Interpretive and Analytical Approaches to Aerial Survey in Archaeology

Abstract

This article discusses two contrasting approaches to archaeological survey using aerial reconnaissance. A more traditional strategy is to look for interesting spots in the landscape with a highly concentrated archaeological record. These are usually called “sites”. This concept is still used in everyday practice, despite its long-standing problematic character. The opposing approach divides the studied region into analytical units, which are sampled for evidence in a standardized manner and only then is the collected information subsequently interpreted. Varying densities of recorded facts across space are now studied rather than the binary categories of “on-site” and “off-site”. In Czech archaeology, this operational difference has often been classified as the “synthesizing” vs. “analytical” research methodology. This debate has been ongoing for quite some time in the context of field-walking and surface collection of archaeological finds. This text examines an analogous problem in the field of aerial survey, where it seems to be closely connected to another long-standing methodological and terminological discussion: the comparative usefulness of “oblique vs. vertical” aerial photography. IANSA 2017 ● VIII/1 ● 79–92 Ladislav Šmejda: Interpretive and Analytical Approaches to Aerial Survey in Archaeology 80 in mutual opposition to each other as regards their technical parameters and practical utility. The aim of this paper is to evaluate oblique and vertical aerial photographs in terms of the two above-mentioned survey strategies: synthesizing and analytical approach. 2. Oblique and vertical aerial photographs As their names suggest, the main criteria for distinguishing between vertical and oblique photographs is the orientation of the camera at the moment when the photograph is taken. Verticals are produced when the camera’s optical axis is oriented downwards, perpendicular to the horizontal plane. For practical reasons, a small deviation (usually less than 3 degrees) of the optical axis from the plumb line is generally tolerated. Obliques are captured by cameras that are tilted significantly from the vertical. We speak about “low obliques” when the optical axis is tilted no more than 30 degrees from the vertical, and “high obliques” that typically point around 60 degrees away from the vertical. In vertical photographs, the nadir (i.e. point on the ground directly below the camera at the time of exposure) is located approximately in their geometrical centre (principal point); while in the case of high obliques the position of the nadir is typically positioned outside the photo frame (Figure 1). Another significant difference is that verticals are often taken in so-called stereo pairs (subsequent frames have significant overlap of their ground coverage), enabling a “threedimensional” perception during visual analysis and offering advanced possibilities of precision mapping (Risbøl et al. 2015). Obliques are very rarely obtained in this way, their analytical potential thus being, technically speaking, more limited. Verticals versus obliques can be compared based on practical considerations of data collection and processing, but not necessarily the most important one for a full appreciation of the actual potential of aerial photographs. No image taken by an optical sensor with a central projection of rays (all conventional cameras) captures the surface of the Earth truly vertically (orthogonally), thus making what we understand as a plan or map. This radial distortion of an image due to the vertical ruggedness of the terrain is explained in Figure 2. There is no simple transformation relationship between the central projection of any photo and the orthogonal map or plan. Correction of this type of distortion can be computed from a series of overlapping images, in which the apparent dislocation of points on the individual photographs can be explained by differences in their elevation. If stereo pairs of photographs are not available, a digital elevation model of the terrain can help to re-project a photo onto a horizontal plane (Hampton 1978). Adjustments of the horizontal positions of captured data must therefore always be computed for both verticals and obliques. For this type of processing vertical photographs are much less problematic, because the perspective distortion as well as displacement due to elevation variances generally increase with the distance from the nadir. In vertical photos, these positional shifts as well as the distortions of shapes and lengths are smaller and more regularly distributed across the photo frame than is the case in high-angle obliques. However, it is clear that all photographs require a geometric correction before they are used for planimetry (measurements of distances, angles and areas). Therefore it might seem more suitable to link the difference between “oblique” and “vertical” imaging more generally with the strategy of data collecting (synthesising/interpretive vs. analytical), rather than with the type and orientation of the camera. 