Lesson Note On Theodolite: Part B

Theodolite:
Axes of theodolite:
The most important relationships are as follows:
1.The axis of plate bubble should be in a plane perpendicular to the vertical axis.(main axis order).
2.The line of sight should be perpendicular to the horizontal axis.
3.The horizontal axis should be perpendicular to the vertical axis.

Instrument setup:
At each station point, before taking any observation, it is required to carryout some operations in sequence. The set of operations those are required to be done on an instrument in order to make it ready for taking observation.

1. Setting
2. Centering
3. Leveling
4. Focusing

Instrument setup:

1.Tripod height –upper about chest height to make observation

easily.Place the instrument over the point with the tripod plate as

level aspossible. Then place the theodolite on the top of tripod.

Theodolite must be hold by hand until the theodolite is attached to

tripod head.

2.Check that the station point can be seen

through the optical plummet. (Rotate the

focus reticle –pull in or out to focus

on the ground-monument)

Then push in the tripod legs firmly by pressing down on the tripod shoe spurs. If the point is now not visible in the optical plumb sight, leave one leg in the ground, lift the other two legs, and rotate the instrument, all the while looking through the optical plumb sight. When the point is sighted,carefully lower the two legs to the ground and reseat them keeping the station point view.

While looking through the optical plumb, manipulate the leveling screws until the crosshair of the optical plummet is directly on the station mark.

3.Level the theodolite circular (pond) bubble by adjusting the tripod legs up or down (approximate leveling). This is accomplished by noting which leg, when slid up or down, moves the circular bubble to ward the bull’s eye. Upon adjusting the leg, either the bubble will move into the circle, or it will slide around until it is exactly opposite another tripod leg. That leg should then be adjusted up or down until the bubble moves into the circle. If the bubble does not move into the circle, repeat the process. If this manipulation has been done correctly, the bubble will be centered after the second leg has been adjusted;

Performa check through the optical plummet to confirm that it is still close to being over the station mark/turn one or more leveling screws to been sure that circular bubble is now exactly centered (if necessary).

The instrument can be now be leveled precisely by centering the plate (tubular) bubble.

a) Set the plate bubble so that it is aligned in the same direction as two of the foots crews. Turns these two screws in opposite direction until the bubble is centered.

b)Turn the instrument 100g, at which plate bubble will be aligned with the third leveling (foot) screw. Finally turn that third screw to center bubble.

Finally,check the axis of plate bubble should be in a plane perpendicular to the vertical axis. main axis order). It is always checked by turning the instrument through 200g. If the plate bubble is centered, the instrument is leveled.

Lesson Note On Theodolite Part A

Theodolite:
A theodolite is an instrument which is used primarily to measure angles, both horizontal and vertical. It is also used for many other subsidiary work during surveying such as setting up of intermediate points between intervisible points, establishment of intervisible points,prolonging a line, laying out traverse etc.
Types of theodolites:
Classification with respect to a construction:

• Open-faced,vernier-equipped engineer’s transit
• Optical theodolites with direct digital read-out sormicrometer-equipped read-outs(formoreprecisereadings)
• Electronic theodolites

Classification with respect to accuracy:

1. One-minute theodolites:the least division of the scale is 1 or 2 minutes
2. One-second theodolites: the least division of the scale is 1 or 2 seconds

Components of theodolite:

The vertical axis of these instruments goes up through the center of the spindles and is oriented over a specific point on the earth surface. The circle assembly and alidade rotate about this axis.

Horizontal axis of the telescope is perpendicular to the vertical axis, and telescope and vertical circle tiltonit.

The line of sight (line of collimation) is a line joining the intersection of the reticle crosshairs and the center of the objective lens.The line of sight is perpendicular to the horizontal axis and should be exactly horizontal when the telescope level bubble is centered and when the vertical circle is set at 100g–300g or 0g for vernier transits.

Plate bubble axis is assumed to be tangent to the plate bubble.

Axes of theodolite:

SS:Vertical(standing)axis

TT: Horizontal (Trunnion)axis

PP: Plate bubble axis

CC:Collimation axis (line of sight)

LESSON NOTE ON SETTING OUT WORKS - FOUNDATION MARKING

SETTING OUT WORKS - FOUNDATION MARKING

AIM

To set out the foundation marking for the proposed construction of building.

APPARATUS USED

1.Theodolite, 2. Chain (or) Tape 3.Ranging rods, 3. Pegs or Arrows, 4. String.

GENERAL

The operation of the marking on the site the centr e lines of the foundation of a building is called

setting out. Setting out of a foundation is the first step in the construction of any structure.

PROCEDURE

1.

A centre line sketch of the building is prepared. (The centres of cross walls are also to be indicated.)

2.

The base line is set out with reference to given reference points.

3.

The ends of the centre line of the walls, points A and B from the base line are marked.

