QUESTION: Explain grid levelling method

     Explain grid levelling method

 

                     In this method, the area to be surveyed is divided into a grid or series of squares. The grid size may vary from 5 m x 5 m to 25 m x 25 m depending upon the nature of the terrain, the contour interval required and the scale of the map desired. 

Also, the grids may not be of the same size throughout but may vary depending upon the requirement and field conditions. The grid corners are marked on the ground and spot levels of these comers are determined by leveling. 

The grid is plotted to the scale of the map and the spot levels of the grid corners are entered. The contours of desired values are then located by interpolation. 

Special care should be taken to give the spot levels to the salient features of the ground such as hilltops, deepest points of the depressions, and their measurements from respective corners of the grids, for correct depiction of the features. The method is used for large scale mapping and at average precision. 

QUESTION: DESCRIBE THE SOURCES OF ERROR IN LEVELING

            DESCRIBE THE SOURCES OF ERROR IN LEVELING

            Many sources of error exist in levelling and the most commonly met in practice are discussed. Firstly, one of the sources of error is errors in the equipment which is collimation error. This can be a serious source of error in levelling if the sight lengths from one instrument position are not equal, since the collimation error proportional to the difference in sight length. The line of collimation should be parallel to the line of sights. 

Hence, in all types of levelling, sights should kept equal, particularly back sights and fore sights. Before using any level it is advisable to carry out a two-peg to ensure that the collimation error is as small as possible. Other than that, compensator not working. The function of compensator is to deviate the horizontal ray of light at the optical center of the object lens through the center of the cross hairs. This ensure that line of sight viewed through the telescope is horizontal.  

If the reading changes to a different position each time the footscrew is moved or thr instrument tapped, the compensator is not working properly and the instrument should be returned to the manufacturer for repair. Parallax  also one of error in the equipment. Parallax must be eliminated before any readings are taken. Parallax is occur when the image of the distance point or object and focal plane are not fall exactly in the plane of the diaphragm. 

To eliminate parallax, the eyepiece is first adjusted until the cross hairs appear in sharp focus. Then, defects on the staff  which is the incorrect graduation staff cause the zero error. This does not effect height differences if the same staff is used for all the levelling but introduces errors if to staves used for the same series of levels. When using a multisection staff, it is important to unsure that it is properly extended by examining the graduations on either side of each joint. The stability of tripods should also be checked before any fieldwork commences .

 

                     Secondly, field errors also source of error. The example of field errors is staff not vertical, failure to hold the staff vertical will result in incorrect readings. The staff is held vertical with the aid of a circular bubble. At frequent intervals the circular bubble should checked against plumb line and adjusted if necessary. Another example of field errors is unstable ground. When the instrument is set up on soft ground and bituminous surfaces on hot days, an effect often overlooked is that the tripod legs may sink into the ground or rise slightly while readings are being taken.This alters the height collimation and therefore advisable to choose firm ground on which to set up the level. 

After that, handling the instrument and tripod as well as vertical displacement, the HPC may be altered for any set-up if the tripod is held or leant against. When levelling, avoid contact with the tripod and only use the level by light contact through the fingertips. Then, instrument not level is also the field errors. For automatic levels this source of error is unusual but, for tilting level in which the tilting screw has to be adjusted for each reading, this is common mistake. The best solution is to ensure the main bubble is centralised before and after reading.

 

                 Thirdly, source of error is the effects of curvature and refraction on levelling. The effect of atmospheric on the line of sight is to bend it towards the Earth’s surface causing staff readings to be too low. This is variable effect depending on atmospheric condition but for ordinary work refraction is assumed to have value 1/7 that of curvature bit is of opposite sign. The combined and refraction correction is c + r = 0.0673 D². If longer sight lengths must be used, it is worth remembering that the effects of curvature and refraction will cancel if the sight length are equal. But, curvature and refraction cannot always be ignored when calculating heights using theodolite methods.

 

                  Lastly, source of error is reading and booking error and also weather conditions. Source of reading error is the sighting the staff over too long a distance, when it becomes impossible to take accurate readings. It is , therefore, recommended that sighting distances should be limited to 50m but, where absolutely unavoidable, this may be increased to maximum of 100m. For weather conditions, when it windy will cause the level to vibrate and give rise to difficulties in holding the staff steady. In hot weather, the effect of refraction are serious and produce a shimmering effect near ground level. The reading cannot be read accurately.

QUESTION : EXPLAIN BASIC RULES IN PRACTICE WHEN CONDUCTING A LEVELING

1.     EXPLAIN BASIC RULES IN PRACTICE WHEN CONDUCTING A LEVELING

    

                Levelling is the process of measuring the difference in elevation between two or more points. In engineering surveying, levelling has many application and is used at all stages in construction projects from the initial site survey through the final setting out. In practice, it is possible to measure heights to better than a few millimeters when levelling  this precision  is more than adequate for height measurement on the majority of civil engineering project.

The basic rules in practice when conducting a levelling fieldwork should be adhered to if many of the sources of error are to be avoided. Levelling should always start and finish  at points of known reduced level so that misclosures can be detected. When only one bench mark is available, levelling lines must be run in loops starting and finishing at the bench mark. 

Where possible, all sights length should below 50m. The staff must be held vertically by suitable use of a circular bubble or by rocking the staff and notong the minimum reading. Backsight and fortsight length should be equal for each instrument position. For engineering application, many intermediate sight readings may be taken from each set- up. Under this circumstances it is important that the level has no more than a small collimation error. 

Reading should book immediately after they are observed and important readings, particularly at change points, should be checked. The rise and fall method of reduction should used when heighting reference or change  points and the HPR method ( height of collimation) should be used for contouring , sectioning and setting out applications.

QUESTION: Describe the methods of inverted staff reading & reciprocal levelling

1       Describe the methods of inverted staff reading & reciprocal levelling

 

                       In levelling there are other levelling method which is inverted staff reading and reciprocal levelling. Inverted staff it may be necessary to determine the reduced levels of points such as the soffit of a bridge, underpass or canopy. Generally, these points will be above the line of collimation. To obtain the reduced levels of such points, the staff is held upside down in an inverted position with its base on the elevated points. When booking an inverted it is entered in the levelling table with a minus sign, the calculation proceeding in the normal way, taking this sign into account. An inverted staff position must not be used as a change point since there is often difficulty in keeping the staff vertical and in keeping its base in the same position for more than one reading. An inverted staff position must be used as a change point because it is often difficult to keep the staff vertical and to keep iys base in the same position for more than one reading.

                           True differences in height are obtained by ensuring that back sight and fort sight lengths are equal when levelling. This eliminates the effect of any collimation error that may be present in the level used and also eliminates the effects of curvature and refraction. There are certain cases, however, when it may be not be possible to take readings with equal sight lengths as, for instance, when a line of levels has to be taken over a wide gap such as river. In these cases, the technique of reciprocal levelling can be adopted.

                     Figure down shows two point A and B on opposite sides of a wide river. The line of collimation has been assumed to be elevated above the horizontal plane. This may not be the case but does not affect the calculations. To obtain the true difference in level between A and B a level is placed at I1 about 5m from A, and a staff is held vertically at A and B . Staff readings are taken at A (a1) and B (b1). The level is next taken to position I2 where readings a2 and b2 are recorded. Since the observations are taken over the same sighting distances with the same level, the effects of the collimation error will be the same for both cases. When repciprocal levelling with one level, the two sets of observations must follow each other as soon as possible so that refraction effects are the same and are therefore eliminated. Where this not possible, two levels have to be used simultaneously. It must be realised  that the levels should have the same collimation error or the true height different will not be obtained.

                  

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