and 2.12(b). 2-24 AMRC 2012
The basic differences relate to construction material, i.e., wood, metal, aluminum, etc.; the leg construction, either rigid or adjustable; and the plate and plate clamp screw arrangement. A basic tripod and the component features are shown in Figures 2.12(a) and 2.12(b). Figure 2.12(a) Basic Tripod Figure 2.12(b) Domed Head AMRC 2012 2-25
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SECTION 2.8 Basic Level Operation 2.8.1 Preparing and Moving the Level At the start of a levelling operation the level is generally removed from its protective case and mounted on the tripod at the location of the survey crews transportation. Levels may not commence at this point but mounting the level properly will prevent accidental damage to the level. Carefully remove the level from its case using both hands and holding the instrument by the levelling head; screw it firmly onto the tripod head. When moving the level from one setup location to another, ensure that the telescope is free to turn on its axis in case it is accidentally bumped. The level can then be carried over the shoulder in open country. The level should be carried under the arm with the instrument head forward when travelling through heavy bush or on a sidehill to prevent impact or damage to the instrument. 2.8.2 Setting up the Level When setting up the level, first spread the legs sufficiently to ensure steadiness and have the head approximately level. Remember that the level must have a firm base to obtain accurate readings. Press the legs far enough into the ground to give the tripod firm support (use your heel if there are lugs or spurs on the legs). If the setup is on a sidehill place two legs downhill and one leg uphill. When the level is to be set up on soft ground, stakes can be firmly set in the ground and the tripod placed on them. You can check for stability by shifting the weight of your body from one foot to the other while watching the level bubble for movement. When the level is to be set up on soft pavement (i.e., in summer temperatures) place tripod legs on boards or plywood. This will prevent the legs from sinking into the pavement. 2.8.3 Levelling the Instrument After the initial setup of the tripod and mounting of the level, the instrument itself must be levelled prior to commencing any rod readings. The type of adjustments required will be dependent on the instrument and tripod combination being used. Instruments with a bull s-eye bubble and domed tripod head are generally Automatic Levels. Levelling the instrument is completed by rotating or sliding the instrument on the dome until the bull s-eye bubble is centred, then tightening the plate clamp screw. If the AMRC 2012 2-27
instrument is an automatic level, readings can be taken after checking to see that the compensator mechanism is functioning (see Section 2.7.3). Many of the Three Screw Tilting instruments have a bull s-eye level which should be centred prior to fine adjustment to level with the tilting screw (bringing split bubble into coincidence). Centring the bull s-eye bubble will generally bring the instrument within the range of the tilting screw adjustment. Three Screw instruments are levelled by turning the instrument so that the plate level is aligned with two of the adjusting screws (see Figure 2.13). The bubble is then centred by rotating Screws 1 and 2 in opposite directions at the same time. Once level in this direction, the instrument is turned 90 degrees so that the plate level is aligned with the third screw (Screw 3), which is then adjusted until the bubble is again centred. The instrument is rotated 90 degrees again to align with Screws 1 and 2, and the bubble re-centred if it has strayed. The steps are repeated until the plate level remains level in all directions. Figure 2.13 When levelling a Four Screw Instrument, the instrument is turned to align with opposing pairs of screws, which are then rotated in opposite directions until the plate level remains level in all directions. It is important to rotate opposing screw pairs in opposite directions, and to keep an even tension on the screws. If they are too loose, the telescope and levelling head will tilt back and forth. If they are too tight, deformation of the parts can occur, or undue strain will be placed on the metal. 2-28 AMRC 2012
Figure 2.14 Adjusting Levelling Screws Figure 2.15 Following these procedures will bring the instrument to a level position, ready for rod readings in any direction. These procedures, together with the instrument procedures described later in this module, will result in accurate rod readings if the instrument is in adjustment and the rod is read correctly. 2.8.4 Focussing the Cross Hairs The proper use of the telescope requires that the sighting cross hairs, which are used for rod readings, must be pre-focussed to the user s eye. All levels have adjustable focussing of cross hairs and this is the first step of level use after the instrument is set up and prior to taking any rod readings. AMRC 2012 2-29
Improper focal length adjustment of the cross hairs results in a fuzzy or double cross-hair image and makes accurate rod readings impossible. This is known as parallax. Turning the eyepiece focussing ring on the eyepiece will bring the cross hairs into sharp focus. This adjustment must be checked during the day s operations; the operator s eye strength may vary during the day. Figure 2.16 Typical Level Cross Hairs 2-30 AMRC 2012
