Documentation of Istanbul Historic Peninsula by Sensor Integration and Data Fusion

The Historic Peninsula of old Istanbul is one of the most important tourism spots of Turkey. Due to its importance these "Historic Areas of Istanbul" were added to the UNESCO World Heritage List in 1985. This area is located on the southern shore of the Golden Horn, which separates the old city center from the northern and younger parts of the European side. The Historic Peninsula ends with the Theodosian land walls in the west. The peninsula is surrounded by the Sea of Marmara on the south and the Bosporus on the east.
Due to the request of many municipality applications and due to the expected earthquake in Istanbul within the next 30 years BIMTAS, a company of the Greater Municipality of Istanbul, started the documentation of all buildings in the area of the Historic Peninsula by terrestrial laser scanning in 2006. The Historic Peninsula comprises an area of 1500 ha and approximately 50,000 buildings in crowded and partial narrow streets, which should be mapped in a time frame of two years, which demonstrates the great ambition of the project.
For this project a new production organisation was built up at BIMTAS using modern 3D mapping and computer technology. The terrestrial laser scanning group includes 24 persons, who use the following technical equipment for data acquisition: five Leica scanners (four HDS 4500 and one HDS 3000), four ILRIS scanners from Optech, four Topcon total stations for geodetic control point measurements and pre-calibrated SLR cameras Nikon D70 with 14mm and 28mm lenses for digital photogrammetric documentation. The HDS 4500 measures distances in the range of up to 50m with phase differences method, while the HDS 3000 and the ILRIS can measure up to 100m with the time-of-flight method. Due to the limited speed of 2000 points per second and limited field of view it turned out very soon, that the HDS 3000 and the ILRIS scanners are not useful for the busy and narrow streets of the project area. These scanners are more suitable for the documentation of landmarks. Thus, all buildings were scanned with a scan resolution of ~15 mm at the object using four HDS4500. For data processing of the scanned point clouds, which includes registration, geo-referencing and segmentation of the point clouds, five licenses of Cyclone 5.2 and four licenses of Polyworks 4.1 are in use in the office. In general, a well spatial (geometrical) distribution of the targets on the object or around the object was guaranteed, in or-der to achieve reliable registration of the point clouds. 80ha of the project area could be scanned within the first six months using the existing production capacity, which clearly indicated, that the scanning would need more than four years for the entire area of the project, if the current scan rate of approximately 0.7ha per day could not be increased.
As a consequence the scan rate was significantly increased by the introduction of a mobile mapping van from the Swedish company VISIMIND AB in June 2007 using a hybrid sensor system on the vehicle consisting of a terrestrial laser scanning system HDS4500, supported by GPS/IMU and digital cameras. The sensor integration and the calibration of the system in the streets of Istanbul took some weeks, but the data acquisition in the field was working since end of June 2007. The laser scanner is fixed with its orientation in the horizontal direction, scanning only in the profile perpendicular to the moving direction. It has been operated with 25 scan profiles/sec – and it has been improved later to a speed of up to 40 profiles/sec (possible maximum by instrument specification: 50 profiles/sec). The distance of the neighboured profiles was 2 cm up to 3cm in the beginning, corresponding to a speed of the van during scanning of 0.5m/sec up to 0.75m/sec or 1.8 km/h up to 2.7km/h. Due to problems with the reception of the GPS signal in the streets of the Historic Peninsula control points were marked on the buildings in a distance of five meter on each side of the street. Nevertheless, the technical parameters of the hybrid systems were opti-mised on the job due to problems with the quality of the pre-processed point clouds.
The geo-referenced point clouds from the laser scanning group are used for line mapping of the facades in a plot scale of 1: 200. The line mapping group consists of 34 operators using currently 34 licences of the MenciSoftware Z-MAP Laser from Italy, which is able to process laser scan data and rectified photogrammetric images simultaneously for line mapping with limited Auto-CAD functionality. It was estimated that approximately 5 million square meters of facades have to be mapped. The current production rate of one operator is 65ha/day, which indicates that the line mapping can be finished within three years.
Additionally, the roofs of the buildings are measured by a roof mapping group of five operators with Z-MAP Foto using scanned colour aerial imagery with a GSD of 10cm. In this part of the project it was an essential task for the quality of the data to combine the two different data sets from aerial imagery and from mobile TLS to one common data set in the same coordinate system without any discrepancies from the different data acquisition source.
The whole production line for the documentation of the buildings in the Historic Peninsula of Istanbul is described in the paper. Although the production line was establish on the job during production, it was always essential to optimise the technical solutions in order to meet the requirements for the quality of the generated data and for the deadlines of data deliveries.