The establishment of two- or three-dimensional control networks is the most fundamental operation
in the surveying of an area of large or small extent. The concept can best be illustrated by considering
the survey of a relatively small area of land as shown in Figure below:
- prevent sag in the tape and their end positions at 1, 2 and B fixed using a plumb-bob and an additional assistant. The steps are then summed to give the horizontal distances.
- Thus by measuring all the distances, relative positions of the survey stations are located at the intersections of the straight lines and the network possesses shape and scale.
- The surveyor has thus established in the field a two-dimensional horizontal control network whose nodal points are positioned relative to each other.
- It must be remembered, however, that all measurements, no matter how carefully carried out, contain error. Thus, as the three sides of a triangle will always plot to give a triangle, regardless of the error in the sides, some form of independent check should be introduced to reveal the presence of error. In this case the horizontal distance from D to a known position D on the line EC is measured.
- If this distance will not plot correctly within triangle CDE, then error is present in one or all of the sides. Similar checks should be introduced throughout the network to prove its reliability.
(3) The proven network can now be used as a reference framework or huge template from which further measurements can now be taken to the topographic detail. For instance, in the case of line FA, its position may be physically established in the field by aligning a tape between the two survey stations. Now, offset measurements taken at right angles to this line at known distances from F, say 20 m, 40 m and 60 m, will locate the position of the hedge. Similar measurements from the remaining lines will locate the position of the remaining detail.
- The method of booking the data for this form of survey is illustrated in Figure 1.6. The centre column of the book is regarded as the survey line FA with distances along it and offsets to the topographic detail drawn in their relative positions as shown in Figure 1.4.
- Note the use of oblique offsets to more accurately fix the position of the trees by intersection, thereby eliminating the error of estimating the right angle in the other offset measurements.
- The network is now plotted to the required scale, the offsets plotted from the network and the relative position of all the topographic detail established to form a plan of the area.
- (4) As the aim of this particular survey was the production of a plan, the accuracy of the survey is governed largely by the scale of the plan. For instance, if the scale was, say, 1 part in 1000, then a plotting accuracy of 0.1 mm would be equivalent to 100 mm on the ground and it would not be economical or necessary to take the offset measurements to any greater accuracy than this.
- However, as the network forms the reference base from which the measurements are taken, its position would need to be fixed to a much greater accuracy.
- The above comprises the steps necessary in carrying out this particular form of survey, generally referred to as a linear survey.
- It is naturally limited to quite small areas, due to the difficulties of measuring with tapes and the rapid accumulation of error involved in the process.
- For this reason it is not a widely used surveying technique. It does, however, serve to illustrate the basic concepts of all surveying in a simple, easy to understand manner. Had the area been much greater in extent, the distances could have been measured by EDM equipment; such a network is called a trilateration. A further examination of Figure 1.4 shows that the shape of the network could be established by measuring all the horizontal angles, whilst its scale or size could be fixed by a measurement of one side. In this case the network would be called a triangulation.
- If all the sides and horizontal angles are measured, the network is a triangulateration. Finally, if the survey stations are located by measuring the adjacent angles and lengths shown in Figure 1.7, thereby constituting a polygon A, B, C, D, E, F, the network is a traverse. These then constitute all the basic methods of establishing a horizontal control network.