Digital Surveying Instruments

2. Digital Pantographs

Total stations can only rarely be of use when recording complicated features such as skeleton graves, ceramic sherd pavements, bog features etc. and then only as a backup for documentation. The length of time needed to record each point drastically reduces working efficiency.

Instead, either photogrammetry techniques or 3D digital pantographs are used. Digital pantographs are freely manoeuvrable in 3D space, can be operated by one person and can record the survey points in real time and convert them into three dimensional coordinates. The surveyor stands directly over the feature and can accordingly trace the feature contours very rapidly. There are several differently constructed digital pantographs on the market but only the three dimensional, polar examples prove themselves to be worthwhile buying in the long run.

In the following article, I will restrict myself to describing a new system, developed by ArcTron Ltd. specifically to meet these requirements.

The Digital Laser Pantograph ALPHA

ArcTron Ltd. has been developing the Digital-Pantograph ALPHA since the end of 1996. ALPHA is a digital laser pantograph which is an "all-round" drawing instrument which can survey vectors in three-dimensional space and convert them into 3D vector drawings on a connected microcomputer. This machine is specially designed for use in archaeology and buildings and monuments heritage and is particularly well suited to the three-dimensional recording of interiors and stratigraphic or stone by stone recordings. It has, for example, many uses when recording inner city excavations and other complicated archaeological features and building monuments. The apparatus works like a total station, i.e. the pantograph measures distances and angles in three-dimensional space after free stationing has been accomplished. This is a surveying technique which has only become possible in the last few years. The precision of the laser beam depends on the colour of the ground. Light ground reflects the light from the laser better than dark ground. The laser works very accurately from a distance of up to 2m (+/- 0.75cm). Inaccuracies of up to approx. 2-5cm first appear when the measuring range is extended to a distance of approx. 5-10 m. More effective measuring lasers can be built into the pantograph but this considerably increases the cost of the instrument.

In order to construct a freely manoeuvrable machine, which can work around spatial obstructions, a robot-like arm was developed. It is flexible, thanks to several hinged joints, and is mounted with a counterweight on a solid and stable tripod. A measuring laser is mounted at the end of the arm and can be tilted and rotated in any direction (Fig. 5).

Fig. 5 - Left: Side view of the digital pantograph ALPHA version 5.0. Right: View of the pantograph from the view of the operator. In this version, the main computer with the graphics interface is mounted on the cantilever arm at eye level.

The instrument illustrated here is only produced in a limited range. The apparatus not only allows the operator to target measurement points precisely but also enables undercuts and overhangs etc. to be surveyed. Rotary encoders offset all movement in the various spatial axes. The entire pool of measurement values is then converted into 3D coordinates by seven microcomputers connected to a central computer (master/slave network). The ingoing 3D coordinates can subsequently be further processed by a portable computer (PocketPC) plugged in to the cantilever arm. Advantages lie in the small size and ease of transport of this component. Another advantage is that the PocketPC is easy to access and operate thanks to it's position at the surveyor's eye level (Fig. 5, right).

The 3D coordinates can then be converted into coherent 3D vector lines in real time and differentially processed using the specially developed 3D software program, ArcDIGIT, on the main computer. This program currently enables free and local stationing and free coding of measurement points, as with a total station, and is continually being upgraded. It features functions for triangulating, closing, copying and erasing lines. The graphics interface enables free, visual monitoring of the measurement data in 3D space. Data can be saved in various formats including all standard total station formats. It is also possible to directly import data into AutoCAD and process it using ArchaeoCAD.

A good example of one way in which this technology can be employed is the survey of a baroque tank cistern in Hemau (Oberpfalz) which was carried out by ArcTron. Using this system, we were able to construct a complete 3D model of the cistern within only a few days, although the result could only be viewed in detail on computer. During a surveying period of approx. 12 hours, over 20 000 3D points were recorded. A specialist CAD program converted the contours of the stones into 3D bodies and using these, a section was generated (Fig. 6).

Fig. 6 Baroque tank cistern in Hemau, near Regensburg, Oberpfalz. Left: Computer supported section through the cistern. Right: 3D measuring with the laser pantograph ALPHA.

The surveying instrument in question is being continually improved and is now available in the 5.0 version illustrated here. The connected 3D software, aSPECT3D, has also been considerably improved upon, and can now generate rendered 3D surfaces actually during the recording process. A further level of complexity is planned which will allow the points of three dimensional objects to be automatically and controllably recorded using additional multiphase motors.

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