The Hatra complex is characterized by a high level of geometrical complexity. To carry out a global 3D survey of the surfaces of the buildings the laser scanning survey has been integrated with 3D acquisition techniques based on terrestrial and drone photogrammetric systems.
The integration between active and passive sensors allowed us to perform the 3D survey in almost any measuring condition, in order to complete, where necessary, the missing information within the laser global point cloud of the Temenos.
Furthermore, the images have been processed to obtain the orthophotos.
Drone survey has been implemented according to domestic rules of Iraq Civil Aviation Administration: flight operations has been carried out by a certified pilot authorized to flight over critical areas, expert in landscape and cultural heritage survey. Flight operations has been performed at maximum height of 60 meters AGL (Above ground level) and in VLOS (Visual Line of Sight) at maximum 200 meters from the pilot. The drone survey has been carried out choosing a consistent height AGL aimed to obtain a ground resolution between 3- and 5-mm pixel, consistent with the resolution of the laser scanning survey described before.
The flight plan has been adjusted according to the heights of buildings in order to keep a constant distance between the sensor and the surveyed surface.
The flights have been planned according to the standard photogrammetric acquisition pipeline, recording regular strips of vertical images with 80%-90% of forward overlap and 60% of side overlap.
A second flight plan has been executed in orthogonal direction respect the previous one, to obtain cross grid images useful to minimize eventual shadow areas due the building elevations. The nadiral flights has been integrated with oblique flights in order to acquire the elevations of whole complex. The image acquisition has been carried out by automatic and manual flight. In case of manual flights, the pilot used the collision sensors to keep a constant distance between the optic sensor and the surfaces. Flight path followed horizontal regular strips to acquire images with 80%-90% of forward overlap and 60% of side overlap.
Terrestrial photogrammetric datasets have been produced by using 35mm DSLR cameras equipped with appropriate lenses to respond in the best way to the environmental conditions of the site.
The cameras have been set up to consider the native sensitivity of the sensors, selecting the Adobe RGB color space and defining the format for saving images as uncompressed RAW files. During the acquisition has been used the Lastolite color checker to adjust the colorimetry of the RAW files during the post-processing phase.
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