SECTION 2.9 Levelling Rods Levelling rods are produced by a variety of manufacturers and although the principle is the same, the markings on the rod faces vary widely. Levelling rods are usually 3 to 4 metres long and made in either two or three sliding or folding sections, which when extended were held in place by clamps. The New York rod is a two piece rod, graduated into hundredths of feet by fine lines. These are too fine to be read at any great distance and, hence, have to be used at close range if accuracy is desired (see Figure 2.17). The most popular Imperial levelling rod was the Philadelphia type, with a white face and black graduations 0.01 feet wide and spaced 0.01 feet apart. The tenths of a foot were marked by black numbers and the foot marks by red numbers (see Figure 2.17). The metric levelling rod is graduated into metres, decimetres, and centimetres and will usually be four metres in length. The millimetres must be interpolated as no graduations exist on the rod. The centimetres are alternate white and black graduations. The decimetres and metres are marked by black numbers. AMRC 2012 2-31
Figure 2.17 Levelling Rods 2.9.1 Rod Levels A rod level is shown in Figure 2.18 and consists of a bracket which can be held to the rod and a circular bull s-eye level. When the bubble is centred the rod is in the plumb position and a correct rod reading can be taken. 2-32 AMRC 2012
Figure 2.18 2.9.2 Waving or Rocking the Levelling Rod If a rod level is not used, the rod man is required to slowly rock or wave the rod in the direction of the line of sight of the level while maintaining a plumb position laterally. When the rod is waved in this manner the rod readings as seen through the level will increase when the rod is not plumb and will decrease as the rod again approaches the plumb position. The correct, or lowest rod, reading will occur as the rod passes through the plumb position as shown in Figure 2.19. Figure 2.19 The rodman must ensure that the rod passes through the vertical position when rocked to obtain the correct reading. AMRC 2012 2-33
2.9.3 Instrument Accuracy and Adjustment Levels should be tested frequently but adjusted only when a particular test indicates the same amount of error at least three times, and whether or not this error will have a material effect on the field results. Quite often the need for adjustment arises from the fact that improper adjustments were made that were not required in the first place. All tests and adjustments should be carried out in the order given for the make and type of instrument so that no previous adjustments will be disturbed. After an adjustment has been made, the proper test should be applied at once. If a number of adjustments have been made, all the tests should be applied again in the proper order, in case some other adjustments may have been disturbed. Tests and adjustments should be carried out according to the manufacturer s specifications where possible. The most common source of level error is where the line of sight of the telescope cross hairs is not in fact level when the plate bubbles have been levelled. This is determined through a routine peg test. 2-34 AMRC 2012
SECTION 2.10 Common Sources of Levelling Error Instrument out of adjustment or worn. Settlement of the tripod in soft ground. Unequal expansion of instrument parts exposed to the sun. Vibrations due to wind. Parallax in the telescope. Bubble not centred. Incorrect reading on the rod (e.g., 6 for a 9). Errors in booking. Mathematical errors. Rod not plumb. Unstable turning points. Error in rod length (rod not fully extended). Foresights and backsights not balanced. Errors caused by climatic or topographic features can sometimes be eliminated by using care in choice of setups. Setting up in a shaded or sheltered area where possible eliminates wind and unequal expansion errors. The remainder of errors are mostly caused by human carelessness and can only be eliminated by proper training of levelmen and rodmen so that proper procedures will be followed. AMRC 2012 2-35
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SECTION 2.11 Self-Test Provided below is a self-test for this module. Perform the test and compare your answers with those given in your student guide. If you have difficulty with the self-test, review the course materials and make notes in order to be able to successfully complete similar questions upon examination. 1. List at least four methods by which distance measurements may be made in surveying applications, and indicate their relative accuracy. 2. Give one advantage of a microwave EDM over an electro-optical. 3. Give the advantages of the modern electronic field book. 4. Name the numbered elements in the following diagram utilizing the correct terminology. 5. Name three types of levelling instruments and then state in your own words the main features of each that ensure a constant horizontal plane elevation, and their relationship to the telescope. 6. What checks must be done with respect to ensuring the horizontal plane elevation after each reading, prior to noting the elevation for each type of level? 7. Why is a rod waved or rocked if a rod level is not used? (Use a diagram to help explain your answer.) AMRC 2012 2-37
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