Description:

Plans are generally produced on a plane rectangular coordinate system, and hence salient points of the design may also be defined in terms of rectangular coordinates on the same system. For instance, the centreline of a proposed road may be defined in terms of coordinates at, say, 30-m intervals, or alternatively, only the tangent and intersection points may be so defined. The basic methods of locating position when using coordinates are either by polar coordinates or by intersection.

By polar coordinates:

 A, B and C are control stations whose coordinates are known. It is required to locate point IP whose design coordinates are also known. The computation involved is as follows:

(1) From coordinates compute the bearing BA (this bearing may already be known from the initial control survey computations).

(2) From coordinates compute the horizontal length and bearing of B − IP.

(3) From the two bearings compute the setting-out angle AB(IP), i.e. β.

(4) Before proceeding into the field, draw a neat sketch of the situation showing all the setting-out data. Check the data from the plan or by independent computation.

Alternatively the coordinate geometry functions in a total station can be used to avoid most of the computations. The field work involved is as follows:

(1) Set up theodolite at B and backsight to A, note the horizontal circle reading.

(2) Add the angle β to the circle reading BA to obtain the circle reading B − IP. Set this reading on the theodolite to establish direction B − IP and measure out the horizontal distance L.

If this distance is set out by steel tape, careful consideration must be given to all the error sources such as standardization, slope, tension and possibly temperature if the setting-out tolerances are very small. It should also be carefully noted that the sign of the correction is reversed from that applied when measuring a distance. For example, if a 30-m tape was in fact 30.01 m long, when measuring a distance the recorded length would be 30 m for a single tape length, although the actual distance is 30.01 m; hence a positive correction of 10 mm is applied to the recorded measurement. However, if it is required to set out 30 m, the actual distance set out would be 30.01 m; thus this length would need to be reduced by 10 mm, i.e. a negative correction.

Setting out from control points

Setting out with a steel tape

 

 

The best field technique when using a steel tape is carefully to align pegs at X and Y each side of the expected position of IP . Now carefully measure the distance BX and subtract it from the known distance to obtain distance X − IP, which will be very small, possibly less than one metre. Stretch a fine cord between X and Y and measure X − IP along this direction to fix point IP. Atotal station may be used to display horizontal distance, so the length B−IP may be ranged direct to a reflector fixed to a setting-out pole.

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