Underground measuring techniques

In order to document the large number of finds in the small Hallstatt mines, researchers use a combination of state-of-the-art techniques and methods employed successfully for many years. Precise measuring is made even more difficult by the fact that the mountains are permanently in motion. However, precision is of utmost importance as it is impossible to gain an overview of the finds within the small excavation area. Documentation is therefore needed to provide researchers with a general picture of the archaeological situation at the site. This is why researchers in Hallstatt are developing new archaeological documentation methods.

Importance of documentation
New documentation methods
Tried and tested documentation methods
Movements within the rock
Geodetic measurements
3D laser scan
 

Importance of documentation

Archaeological excavations aim to uncover and recover from the soil remains of human occupancy, either to increase knowledge or to protect the finds from destruction. Archaeology is thus one of those scientific fields that will necessarily be destructive of its sources while reading from them, and it is therefore imperative that the destroyed site be documented, in all possible ways. To meet the requirement for up-to-date documentation, new methods have been used in the archaeological research work undertaken in the last ten years in the Hallstatt mine – without, however, abandoning traditional and proven methods.
 

New documentation methods

One of these innovations is geodetic below-ground measurement, performed today by means of digital total stations. Such measurements form the basis of any further documentation of finds and features according to their actual positions and create the database required to set up a Geographic Information System (GIS). The documentation of the mines, of the original prehistoric surfaces and of the finds, is further supported by digital photography and 3D laser scanning.
 

Tried and tested documentation methods

The drawing of finds and features and measurements with measuring tapes and mechanical measuring devices, the so-called Schnurzeug, are among the proven methods applied on the site. A basic feature of any archaeological documentation is a survey grid into which all features and finds can be entered. Such a grid is already available in Hallstatt, and the Salinen Austria AG has been kind enough allow access to the benchmark data that gives the absolute fix on our datum points. This network has been set up in the Austrian national coordinate system, and it is therefore possible to project every findspot below ground onto its position above ground. Several key datum points have been marked in the individual sites in the mine, and further measurements are performed on the basis of these reference points.
 

Movements within the rock

One of the major challenges of documenting archaeological finds is that the rock they are embedded in – within the Haselgebirge mountain range – is very malleable and is almost constantly in motion. Since the datum points are anchored in this rock, the motion causes them to slowly migrate, inevitably causing small absolute errors at a planimetric level. Unfortunately, there is no workable solution to this problem, and we have to ignore it. Besides, the resulting errors are not highly significant, because the findspots to also migrate along with the rock. Due to the excavation method, differing in many respects from excavations above ground, original prehistoric surfaces can rarely be documented. This is why it is mainly the dimensions of the investigation tunnel that are measured, while the actual prehistoric deposits layers can merely be recorded along the tunnel walls. The planar documentation of these layers is possible in a minority of cases.
 

Geodetic measurements

Immediately after a find has been uncovered, its position is accurately measured and documented by digital photos, drawings and sketches of the item, driving and profile walls. All this data is collected in a three-dimensional digital form in a GIS, and can at any time be accessed by PC and analysed. On the basis of this data, it is possible to establish 3D models of the findspots below ground, thus generating an image of the prehistoric Hallstatt mines that would otherwise be unattainable.
 

3D laser scan

Back in the early 1990s first attempts were made to document these unique finds in the mines of Hallstatt. The various mineworking traces were documented by drawing and detailed photogrammetric imaging, and the overall form was determined by volumetric survey of the cavity. The technology used was quite new at the time: reflectorless laser distance measurement in combination with digital theodolite.  Over weeks of demanding work, some 10,000 points were recorded and used to establish the first threedimensional model of the reopened mining chambers. By the turn of the millennium, this technology had developed into 3D laser scanning. 3D laser scanners measure the horizontal and vertical angles and the distance to a target point.

Unlike the theodolite, the laser scanner does not have to be initially aligned with a target point, but scans the entire view area of the instrument within a determined angular grid. The infrared laser beam is moved up and down via a rotating mirror, while the scanner turns around its own axis, making two to three million measurements in just a few minutes. It is combined with a high-resolution digital camera, thus allowing the simultaneous recording of the topographic form and the colour texture of the scanned surfaces in a coloured three-dimensional point cloud. Several scans are required to reach the areas that are not visible from a single point of view. The combination of the various scan positions results in a three-dimensional model of the scanned cavities in centimetre resolution. We thus obtain virtual pictures of the cavities in the mountain as well as models at various scales illustrating the impressive technical achievement of our ancestors.

(Loecker, K. – Neubauer, W. – Loew, C.)
  
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