Section Documentation

In some ways, documenting sections using a total station is the same as recording plana. All preparatory work (marking out, section nails etc.) should also be carried out before surveying begins. It is completely unnecessary to include levelling of spot heights and measuring strings in this preparation. The surveying procedure can begin directly after the total station has been stationed. It is absolutely necessary that the section cuts are straight if they are to be successfully recorded. Sections that have bent or otherwise curved sides cannot be precisely surveyed but, on excavation, the loss of such information will not be drastic. If the section is marked out on an angle instead of straight, it can be documented in a sloping plane. Three points must be surveyed in order to determine a section plane. These will later be used to orientate the user coordinates system in three-dimensional space when processing the data in a CAD program. Individual excavation boundaries, layer contours and scattered finds etc. can then be recorded using appropriate codings. It is not practical to set up and re-station your total station for every single section. In cases such as these, sections can be measured using rods, which can be levelled with a circular spirit level and which reach up to approx. 1m above the section border (Fig 11). In this way, several sections can be measured from one total station location.

Fig.11 Section surveying using a simple section measuring rod with a circular spirit level.

An alternative to section rods is the section slide, developed by ArcTron (Fig. 12). This is especially useful when measuring larger trench sections. The section slide can be set up exactly horizontally and has a measuring pole with a reflector that can be minutely adjusted so that it stands precisely vertically. All contours and other section details can then be surveyed using a scanner pen.

Fig. 12 Top: Using the section slide for a total station survey of a deep and complex section feature. Bottom: Section survey using the section slide and a total station.

When measuring long sections, e.g. ramparts, ditches or masonry, the total station can be set up directly in front of the section and the contours can be surveyed either using a small, specialist prism mirror or a reflective foil attached to a section nail. Two people are required to carry out this procedure. Some total stations can ascertain the level of a section by themselves and this allows one-man operation of the total station (known as façade recording). Such recordings can be carried out very quickly as the total station only has to ascertain the angle of measurement. Sections can also be surveyed more rapidly and by a single person if a digital pantograph or reflectorless total station is used. Digital pantographs are presently the best-suited equipment available for section recordings. The automatically processed section recordings are then given the necessary layout functions (frames, keys etc.), printed out and subsequently coloured according to the feature. Sections that have been recorded in three dimensions can also be entered into the main plan three dimensionally. This means that they can be viewed from any desired angle to check or visualise layers which are connected to other sections, for example.

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