#engineering survey

Surveying is a vital and preliminary part of any construction project. It involves examining and recording the characteristics of the designated area or a certain piece of land, so that a map, plan or detailed description of it can be furnished.

It involves measuring work, mostly, and the linear measurements, that is, measuring the straight-line distance between point A and point B, can be carried out via three main ways: Direct, Electromagnetic or Optical. We will talk about the direct method today, which basically is employing some physical instrument like a chain or string or tape measure to gauge the distance between two points.

Surveyors may use the following ways to directly measure a linear distance: Pacing, Passometer, Pedometer, Odometer, Chaining. Let us discuss them in detail below.

1. Pacing Method: Pacing is the most ancient and basic ways of measuring a linear distance in surveying. It involves walking in a straight path from point A to point B and counting the steps, and then multiplying the number of steps by the average step length.

Generally, the average step length is taken as 80 cm or in some places, 2.5 feet. However, the surveyor must learn to keep his step length as close to the average as possible via practice if s/he were to employ this method in direct surveying.

Needless to say, this method can be fairly inaccurate. Even by good practice, some 1% error margin cannot be negated. Also it can get tiring and the error margin increases with longer distances. Also if the ground is uneven or sloped, the paces will inevitably vary. Thus the Pacing method is best reserved for use when you’re doing basic reconnaissance and/or lacking proper measurement tools.

This construction video tutorial is very useful for land surveyor. The tutorial sheds light on the rise and fall table in land surveying. Get some useful tricks to resolve the rise and fall table.

Rise and fall method is applied to resolve the levelling to detect the variation in elevation and elevation of points..

By resolving this table, it is possible to work out the reduced level of each point captured at staff rod with leveling machine. In order to work out the reduced level of an individual point, it is necessary to compute the rise and fall of points. After that, for reduced level, the following formula is applied :-

new R.L = old R.L – Fall new R.L = old R.L + Rise

The table contains various heads like number, B.S (back sight), I.S (inter sight), F.S (fore sight), Rise, Fall, R.L (reduced level) and remarks.

B.S - Back sight (BS) is the abbreviation for "back sight reading". It means the first staff reading captured by the surveyor as soon as the levelling instrument is arranged and levelled. B.S is normally captured on the point of known reduced level as on the benchmark or a change point. The normally applied datum is mean sea level.

Fore sight (FS) is the abbreviation for "fore sight reading". It means the last staff reading captured prior to modify the instrument to the other position. It refers to the staff reading captured on point whose RL is to be ascertained. This sight is treated as negative and assumed from Height of Instrument to find out RL of the point.

Reduced Level (RL) in surveying stands for equating elevations of survey points relating to a common assumed datum. It refers to a vertical distance among survey point and selected datum plane. Therefore, it is taken as the base elevation that is applied as reference to consider heights or depths of other important places.

To get more detailed information, watch the following video tutorial.

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