Reconnaissance is a vitally important part of any survey project, as emphasized in Chapter 1. Its purpose is to decide the best location for the traverse points.
Successive points in the traverse must be intervisible to make observations possible. If the purpose of the control network is the location of topographic detail only, then the survey points should be positioned to afford the best view of the terrain, thereby ensuring that the maximum amount of detail can be surveyed from each point. If the traverse is to be used for setting out, say, the centre-line of a road, then the stations should be sited to afford the best positions for setting out the intersection points (IPs) and tangent points (TPs). The distance between stations should be kept as long as possible to minimize effect of centring errors.
Finally, as cost is always important, the scheme should be one that can be completed in the minimum of time, with the minimum of personnel. The type of survey station used will also be governed by the purpose of the traverse points. If the survey stations are required as control for a quick, one-off survey of a small area, then wooden pegs about 0.25 m long and driven down to ground level may suffice. A fine point on the top of the peg such as the centre of a nail head may define the control point. Alternatively, longer lasting stations may require construction of some form of commercially manufactured station mark. Figure shows the type of survey station recommended by the Department for Transport (UK) for major road projects. They are recommended to be placed at 250-m intervals and remain stable for at least five years. Figure 6.15 shows a commercially available earth anchor type of station. Road, masonry or Hilti nails may be used on paved or black-topped surfaces.
Anchored survey station