possible lines of sight. All the horizontal angles shown would be measured to the required accuracy to give the shape of the network. At least one side would need to be measured, say AB, to give the scale or size of the network. By measuring a check baseline, say ED, and comparing it with its value, computed through the network, the scale error could be assessed. This form of survey is classical triangulation and although forming the basis of the national maps of many countries, is now regarded as obsolete because of the need for lines of sight between adjacent points. Such control would now be done with GPS. If the lengths of all the sides were measured in the same triangular configuration without the angles, this technique would be called ‘trilateration’. Although giving excellent control over scale error, swing errors may occur. For local precise control surveys the modern practice therefore is to use a combination of angles and distance. Measuring every angle and every distance, including check sights wherever possible, would give a very strong network indeed. Using sophisticated least squares software it is now possible to optimize the observation process to achieve the accuracies required. Probably the most favoured simple method of locating the relative coordinate positions of control points in engineering and construction is traversing. Figure 1.6 illustrates the method of traversing to locate the same control points A to F. All the adjacent horizontal angles and distances are measured to the accuracies required, resulting in much less data needed to obtain coordinate accuracies comparable with the previous methods. Also illustrated are minor or supplementary control points a, b, c, d, located with lesser accuracy by means of a link traverse. The field data comprises all the angles as shown plus the horizontal distances Aa, ab, bc, cd, and dB. The rectangular coordinates of all these points would, of course, be relative to the major control. Whilst the methods illustrated above would largely supply a two-dimensional coordinate position, GPS satellites could be used to provide a three-dimensional position. All these methods, including the computational processes, are dealt with in later chapters of this book