Simple survey figures have their limitations. If, as in intersection or resection, two angles are measured to find the two coordinates, easting and northing, of an unknown point then only the minimum number of observations have been taken and there is no check against error in either observations or the computations.
A fully observed traverse is a little better in that there are always three more observations than the strict minimum. If the traverse has many stations then this redundancy of three is spread very thinly in terms of check against error. Asurvey is designed for a specific purpose so that a technical or commercial objective can be achieved at a minimum cost. The major questions the planner will ask are: What is the survey for? How extensive must it be?With logistical constraints in mind, where is it? How precise and reliable does it need to be? The first of these four questions puts the last into context. The second and third require administrative answers which are outside the scope of this book. The last question is entirely technical and is the most difficult to answer.
In this context the terms precise and reliable have specific meaning. Precision is a measure of the repeatability of the assessment of a parameter under question. Precise observations usually lead to precise coordinates. Accuracy is a measure of truth. Precision is related to accuracy in that it is a practical best
estimate of accuracy because true values of survey quantities are never known, they can only be estimated. Measurement is an estimation process. It is therefore quite possible to have a set of measurements that are very precise but wholly inaccurate. Reliability is an assessment of the fact that what has been found is what it appears to be.
A distance measurement made with a tape from one control station to another could be in yards or metres, the difference between them is only about 10%. A way to be assured that the measure is in the units that you believe it to be in, would be to include measurements from other control points in the solution, so that it will be apparent that the suspect measurement does or does not fit at a certain level of statistical confidence.
Precision and reliability
On the other hand the coordinates of E may be precise because of the quality of the instrumentation that has been used but they will have zero reliability because a gross error in either of the two observations would not be detected. Consider the independent check in the context of this situation. If precision is the measure of repeatability, then reliability is the measure of assurance of absence of error.
Statements of precision and reliability may be obtained from the least squares adjustment of a survey that has been carried out. Precision and reliability may also be estimated for a survey that has yet to be undertaken, provided the number and quality of the proposed observations is known. These will then lead to an estimate of the cost of the work. If the estimate is not acceptable, then it may be possible to redesign the survey, by network analysis, or failing that, the requirements of precision and reliability may need to be reconsidered.