Photogrammetric methods for recording archaeological structures at Zeviya Tivilki on the Upper Tigris, Turkey

Erkan Atay & A. Tuba Ökse

Introduction

The construction of large dams — and consequently the creation of extensive reservoirs — has obvious implications for the archaeological heritage and requires rapid archaeological intervention within the salvage projects set up in mitigation. This is the case of the Ilisu Dam, which will affect the Upper Tigris region in a few years. The time allowed to excavate the sites in the construction area (Ökse et al. 2008) is even more limited, requiring us to carry out the investigations very rapidly. As a result, we have developed a way of recording structures, using innovative methods of photogrammetry, to document quickly the plan and profiles of a single-storied building complex at the Iron Age settlement site of Zeviya Tivilki (Ökse et al. 2010). The complex consists of foundations of rough stones set directly on the bedrock.

Figure 1
Figure 1. The hanger.
Click to enlarge.
The equipment

A digital camera is attached to a portable hanger for recording plans, and to a tripod for recording profiles of trenches and architectural features. The hanger allows overhead photographs to be taken (Figure 1). It consists of a vertical upright and a horizontal arm made of lightweight aluminium. The upright is constructed of two pieces of 4 x 8cm aluminium profile at least 6m long, held together by hinges. A pod fixes the lower end to the ground. The movable arm passes through the upright 80cm below its upper extremity. This arm is hinged and can be closed for transport.

To attach the remote control camera a free-moving nut and bolt is attached to the end of the arm. A 23cm long and 3cm wide flat iron rod is fixed to this; the camera's pod is screwed to its other end, so that the camera can swing like a plumb-bob to adjust itself vertically for overhead photographs. The arm is raised or lowered by means of a cord running through pulleys on the arm and at the top of the upright. A system used at Tell Atchana (Akar 2009) is operated by a camera running along on a metal wire between two bars (http://www.alalakh.org/photosystem.asp); but we believe that the hanger provides an easily handled alternative.


Figure 2
Figure 2. Erecting the hanger.
Click to enlarge.
Figure 3
Figure 3. Positioning the hanger.
Click to enlarge.

Figure 4
Figure 4. Portable handling of the hanger.
Click to enlarge.

To cope with the distortion caused by wide-angle lenses, a long focus lens with a narrow angle view is preferred. On the other hand, too long a focus flattens the image. Taking into consideration the time and labour required to photograph the area in smaller segments, we opted for a multi-purpose 18–200mm zoom lens at Zeviya Tivilki. The photographs were taken at 300 dots per inch (dpi), providing sufficient image resolution, and saved into Tagged Image File Format (TIFF). By connecting the camera to a computer via a device such as a WT-4 wireless transmitter, the pictures are transmitted directly to the computer, enabling us to simultaneously control the quality of the image.

Figure 5
Figure 5. Merging the photographs and resulting plan.
Click to enlarge.

Figure 6
Figure 6. Recording a profile (scale rods on photographs: 1m).
Click to enlarge.
The plan

To create a plan, the area should be gridded and divided into sub-units depending on the inclination and dimensions of the area in order to achieve the best right-angle overhead shots (Figure 5). If the shooting angle is not correct, this will cause distortion. The coordinates of the sub-units in the field must be clearly visible as reference points in the photographs to allow subsequent digital merging of the photographic segments. The archaeological features must of course be carefully cleaned, so that the contours and details can appear in the photograph. The soil should be slightly dampened so as to enhance their different colours.

The profiles

For photographing profiles the camera is placed on a tripod positioned facing the profile at a place allowing right-angle shots to be taken. Very long and high profiles should be divided into sub-sections to avoid distortion on the sides. Coordinate points should be marked at a suitable elevation along the profile, so as to correlate the image to the datum during merging. The contours and details of the wall or trench profiles should be cleaned carefully and slightly dampened in order to obtain the clearest images. The photographs should be taken adjusting the focal points of each segment separately, and the edges should be superimposed during merging so as to avoid any distortion of the composite image. The key point here is to take photographs of each segment with respect to protruding surfaces in order to avoid blurred images; in other words, each section should be photographed adjusting the focal distance according to its surface character.

Digital processing

The photographs bearing at least four coordinate points are digitised using a Computer Aided Design (CAD) program, such as Netcad (used in our project) or Autocad. Obviously different programs will have their own procedures, so here only general indications are given. The image files are imported into the CAD environment; the reference points on the photograph must be registered and their values given. A tolerance of up to 1cm is acceptable for the merging process. After this transformation, the photograph is saved in the CAD program and made ready for drawing.

We believe that the equipment and system described above and first used by us during the 2009 excavations at Zeviya Tivilki offer a rapid and workable means of recording structures on archaeological sites.

Acknowledgements

We wish to thank our team members Murat Eroglu, Ahmet Günes, Yeliz Atay, Aziz Ayhan Bayraktar, Sibel Torpil, Adem Öncü, Adem Yücel, Yücel Erdas, Vildan Gürdil Öncü, Hüsniye Levent, Zerrin Hamioglu and Ibrahim Halil Can who contributed to the development and processing, cartographer Hamit Akan for the digital plan and Tulu Gökkadar for translation.

References

Authors

* Author for correspondence

  • Erkan Atay
    Museum of Mardin, The Ilisu Dam Construction Area Co-ordination Centre, Mardin, Turkey (Email: erkanatay10@mynet.com)
  • A. Tuba Ökse*
    Department of Archaeology, Kocaeli University, Kocaeli, Turkey (Email: tubaokse@yahoo.com)