PRINCIPLES OF SURVEYING
In spite of the diverse nature of land surveying, it is possible to define certain basic principles which are common to all branches of the subject. These principles have proved over the years to be vital if accurate surveys are to be conducted.
The first and most important principle is the provision of an initial framework before observing and fixing the detail of a survey. This process is often known as providing control. It is essential to ensure that the positions of the control points are known to a higher order of accuracy than those of the subsidiary points. By satisfying this principle it is possible to ensure that errors, which inevitably occur, do not accumulate but are contained within the control framework.
A second and perhaps more obvious principle is that of planning. All too often it is tempting to rush into a survey without consideration for an overall plan. Of particular importance is the need to define a job specification. This is indispensable since the relationship between cost and accuracy is not linear and an increase in accuracy may have a disproportionate effect on cost. For example, if a distance of 50Om is to be determined to an accuracy of either 5 or 0.5 mm, the cost ratio of the respective accuracies may be of the order of 1:300. It is important, therefore, to choose techniques and instruments appropriate to the survey specification. Of equal importance is the need to plan the reconnaissance stage. Before starting a task it is essential to examine the area carefully, considering all the possible ways of doing the survey and then selecting the most suitable method. Remember, ‘time spent on reconnaissance and planning is never wasted’.
A third principle is the need to ensure that sufficient independent checks are incorporated into the survey to eliminate or minimize errors. It is important that the checking system is included at all stages of the survey from fieldwork and computations to the final plotting. In addition, the checking system should be independent and not solely a repeat of the initial measurement. Examples of independent checks are:
• measure both diagonals of a quadrilateral
• measure distances in both directions
• measure angles using different parts of the theodolite circle
• use the summation check on angle observations of geometric figures, e.g. sum of interior angles (2«-4) x 90°
• leveling booking cross-checks
• plot positions of important points by using angles and distance and also using coordinates.
The final principle is that of safeguarding. Safeguarding is equally important at all stages of the survey, and refers to the process of ensuring that the survey results can be replicated if accidental, or other, damage occurs to the survey markers or field observations. Thus, it is important when constructing permanent survey markers to take ‘witness or reference measurements’ to points of prominent detail in the vicinity of the point. Linear measurements of this type enable the point to be relocated if it is damaged, or alternatively if it is difficult to find.
The latter situation can often occur with road projects. In many instances there may be a gap of many years between initial survey and final setting-out. During this time the permanent survey markers may become overgrown with vegetation and hence difficult to locate. The use of witness marks and measurements can often be of crucial importance if the permanent marks are to be relocated.
Safeguarding of field observations is also of paramount importance. Thus, it is considered good practice to produce abstract sheets from the surveyor’s fieldbook at the end of each day. These abstract sheets should summarize the major results from the fieldbook (e.g. rounds of angles, mean distance etc.) and should be carefully filed in the survey office. By such a process the possibility of several days’ work being lost if the fieldbook is damaged or misplaced can be eliminated.