PRINCIPLES OF SURVEYING (StudyCivilEngg.com)

PRINCIPLES OF SURVEYING

SUBJECT : SURVEYING

Surveying consists of various operations for taking measurements in the field. However, the underlying principles in all the different operations and techniques are the same. These principles have been developed by well-known surveyors from time to time based on their experience. These principles give a discipline and unity to the subject. The following principles suggest the most expeditious and effective basis for conducting surveys. A good surveyor should always bear these principles in mind. 

Always Work from the Whole to Part

The first principle of surveying is to work from the whole to the part. The surveyor should first establish accurately a large main framework consisting of widely spaced control points. Between the large main framework, subsidiary small frameworks can be established by relatively less accurate. surveys. The errors in small frameworks are thus localised and are not magnified and the accumulation of errors is controlled. In the reverse process of working from the part to the whole, the small frameworks will be expanded to the large framework and the errors will get magnified. For illustration, let us consider three points A, B and C which form the large main framework ABC (From the below figure). 

The relative positions of these points can be determined accurately. These points are located with a very high standard of accuracy by measuring the distances AB, BC and CA and plotting on a plan to some scale. The interior small framework DEF can be established by measuring distances DE, EF and FD, with relatively lower standard of accuracy. The smallest framework GHJ can be established even with still lower standard of accuracy. Thus the errors in the location of the small triangle GHJ are localised and do not affect the position of the large triangle ABC. 

In the reverse process of working from the part to the whole, the positions of the points G. H and J of the smallest triangle would be fixed first. From the located positions of G. H and J, the positions of the points D, E and F would be determined by measuring the distances DE, EF and FD. Finally, from the located positions of D, E and F, the positions of the points A, B and C are obtained. The location of the points A, B and C fixed by this method is likely to be less accurate than that when working from the whole to the part. This is due to the fact that all surveying instruments are subject to some errors, and there is always some error in plotting and extension of lines. 

A single large triangle involves least number of measurements and provides better results compared with those obtained by the expansion of small triangles involving more number of measurements to a large triangle. For accurate survey, the distance between the control points of the main framework should be as large as possible. The control points should involve the minimum possible number of measurements, and they should be located with a very high standard of accuracy. The measurements of a subsidiary framework can be relatively less accurate.

Always Choose the Method of Survey that is the Most Suitable for the Purpose 

Any desired accuracy can be attained in the field using the modem methods of surveying. However, the cost of surveying increases rapidly with an increase in accuracy, because high accuracy requires very costly precise instruments, more field work and more extensive computations. Always choose the method of survey so that the desired accuracy is achieved at a minimum cost. If the standard of accuracy achieved is less than the specified, the results will be useless. 

On the other hand, if the accuracy attained is more than the specified, it would lead to wastage of time, money and effort. In the interest of speed and economy, the surveyor should work as close to the required accuracy as possible. Every survey is conducted for some specific purpose. For example, the land survey of a rural farm where the cost of land is low, may be done to an accuracy of, say 1 in 1000, using a chain. On the other hand, the survey of a residential plot in a big city where the cost of land is high may require an accuracy of 1 in 10000 or more and a very accurate steel tape or an invar tape will be needed. The relative standard of accuracy of the linear and angular measurements should be consistent.

Always Make Provisions of Adequate Checks

No measurement is dependable if it has not been properly checked. There is always a possibility of making errors while taking the measurement, recording the observations, computing and plotting the results. The survey should be conducted so that the errors do not pass undetected. There should be a suitable provision of checks. As far as possible, the survey work should be self checking. For illustration, let us consider two points B and C already fixed (From the below figure). The point A can be located by measuring the distances AB and AC. The triangle ABC can be plotted even if an error had been made in the measurement of the distances AB and AC. The existence of such errors would not be detected in the plotting. However, if an other line AD is also measured in the field, it would serve as a check. The error would be revealed if the plotted distance AD on the plan does not correspond to the measured distance in the field.

In good surveys, the methods of taking observations, recording and computations are so designed that the errors are revealed in the process. All measurements should be checked at the time of recording. This can be done by adopting a method which automatically reveals mistakes. Alternatively, two different parties should take the same measurements independently or the same party should also take the measurements in the reverse direction. The survey must be arranged so that the results can be checked by office computation and plotting.

Always Record Field Data Carefully

All the measurements taken in the field must be recorded in a proper field book at the time of measurement. Nothing should be left to memory. The record must be in a standard form and clearly written. The record may be in the form of tables, sketches, description or a combination of all the three. The field book is the original record. It should be kept in safe custody. The entries in the field book should preferably be made with a fine black ball-point pen to ensure permanency. 

Alternatively, a hard pencil 3H or 4H should be used so that a permanent impression is left on the paper. If there is a mistake in making an entry, a line should be drawn to cross it and the new value should be written above it. No overwriting is permitted. The field record should be accurate, legible, clear, true and properly arranged.

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