Digital Surveying Equipment

1. Tachymeters / Total Stations

The contours of features which have been uncovered and marked out in plana, layers or sections are recorded using total stations (also called tachymeters or tachyometers).

Fig. 3 Examples of various total stations commonly employed in fieldwork and currently used in connection with ArchaeoCAD (left to right: total stations made by Leica, Sokkia, Topcon, Zeiss).Despite differences in detail, all of them possess an integrated "field book" with a data memory.

Electronic total stations are extremely well suited to this purpose. They are easily assembled and stationed and are then immediately operational for three-dimensional surveying. The laborious task of marking out a measuring grid can be avoided. This job is almost inescapably inaccurate when carried out manually, for instance on steep hill sides. Levelling is another time consuming job that can be carried out by the total station. Another advantage is almost total independence from weather conditions. Measurements can be taken in rain and even in light snow, of which the same cannot be said for graphic documentation methods. All standard models that have an integrated or external data memory (field book) for coding the survey data are suitable for tacheometric surveying.

A total station is assembled like an electronic theodolite but has an additional device for measuring distance so that the zenith angle, horizontal direction and the distance to the measuring point are recorded. The measurement is taken using an optoelectronically aligned infrared beam, which measures distances. The infrared beam hits a prism reflector or a reflective foil placed at the point to be measured. This means that two people are involved in taking measurements. One operates the total station, codes the measurement data, aligns the eyepiece and initiates the surveying procedure and the other measures off the desired points with the reflector (Fig. 4).

Fig. 4 Recording a planum with a total station. In the foreground, coding is being entered into the total station. In the background, object contours are being measured using a prism rod.

There are also motorised and reflectorless models on the market and some which work with a combined laser, but these are much more expensive. Their greatest advantage lies in the fact that a single person can operate them. In practice, however, it has always been our experience that two people are nearly always needed, as measurements often take place over considerable distances and the points can only be poorly observed, or not at all, by a person working alone. The acquisition of a reflectorless total station is nevertheless well worthwhile when it comes to recording larger sections and, in particular, large pieces of masonry and stone features. At the moment, ArchaeoCAD supports total stations made by Zeiss (across their whole product range from routine and compact to motorised and reflectorless total stations) as well as total stations made by Leica, Topcon, and Sokkia. Other models can also be easily integrated into the system, for instance at the beginning of a project when a comparatively inexpensive total station is perfectly adequate. Total stations are usually equipped with additional programs which allow free stationing and adjustments, i.e. the apparatus can calculate it's exact location coordinates using two or more fixed main measuring points (free stationing) and can register and equalise deviations within a certain area (adjusting). Various mathematical procedures are employed to do this, which will not be focused on here.

When positioning the equipment, care should be taken to ensure that the measuring points used for orientation are at right angles to the apparatus. In this way, stationing at an incorrect or too acute an angle is avoided from the start. The appropriate instruction manual or a general surveying technology handbook should be referred to for more information, as these instruments are also equipped with numerous additional specialist functions. These specialised functions could include staking out using coordinate lists or functions for recording facades etc. On an archaeological excavation, the contour points of, for example, a settlement pit can be recorded using the mirror at intervals of approx. 10-30cm. These contour points can be coded with additional information and saved in the total station. The apparatus operates very rapidly (approx. 1-3 seconds for each measurement) and very accurately (+/- 0.25cm). Up to 2000 points can be measured in a single day's surveying on a settlement site, which is equal to roughly 400 postholes. Specific measurement results e.g. scattered finds measurements with finds label numbers, object codings for post holes, levelling values, excavation boundaries etc. can be differentiated using coding. The recorded points can then be transferred to computer via a COM interface and processed. The additional programs that come with the total stations generally allow coordinate lists to be processed. They can be imported into CAD programs in various ways. It is advantageous to use programs which can automatically connect the coded coordinate points with contour lines and arcs, enter symbols, draw excavation boundaries and link the plan to a layer structure that is specially designed for archaeological excavations (see chapter 7).

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