Introduction to Theodolite for Engineers PDF

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Introduction to theodolites used for engineers with equipment, photos and additional information...

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WESTERN SYDNEY UNIVERSITY

SURVEYING FOR ENGINEERS

Unit 300738

COURSE NOTES AND RESOURCES

WEEK 2.

Autumn 2021 .

School of Engineering ___________________________________________________

INTRODUCTION TO THE THEODOLITES THEODOLITES ARE USED FOR:

General purpose angle measurement

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Provision of control surveys

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Contour and detail mapping

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Setting out and construction work

FACTORS WHICH INFLUENCE THE USE OF THEODOLITES 

A clear line of sight between the instrument and the measured points is essential.

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The precision of the instrument is dependent on the repeatability of the angles measured.

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A well-defined point or target is required to obtain the maximum accuracy.

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The accuracy of angles measured is subject to instrumental errors and correct field procedures.

Theodolite can be differentiated either by their angle reading systems or the precision of their angle reading system, (which are generally related). Two types of theodolite commonly used and defined by their precision are the 1" total stations which we will be using and 20" theodolite. There is a digital readout to a screen showing angles both vertical and horizontal.

DIGITAL THEODOLITES Digital theodolites use a microprocessor to convert sightings directly to a digital screen (Liquid Crystal display). A major benefit of this type of theodolite is the data is in digital form, which offers storage and manipulation of the measurements digitally. These theodolites have further features such as Electric Distance Measurement (EDM) for distance measurement. These on board micro-processors interface readily with data capture and download directly to computers.

CONSTRUCTION Digital theodolites are almost identical to the old optical instruments in every way except for the digital readout of the horizontal and vertical angles. All theodolites have a plate bubble which is used to set the instrument up perfectly horizontally, the tilt sensors will only tell you if you are way off level. The total stations we will be using have an electronic display to mimic the look of the plate bubble. All theodolites have a trunnion axis, horizontal plate clamps, vertical clamp, and slow motion screws and are mounted in a tribrach. If an EDM is included in the theodolite then it is possible to determine all elements of a three dimensional vector, that is direction, elevation and length. The micro-processor interprets

this data and it can be downloaded to the surveyors computer, where survey drawings can be directly produced.

TARGETS A target mounted on a tripod may be used to accurately sight a theodolite to another survey station or measurement point, as seen here.

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Gun sight Telescope (Focus) Telescope ( Eye piece) Vertical slow motion screw Vertical circle clamp or tangent screw Plate bubble Display panel Controls Horizontal slow motion Horizontal circle clamp or tangent screw Base plate

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Battery locking lever Battery pack Objective lense Vertical circle Optical plummet Power on-off switch Bulls eye bubble Foot screw

PARTS OF THE THEODOLITE THE HORIZONTAL CIRCLE Is a ‘full circle’ protractor graduated with angles from 0 to 360, divided into degrees, minutes and seconds. The protractor to measure the horizontal angles is called the horizontal circle and the axis about which it rotates, is called the vertical axis. (fig 1.1).

THE TELESCOPE The theodolite is fitted with a telescope which can move up and down, and face any direction. To do this it needs a horizontal and vertical axis. Just remember that the horizontal angles are turned around the vertical axis and the vertical angles are turned around the horizontal axis. The horizontal axis is also called the alidade axis. There is yet a third axis, the line through the centre of the telescope; this is called the telescope axis.

THE VERTICAL CIRCLE To measure vertical angles there is another protractor fixed in the vertical position, called the vertical circle. The horizontal axis passes through the centre of this circle. (fig 1.2). All these axes and circles need brackets or components to support them. Their names are as follows:—

THE TRIBACH First of all there is the part which mounts on the tripod legs. This is the tribach which has the three foot screws used for levelling up the instrument.

STANDARDS Fixed to the tribach are two supports for the horizontal axis. These are called the standards (standard in this sense means supports or posts). The vertical circle is fixed in that plane to one of these standards. CLAMPS There are clamps which allow you to fix the horizontal circle to the bottom plate and one clamp to fix the telescope to the vertical circle. The clamp which locks the horizontal circle to the bottom plate is called the bottom plate clamp.

TANGENT OR SLOW MOTION SCREWS For each of the two clamps in the theodolite there is a tangent or slow motion screw for fine adjustment after you have clamped everything. Our theodolites have a digital readout which gives us accuracies up to 20 seconds of a degree.

TYPES OF ERRORS EXPERIENCED WITH THEODOLITES There are a number of things that can go wrong, which we call ‘errors’. The two broad descriptions are known as: 

Systematic errors – errors over which we have no control.

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Gross errors – just plain errors through using poor techniques

Systematic errors are caused by slight maladjustments in the instruments. Gross errors are caused by the operator turning the wrong knob, sighting to the wrong mark, not being accurate enough when setting up, forgetting the correct levelling procedures, or just inattention. Twenty seconds (20”) is the angle given by ten millimetres, 100 metres away. Some theodolite can read to an accuracy of 5 seconds of arc, equivalent to an angle made by 2.5 millimetres width 100 metres away. See figure 1.5. Our total stations measured to 1” of an arc. There are techniques we will learn in using a theodolite which can indicate immediately if a gross error has occurred or in the case of a systematic errors, the techniques used will cancel the error, if it should exist.

