Internet Archaeol 15. Armin Schmidt. Remote sensing

2. Remote sensing

2.1 Measurements and data recording

In many cases the availability of data governs the archaeological questions that can be resolved. Nevertheless it is useful to consider the principles underpinning remote sensing imagery to identify requirements for data recording and digital storage. Most archaeologists use aerial photographs to identify individual features and the detail visible on conventional photographs is sufficient to resolve even small anomalies. It is the high spatial resolution of these images that allows such 'specific' use. In contrast, satellite scenes have suffered for a long time from a much coarser resolution but instead offered 'synoptic' information through broad overviews of landscapes or multispectral data for the classification of the ground's cover.

The high spatial resolution of aerial photographs can be attributed to two factors: the relatively low height of recording and the use of photographic film as a medium for 'data storage'. The disadvantage of the former is the concentration on specific features with the consequent lack of a synoptic overview, while the use of analogue recording media necessitates an additional processing step before digital data are available for computer manipulation. This processing can either be accomplished by scanning negatives and prints, or by manually digitising individual features identified on photographs. Using digital cameras would eliminate this intermediate step and facilitate long-term archiving without degradation of quality. However, to achieve an image quality that is comparable to photographic film, further improvements in the resolution of digital cameras and the storage of the large resulting data volumes are required. While it is always commendable to collect primary data with the highest possible quality (e.g. high resolution on photographic films), the much simpler handling of digital images must also be considered. It will be interesting to see how much further digital imaging has to develop before it becomes the primary recording tool.

Ground resolution of satellite images has improved considerably over the years. In 1986 the first SPOT satellites were started, and delivered data with 20m and 10m resolution for multi-spectral and panchromatic scenes respectively. Later, the Russian KVR-1000 satellite amazed users with panchromatic images of about 2-5m resolution and the launch of ICONOS in September 1999 made satellite images available with just 1m small pixels (Fowler 2000). It has therefore become possible to use satellite images for the specific identification of archaeological features. It should be noted that the impressive improvement of satellite imaging sensors was only feasible with the simultaneous development of computer systems, capable of handling the increased amount of data.

The limited resolution of satellite imagery has often been compensated for by the availability of multispectral data. By recording the intensity of reflected electromagnetic radiation at various wavelengths for each measured pixel, it is possible to characterise ground features (e.g. grass vs. wheat). Such a synoptic approach allows new insights into an archaeological landscape beyond the identification of individual features. A good example is Fowler's (1994) investigation of the area around Stonehenge where he was able to identify different types of grass covers in individual fields from LANDSAT TM multispectral data. The implications for heritage management are significant and similar applications to the wetlands of Cumbria also showed promising results (Cox 1992). Multispectral sensors mounted on aeroplanes achieve high spatial resolution, and early applications for the investigation of ancient Maya canal networks appeared very promising (Adams 1980). More recently, the airborne CASI sensors (Compact Airborne Spectrographic Imager) have opened new avenues for archaeological applications (Holden 2001). The development of improved multispectral systems was fuelled by the great success of computer-classifications, showing again the important contributions from informatics.

Most remote sensing techniques are passive as they only record the electromagnetic radiation, which is naturally reflected from the ground. In contrast, synthetic aperture radar collects data actively. A beam of high frequency electromagnetic waves (above 1GHz) is emitted from the air- or space-borne system and the returns are recorded. 'Synthetic aperture' describes the enhancement of sensor resolution by using the overlapping returns along the flight path to construct a more focussed image (Rees 1990, 159). These systems possess two major advantages, namely a defined reference signal and a potentially higher depth penetration.

The primary radar waves are transmitted with a known timing to which the received signals can be related. The recorded data therefore contain not only amplitude (i.e. intensity) but also phase information. If scenes from two adjacent flight paths (or from two tandem sensors) are recorded, these paired images can be overlaid to produce interferences or 'fringe patterns', similar to a hologram. Each fringe corresponds to a contour line and, after advanced computations, detailed topographical maps can be created. The height accuracy is determined by the wavelength of the radar signals. It should be possible to design airborne high frequency interferometric radar systems that allow the remote mapping of archaeological sites at reasonable horizontal and vertical resolution to calculate detailed digital terrain models (DTMs) that are akin to ground-based earthwork surveys.

Another advantage of the active nature of radar imaging is to make it possible to observe the polarisation of the received electromagnetic waves. This information is already used for the characterisation of some targets (e.g. trees vs. buildings) but its potential has not been fully realised in archaeology. The advanced processing of relevant radar data could help to identify classes of archaeological features automatically (e.g. grass covered earthworks vs. standing remains).

Radar waves have a shorter wavelength than optical light and can therefore penetrate deeper into dry ground. This has been shown with data from the Spaceshuttle-borne SIR-B sensors that revealed palaeo-rivers underneath the Southern Egyptian Sahara (McHugh et al. 1988). When excavating alongside the shores of these 'radar rivers' a large number of Acheulean stone tools were found, confirming the potential of the method for site prediction. The potential of radar waves to penetrate about 1.5-2m into dry ground may help to reveal even structural archaeological remains that are covered by sand if high resolution airborne sensors are used.

Another method for the creation of DTMs is the use of airborne laser ranging equipment (LIDAR). The early tests for archaeological applications were disappointing (J. Orbons pers. comm. 1999) but later results showed some promise with height accuracies of about 0.1m and the possibility for a spatial resolution better than 1m (Holden 2001). A combination of laser ranging data with stereo-pair images may allow a considerable enhancement of results.