4.

As the end marks A,B,C,etc. are disturbed during excavation, stakes are fixed at L,M,N etc., a little

away (about 2 to 3 m) for end mark and tied accur ately using a string.

5.

The centre line for all other walls AD,BC,etc  are marked by dropping perpendicular. For small

buildings the perpendiculars may be set out by using a chain or a tape by ‘3-4-5’ method. For an

important and big building when sides are long a Theodolite may be employed to accurately set out the

perpendiculars and to range the lines.

6.

For every wall, the pegs are driven a little away for marking the end and tied accurately using a string.

7.

Diagonals ar e measured and checked with their corresponding calculated lengths.

8.

Width of foundation from the centerline are marked and the corners 1,2,3,4,5 etc., are fix up. Pegs are

driven at these corners. The cord is stretched and lime is spread along the chords.

RESULT:

Thus the trench plan being marked on the ground, and excavation may be started.

What Is Visual Survey?

What Is Visual Survey?
This is also called reconnaissance survey.
⇒It is the preliminary inspection of an area to be surveyed.
⇒ It is a see-for-yourself walk-over of the ground to be used for a fish pond or a
fish farm. It is first done with a view to visualise the work to be done.
⇒ It is the venture taken to note and identify all the parameters to be measured or surveyed.
⇒ It is a rough sketch of the field or fields in which all positions and stations are made in the field book.
⇒It is preliminary work done whereby the routes of the main chain lines are noted.
What do you do during visual survey?
(i) The purposes of the survey should be noted.
This includes (a) is it for pond construction?
(b) Is it for damming? (c) Is it for irrigation purposes; (d) Is it for Hydro-electric
power (HEP)?

The purpose will determine the extent of the reconnaissance survey.
(ii) The water parameters to be measured should be noted as from the
beginning. Such parameters include : (a) Water level; (b) Geological attributes;
(c) Soil conditions (texture, structure and permeability); (d) Water pH, hardness,
alkalinity, chloride, phosphate, ammonia, sulphide, sulphite, dissolved oxygen
etc

Lesson Note On Digital Levelling Operating features

Digital Levelling Operating features
The resolution for most instruments is 0.1mm for height and 10mm for distance or better with instrumental ranges up to 100 m. At such distances the effects of refraction and curvature become significant. The effect of curvature can be precisely calculated, the effect of refraction cannot. Most digital levels can also be used as conventional optical automatic levels but in that case the standard error of 1 km of double-run levelling becomes less. Although the digital level can also measure distance, the precision of the distance measurement is only of the order of a few centimetres.

Advantages of digital levelling
One advantage claimed for digital levelling is that there is less fatigue for the observer. While it is true that the observer does not have to make observations the instrument still needs to be set up, pointed at the target and focused. The digital display needs no interpretation such as reading the centimetre from the E on a conventional staff and estimating the millimetre. Measurements are of consistent quality, subject to the observer taking the same care with the instrument to ensure consistency of target distances and illumination of the staff. Also the staff holder must not move the staff between the forward reading in one bay and the back reading in the next, and that the staff must be kept vertical.

There is an acceptable range of illumination, but too much or too little light may make observations
impossible. Some, but not all, digital levels will recognize when staffs are inverted, others will indicate an error if not told that the staff is inverted. Like any automatic level, the digital level will need to be at least coarsely levelled for the compensator to be in range.
Although exact focusing may not be required, the instrument will not work if the focusing is too far out but if the instrument has automatic focusing this would not be a problem. Automatic data storage eliminates the need for manual booking and its associated errors, and automatic reduction of data to produce ground levels eliminates arithmetical errors. However, checks for levelling circuit misclosure need to be made or at least checked and an adjustment to the intermediate points for misclosure needs to be made.
As with all surveying instruments the digital level should be allowed to adapt to the ambient air
temperature.
The scale of the height measurements is primarily fixed by the scale of the staff. An invar staff will
vary less with change of temperature. The scale will also be dependent on the quality of the CCD. How the dimensional stability of CCDs may vary with time is not well known.
There are a number of menus and functions that can be called on to make the levelling process easier,
in particular the two-peg test for collimation error and calibration.

Overall, digital levelling is generally a faster process than levelling with an automatic level. Data can
be directly downloaded to a suitable software package to enable computation and plotting of longitudinal sections and cross-sections. The digital level can be used in just about every situation where a conventional level can be used, and should the batteries fail it can be used as a conventional level if necessary.

Lesson Note On Factors affecting the measuring procedure In Digital Levelling

Factors affecting the measuring procedure
Every operation in a measurement procedure is a possible error source and as such requires careful
consideration in order to assess the effect on the final result.