3. Scale of photographs Archaeologists, and especially those insufficiently acquainted with vertical aerial photos, sometimes highlight the issue Figure 1. Footprints of oblique (A) and vertical (B) aerial photographs covering an archaeological site. The crosses mark the nadirs of individual photographs, i.e. the points directly below the camera positions. Note that they are located outside the covered area in the case of obliques, while they coincide with the centres of vertical photos (after Hampton1978, Figure 9). IANSA 2017 ● VIII/1 ● 79–92 Ladislav Šmejda: Interpretive and Analytical Approaches to Aerial Survey in Archaeology 81 that the nominal scale of available vertical images is smaller than that required for fine-grained studies of archaeological heritage and that no details are visible. In many cases this is true of imagery taken for purposes other than archaeology, but in principle there should be no dramatic differences in this respect between vertical and oblique photographs, and this can be easily exemplified. To better understand this, we can consider imaging on film to illustrate the principle, even though film has largely been replaced by digital technology nowadays (Verhoeven 2007). We know that the nominal scale of an image on a film depends on the ratio between flight height (altitude above the terrain) and the focal length of the camera. When photographing the landscape using a common hand-held camera with a standard lens of focal length f=50 mm from an altitude of 500 m, we get an image on the negative at a scale of 1:10,000 (500/0.05). For hand-held oblique photography, the use of a lens with a significantly longer focal length (a so-called telephoto lens) is mostly impractical in aerial prospection because such an arrangement can capture only small views and the image is too enlarged to be held steadily in the viewfinder because of constant vibrations and turbulence affecting the aircraft and its crew during the flight. In addition, the necessity to use a fast shutter speed in order to avoid blurred images calls for a wide aperture, which may in some cases decrease the sharpness of certain parts of the picture. Hence in oblique photography we can hardly obtain a significantly higher nominal scale than the value stated above. Obtaining vertical images at approximately this same scale is not particularly a problem (for example, with the once common wide-angle aerial camera with f=152 mm from an altitude of 1,520 m above the ground). To give an example from central Europe, a limited number of verticals with this scale are available in the military archive of the Czech Republic in Dobruška (Břoušek, Laža 2006), although more frequently we can find photos there with a nominal scale ranging from 1:20,000 to 1:30,000. Nevertheless, large format negatives (18×18 cm or more recently 23×23 cm) can be enlarged without any significant loss of detail. Thus, we can conclude that in the end, we are working with enlarged oblique and vertical photographs of comparable scales (see also Doneus 1997; Palmer 2005, 103–104). Furthermore, the scale of oblique photographs dramatically decreases from the foreground to the background of the image, which, together with the distortion of shapes due to perspective, usually leaves parts of oblique photographs useless for detailed analysis. Oblique photography using medium or large format film still has the advantage that we can get a greater enlargement of the details on the positive compared to vertical imaging from a greater height, but today most oblique photographs are probably taken on small format film or, increasingly, by a digital sensor, the resolution of which has only slowly been improved to approach the standard common in analogue photography. Past studies have concluded that the necessary density of data was not present in the primary digital record Figure 2. The concept of radial distortion of an image due to vertical ruggedness of the terrain on an aerial photograph. There is no simple transformation relationship between the central projection of the photo and the orthogonal map or plan. The correction of the distortion can be derived from a series of overlapping images, in which the apparent dislocation of points a, b, c on the individual photographs can be explained by differences in their elevation. Using the method of intersecting radial lines, their correct locations A, B, C on the map can be derived (after Hampton 1978, Figure 17). IANSA 2017 ● VIII/1 ● 79–92 Ladislav Šmejda: Interpretive and Analytical Approaches to Aerial Survey in Archaeology 82 due to obvious technical limits and that digital imaging could not at that time surpass traditional film (Owen 2006; Verhoeven 2007). However, the emphasis on a completely digital workflow is strong, and there are also further benefits stemming from the use of digital technology for data collection, which will likely dictate future