Topographical survey (1990-2005)

Printer-friendly versionSend to friend

Introduction

Approximately 80 ha of mountainous area were ready to be mapped on a large scale. The excavation site of the old Greek-Roman city of Sagalassos extends for about 1.3 km East-West and 1.1 km North-South. There is a difference in height of 350 m between the highest and lowest spot on the site. The city is located at the foot of a steep flank that continues hill-upwards for several hundreds of meters until a high mountain ridge is reached.

In the 80's, the central zone of about 40 ha had been topographically measured on a scale of 1:1000 by a group of geographers from a British university. The interval between contour lines was then 2 m. The first aim of the topographical survey was to complete the existing map, primarily in Eastern direction, East of the theatre what is now known as the potters' quarter, as well as South and West of the centre that had already been mapped before. These new surveys were planned to be more detailed in scale (1:500) with an interval between contour lines of only 1 m. In a second stage and depending on the amount of resources available, the remapping of the central part on a larger, more detailed scale was planned as well.

The topographical mapping of the whole area was completed in 2005 after 16 campaigns by various teams under the direction of Prof. F. Depuydt (K.U.Leuven). The survey was done by 2 to 4 topographers each campaign. These were, in alphabetical order: W. Carpentier, F. Depuydt, A. Goemans, A. Goyens, D. Grobben, M. Loopmans, H. Roose, M. Schreurs, J. Theelen, K. Toebat, V. Van Acker, S. Van Eyck, A. Van Rompaey and H. Vanfleteren.

Topographical instruments

The method of working was set by the financial resources and technical possibilities of the early days. They gradually evolved in the course of the 16 successive campaigns. Initially, the set of instruments were optical theodolites (Wild and Sokkia) and tilting levels (Zeiss and Sokkisha) together with a semi-automatic EDM for detailed surveying (Sokkia MiNi AR). Later on, a Total Station (Sokkia) and GPS-system were used as well.

Local coordinate system

At the start of the topographical surveying, the relative coordinate system set by the English topographers for their map was adopted and used for all new surveys. This local coordinate system's cartographic north was oriented about 8° west of the real geographic north but was used nonetheless. The relative origin was located in the northwest of the excavation area. From there, a network of rectangular zones of 100 m by 100 m was set out to help position the archaeological excavations and locate the finds. In 1998, halfway through the topographical survey program, GPS measurements were used to redefine the local coordinate system. The cartographic north was 'corrected' and aligned to the real geographic north and the coordinate system's origin was shifted to such an extent that measurements in the old and new system would not be mixed up. The new origin with coordinates x: 5000 m, y: 5000 m was now located more in the center of the excavation site.

The new local coordinate system is still used today for all large scale topographical surveying on the site of Sagalassos, either by archaeologists, architects or researchers from other disciplines. For the wider area, outside the city, other coordinate systems like UTM-36N and geographical coordinates are used (WGS-84 datum). Transformations between all coordinate systems in use have been defined to help combine field measurements.

Topographical surveying method

The network of fixed geodetic points, an extension of the English foundations, was created using triangulation techniques such as trilateration and resection, realized with the before mentioned optical instruments and in a later stage verified using a total station and GPS. The intended absolute precision of 15 cm for the materialized topographical fixed points and a relative precision of 20 cm for the detailed mapping were achieved.

About sixty geodetic points, most of them materialized and numbered in the field on the rocky surface, were also used to position the Sokkia MiNi AR to do the detailed mapping. Using this semi-automatic instrument, the operator measured the distance to and difference in height with a prism mounted on a surveyor's staff in the field. Location and height were then immediately indicated on a map mounted on a plane table. Using plane table surveying, the map making was done in the field and corrections could be taken into account almost instantly. This was a major advantage compared to more traditional techniques using total station or GPS where field measurements are typically only visualized on a computer screen later on.

About ten partial maps with scale 1:500 were created in the field. These were then copied in the cartographic laboratory of the K.U.Leuven to seven large master maps of 1.5 m2 each. The scale for these master maps was kept to 1:500. This compilation of maps was realized by R. Geeraerts and A. Goyens. The whole surveyed area of the city was then digitized under the supervision of J. Theelen, who is together with Prof. F. Depuydt doing the map realization. The digital data and topographical map are also used as a base to incorporate the detailed masterplans of all archaeological sites at any time.