(1) Pointing and focusing
Obviously the instrument will not work if it is not pointed at the staff. The amount of staff that needs to be read depends on the range of the instrument to the staff. However, there will be a minimum amount necessary at short ranges. It may not be critical to have the staff pointing directly at the instrument.
The precision of the height measurement may be independent of sharpness of image; however, a clear, sharply focused image reduces the time required for the measurement. If the image is too far out of focus then the instrument may not read at all. Some instruments have an auto-focus function to avoid potential focusing problems.
(2) Vibrations and heat shimmer
Vibration of the compensator caused by wind, traffic, etc., has a similar effect on the bar code image as that of heat shimmer. However, as digital levelling does not require a single reading, but instead is dependent on a section of the code, the effects of shimmer and vibration may not be critical.
Similarly, scale errors on the staff are averaged.
(3) Illumination
As the method relies on reflected light from the white intervals of the bar code, illumination of the staff is important. During the day, this illumination will be affected by cloud, sun, twilight and the effects of shadows. Up to a point these variations are catered for by the instrument but under adverse conditions there may be an increase in the measuring time.
(4) Staff coverage
In some conditions part of the bar code section being interrogated by the instrument may be obscured.
Consult the manufacturer’s handbook to ensure that sufficient of the staff is showing to the instrument.
(5) Collimation
The collimation value is set in the instrument but can be checked and changed as required. The method of determining the collimation is based upon one of the two peg methods described earlier. Once the collimation value has been determined it is applied to subsequent readings thereby minimizing its effect.
Note, however, that it can never be completely removed and appropriate procedures according to the
precision required must still be applied.
(6) Physical damage
It is likely that the instrument will be seriously damaged if it is pointed directly at the sun.

LESSON NOTE ON DIGITAL LEVELLING

DIGITAL LEVELLING
The digital level is an instrument that uses electronic image processing to evaluate the staff reading. The observer is in effect replaced by a detector which derives a signal pattern from a bar-code type levelling staff. A correlation procedure within the instrument translates the pattern into the vertical staff reading and the horizontal distance of the instrument from the staff. Staff-reading errors by the observer are thus eliminated.
The basic field data are automatically stored by the instrument thus further eliminating booking errors

Instrumentation
The design of both the staff and instrument are such that it can be used in the conventional way as well as digitally.
(1) The levelling staff
The staff is usually made from a synthetic material, which has a small coefficient of expansion. The staff may be in one or more sections. There are precise invar staves for precise levelling. On one side of the staff is a binary bar code for electronic measurement, and on the other side there are often conventional graduations in metres. The black and white binary code comprises many elements over the staff length. The scale is absolute in that it does not repeat along the staff. As the correlation method is used to evaluate the image, the elements are arranged in a pseudo-random code. The code pattern is such that the correlation procedure can be used over the whole working range of the staff and instrument. Each manufacturer uses a different code on their staffs therefore an instrument will only work with a staff from the same manufacturer.
(2) The digital level
The digital level has the same optical and mechanical components as a normal automatic level. However, for the purpose of electronic staff reading a beam splitter is incorporated which transfers the bar code image to a detector. Light reflected from the white elements of the bar code is divided and sent to the observer and to the detector. The detector is a form of charge couple device (CCD) which turns the black and white staff pattern into a binary code. The angular aperture of the instrument is quite small,

of the order of 1◦–2◦, resulting in a short section of the staff being imaged at the minimum range and
up to the whole staff at the maximum range. The bar code image is compared with a stored reference
code to find the height collimation on the staff. The instrument may not need to see the part of the
staff where the cross-hairs lie. The distance from instrument to staff is dependent on the image scale of the code.
The data processing is carried out within the instrument and the data are displayed in a simple format.
The measurement process is initiated by a very light touch on a measure button. A keypad on the
eyepiece face of the instrument permits the entry of further numerical data and pre-programmed commands.
The data can be stored and transferred to a computer when required. The instrument may have an interface, which permits external control, data transfer and power supply.

Measuring Procedure
There are two external stages to the measuring procedure; pointing and focusing on the staff and triggering the digital measurement. The whole process takes a few seconds.
Triggering the measurement determines the focus position, from which the distance to the staff is
measured, and initiates monitoring of the compensator.
Acoarse correlation approximately determines the target height and the image scale and a fine correlation using calibration constants produces the final staff reading and instrument to staff distance.
For best results a number of observations are taken automatically and the result averaged. This reduces biases due to oscillations of the compensator and air turbulence within the instrument.
The results may be further processed within the instrument, displayed and recorded. The programs
incorporated will vary from instrument to instrument but typically may include those for:
(1) A single measurement of staff reading and horizontal distance.
(2) The start of a line of levelling and its continuation including intermediate sights. Automatic reduction of data. Setting out of levels.
(3) Calibration and adjustment of the instrument (two-peg test).
(4) Data management.
(5) Recognition of an inverted staff.
(6) Set the parameters of the instrument; a process similar to the initializing procedures used when setting up electronic theodolites.