SETTING UP In the same way as you set up the automatic level last week, the same procedures apply to the use of a theodolite: 

Firstly set up the tripod legs, extending the legs not quite to their full length and have them approximately 1m apart from each other.

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Open up the instrument case, noting particularly how the instrument is positioned.

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There is a particular way that the theodolite can only be put back. With our total stations the battery must be uppermost when it is correctly placed in the instrument case. Also the three levelling screws on the tribach must be wound up but not too tightly. If they are extended you can’t return the theodolite to its correct position in the carry box.

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Take the instrument out using both hands and lift it onto the tripod head. While holding it with one hand, use the other hand to attach the instrument to the tripod with the retaining screw under the tripod head. Remember you can tighten this so tight that it will almost be impossible to undo. It is only meant to be finger tight.

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Only once the retaining screw is gently but firmly tightened, through adjusting the legs you can try to get the cats eye (bulls eye) roughly level. Then firmly push the legs of the tripod into the ground.

THE INITIAL SET UP 

Slacken off the various clamps so that the instrument is free to rotate.

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Carry the instrument vertically, to the mark over which you are to set up.

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Position the instrument roughly over the mark.

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Stand back a pace or two and sight down by eye to see how close you are.

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If you are not over the mark by eye, move the instrument and have another check. You should be within about 5 to 10 mm of the mark.

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Now you can use a couple of devices or techniques to make this process quicker: 

One is the optical plumb. (See item 17 on Pg. 2) This allows you to sight down through the vertical axis of the instrument.

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The other is the ‘quick-set’ bubble, a small circular bull’s eye (cat’s eye) bubble mounted on the tribach, usually near one of the foot screws.

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When you have set up the instrument approximately over the mark, walk around the legs and stand on each foot step of each leg of the tripod and push the point of the legs firmly into the ground by standing on it, put all your weight on it – it must be firmly in the ground. Even if you are set up on concrete, still stand on each footstep; this will ensure that the tripod feet are secure and will not slip.

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Make sure the foot screws on the tribrach are in their halfway position. There is usually a groove in the thread to indicate the midpoint. Sight through the optical plumb to see the ground mark. If you can’t, it may mean moving your instrument. The centre of the optical plumb is shown by a circle engraved on the glass in the sight. Now use the foot screws to centre the ground mark in the optical plumb.

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The instrument will now not be level; this can be verified by a quick glance at the circular bubble. This is the tricky bit; you can’t use the foot screws to level the instrument as this will move the ground mark off the centre of the optical plumb. This is why it’s important when setting up the legs of the theodolite (before taking the instrument out of its carry box) not to extend the legs to their full length.

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Note on which side of the circle the bubble is located. Adjust the length of the tripod nearest the bubble to move the bubble towards the other two legs, this sound easy but we have to be careful adjusting the length of the legs. This is achieved by putting one foot on the step of the leg, put one hand around the top of the bottom part of the leg. With your other hand slacken off the clamp so that you can slide the top part of the leg either up or down carefully seeking to centre the bubble in the cats eye. Repeat this process on one of the other leg to centre the bubble if required.

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With the cats eye bubble levelled, have a look through the optical plumb and the mark on the ground should still be centred. If not, slacken off the retaining screw under the tripod head and move ever so slightly to centre exactly over the ground mark.

THE FINAL TOUCHES The next step is to accurately level the instrument as the bulls eye bubble is only an approximate level. Last week when we introduced an automatic level, the bulls eye was sufficient, as the instrument had a prism inside which did the final levelling for you. Between the standards is the bubble now that we level to. Turn the instrument to align the bubble between two foot screws. Through the process of turning the foot screws in opposite directions you can centre the bubble as seen in figure 1.6, below.

Rotate the bubble through 90 and adjust with your hand the levelling screw to again centre the bubble. Rotate the instrument through a further 90 and if the bubble is not level adjust it by half the distance of what it is out by. One last rotation of 90 adjust the bubble if necessary by taking it back by half the distance it is out by. This should be the last time you need to adjust the bubble. Do not look back at the cat’s-eye bubble, it is very inaccurate. Finally check through the optical plumb and the theodolite should now be centred over the point, if not slacken off the retaining screw by half a turn and slide the instrument very gently over the top of the legs to centre it exactly over the ground mark. If you look back at the bulls eye it may well be now off centre, ignore it as it is only an approximate levelling device.

ADJUSTMENTS IN THE INSTRUMENT TEMPORARY ADJUSTMENTS The first temporary adjustment is to ensure that the cross hairs are as sharp and dark and not grey in colour, this can be adjusted to suit your eyesight. If they are not perfectly focused (sharp) then every reading on the staff will have a built in error due to parallax error. Next is to look through the telescope at a wall or some flat surface and use the focusing ring to bring the wall perfectly into focus. Do not do this in reverse order. The optical plumb also has a focusing ring and it is important that it is properly adjusted. The levelling plate bubble also can be checked through the process that we went through earlier in setting up by adjusting the level by half the distance it is out by each time.

