Appendix B: Using Photogrammetry and Geographic Information Systems (GIS) to Draft Accurate Plans of Qazion

Surveying the site of Qazion presented some not inconsiderable challenges. Given its location within the boundaries of an active Israel Defense Forces military training ground, access to the site was limited to specific days when live shooting exercises were not scheduled. Since conventional survey techniques for a site of this size and complexity could not have been completed within the allotted time, it was realized that a methodology had to be developed that permitted the off-site drafting of architectural plans using high-resolution aerial photography. The resulting solution necessitated recording the site by low-level aerial photography, which would provide the basis for generating accurate architectural plans. In working within the requirements to undertake on-site recording in restricted access windows, the strategy required two on-site visits, one to capture the aerial photography and another to establish survey control, a series of reference points used to geolocate the aerial photographs across the site with reference to the imagery.

Surveying and Architectural Drafting

During the initial visit to Qazion, aerial photographs were taken using a camera suspended beneath a blimp-shaped moored balloon from elevations reaching up to 60 meters (Fig. 1). The photographs were taken from a limited number of camera positions and concentrated on the Main Cultic Structure. Since the photography was undertaken before the full-scale development of a survey strategy, the range of photographs was not tailored to fit with any particular aerial mapping technique. This issue had implications when devising a specific survey methodology.

The site was revisited six months later in order to both establish survey control and create a ground-level photographic record of all major architectural forms. The survey was executed using two total stations linked to a control network established by closed traverse methods and geolocated in real space through cross-referencing the network to a projected coordinate system(WGS 1984, UTM Zone 36N) using a survey-grade GPS unit. Having established the spatial network, extant masonry was compared with similar points visible on aerial photographs. Only points in the field that clearly correlated with features depicted on the aerial imagery were surveyed (Fig. 2). The point coordinates were recorded with a total station and their positions were annotated on the balloon images. In all, 536 points were identified and together provided an accurate survey control for the photographs. Additionally, nearly 300 ground-level photographs were taken to act as an added reference for the processing of the balloon imagery and subsequent generation of architectural plans.

The first task, with respect to the off-site analysis, included processing the balloon imagery and associated

Fig. 1.

The SkyView blimp-shaped moored balloon employed to take high-resolution low-level aerial photographs of Qazion. (Photo by A. E. Killebrew.)

Fig. 2.

An aerial photograph depicting the Main Cultic Structure with survey control points indicated. (Graphic design by J. Quartermaine; photo by SkyView Photography.)

control points into an accurate photorealistic rendering to serve as a platform from which the architectural plans would be created. Given the quality of the Qazion imagery and subsequent survey, it was decided to try both two- and three-dimensional techniques to produce such a rendering.

Agisoft PhotoScan

The three-dimensional technique entailed the application of photogrammetry, which is an old and long-established survey method that uses photographs to make reliable measurements, and has been enhanced by software packages such as Agisoft PhotoScan. This package has great potential for archaeologists, as it automatically establishes relative camera positions by matching points of detail in the overlap of aerial images using an algorithm known as Structure from Motion (SfM). The program then uses these points to create an accurate three-dimensional model of the ground surface. The model is then textured by draping the aerial photographs over the model resulting in a photo realistic three-dimensional rendered output of the ground surface. Because of the program’s potential and the success of recent archaeological case studies, PhotoScan...