The last adjustment is to ensure that the two clamps, one for the vertical circle and the other for the horizontal circle, when clamped, that the fine tuning screw actually work. It will never work if it is not clamped properly.

THERE ARE TWO FACES USED IN THEODOLITE’S. The standard first reading is always a “face left” (FL) reading. To enter the “face right” (FR) position you must plunge and reverse the telescope. The instrument always has the zenith reading directly above the instrument and so when viewing the horizon in the “face left” (FL) position the vertical circle will read 90. When you plunge and reverse the instrument putting it into the “face right” (FR) position when you view the vertical circle it will now read 270. This is a simple check to make sure that you are definitely in face left & right positions.

ADJUSTING FOR INSTRUMENT ERRORS The theodolite is a scientific instrument; and rough handling can throw the readings of. Errors in angular measurement caused by instrument maladjustment are called systematic errors. Below is a list of things which can get out of adjustment starting from the top, the telescope: 

Out of collimation in altitude

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Out of collimation in azimuth

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Horizontal axis

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Vertical circle not correctly orientated to the zenith (index error)

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Vertical axis

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Plate bubble

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Optical plumb not exactly on line with the vertical axis

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Bulls eye bubble not in adjustment

The checks for each of these errors is outside this course. If you think there is anything wrong with the theodolite, report it immediately, and a different instrument will be provided.

MINIMISING ERRORS Take a lot of care when using the theodolite to make sure that: 

The instrument is exactly over the mark

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The instrument is accurately level

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The target you are sighting to is exactly over the correct mark

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The crosshairs are sharp and exactly on target

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You have read the correct values in the screen

If you get a chance to observe a surveyor working in the field notice how they carry out their observations. By observing all angles on both face left and right of the instrument, all collimation errors, errors of eccentricity of axis and even human error can be located, minimised and eliminated.

OBSERVING ANGLES Theodolite are used for measuring both horizontal and vertical angles. This feature allows us to observe tall objects such as the height of a water tower. We can observe horizontal angles determining the bearing of certain objects that we are shooting onto, remembering all bearings are taken from the North point. We can also measure angles between two items

we are sighting onto. We can accurately shoot onto a target, using the cross hairs and focusing rings, record the FL angle and then, through plunging and reversing the telescope, we can then observe the FR angle, of that same target. If the difference between the two readings, is not greater than 40 seconds of a degree, we then take the average of the FL and FR angles. In order to shoot FR, after we have shot FL, we have to unclamp the horizontal plate, this is why when we shoot back to the object, we get the cross hairs exactly back to the same point of the first observation (FL), or otherwise our FL and FR angles will never be within 40” of a degree. This is the skill that we will be learning, so that every time we shoot FL and FR, we get the cross hairs exactly back onto the same point. The reason for taking the average of the FL and FR angles, allows us to average out any systematic error that may exist in the theodolite. When we read the FL angle, we record it on our log book and we note that this is target ‘A’ that we are shooting onto, then record the FR angle after plunging and reversing. There and then, we check to see if the angles are within 40” of a degree, not later. If the difference is greater than 40”, then re-shoot until they are within 40” of a degree. Whether we are observing horizontal or vertical angles, the same technique is used for both. At all times when we have clamped either the vertical or the horizontal circle, we use the tangent screw to fine tune the crosshairs, so that we are accurately shooting at the target, whether it is FL or FR. In order to read an angle, you need two targets to shoot onto, target ‘A’ (RO) and target ‘B’, as seen in figure 2.3. Target ‘A’ is our referenced object (RO) or target from which other angles are being calculated. Step 1.

Once the instrument is set up over the station, make sure that all the clamps off and that the instrument is in the face left position.

Step 2.

Set the horizontal angle to zero, once you have carefully shot onto the reference object (RO).

Step 3.

Plunge and reverse the theodolite and shoot back on to the same reference object, check that the difference between the FL and FR is not greater than 40 seconds of a degree. If it is, go back to the face left and re-shoot, once again zeroing the horizontal angle. Keep repeating this process tilt you are less than 40 seconds of a degree out.

Step 4.

We now shoot onto the other target (B). Once again we observe the face left reading, first and record it. We then plunge and reverse the telescope and observes the face right reading. We further check that the readings are not out by more than 40 seconds of a degree.

Step 5.

We now have observed the internal angle between our reference object and other target.

SOME PROCEDURE IS TO ADOPT TO MINIMISE ERRORS 

When turning through angles, always turn in a clockwise direction. This is more than a convention; it is a way of retaining your sanity. Remember FL turn right.

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Use the gun sight (item 1 on Pg 2) to shoot approximately to the target, then look through the telescope and accurately using the tangent screw line up the cross hairs onto the target. Then lock the horizontal plate and read off the angle on the digital readout. Look back through the telescope to check that the crosshairs are still exactly on to the target, otherwise do it again.

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When shooting onto your reference object, and setting the theodolite to 0°, do not readjust anything or touch the tangent screw or otherwise, you will have to zero the theodolite again.

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When shooting onto other objects, you are observing angles from your reference object. T...