An Objective Assessment of Hyperspectral Indicators for the Detection of Buried Archaeological Relics

Abstract

Hyperspectral images can highlight crop marks in vegetated areas, which may indicate the presence of underground buried structures, by exploiting the spectral information conveyed in reflected solar radiation. In recent years, different vegetation indices and several other image features have been used, with varying success, to improve the interpretation of remotely sensed images for archaeological research. However, it is difficult to assess the derived maps quantitatively and select the most meaningful one for a given task, in particular for a non-specialist in image processing. This paper estimates for the first time objectively the suitability of maps derived from spectral features for the detection of buried archaeological structures in vegetated areas based on information theory. This is achieved by computing the statistical dependence between the extracted features and a digital map indicating the presence of buried structures using information theoretical notions. Based on the obtained scores on known targets, the features can be ranked and the most suitable can be chosen to aid in the discovery of previously undetected crop marks in the area under similar conditions. Three case studies are reported: the Roman buried remains of Carnuntum (Austria), the underground structures of Selinunte in the South of Italy, and the buried street relics of Pherai (Velestino) in central Greece.

Deploying multispectral remote sensing for multi‐temporal analysis of archaeological crop stress at Ravenshall, Fife, Scotland

“…At present archaeological crop mark identification relies largely on traditional observer‐directed manual approaches, which brings with it the issues of confirmation bias and the limitations of an entirely human based visible spectrum approach to image analysis. As semi‐automated approaches to analysis are explored, it will be vital to have data that can support discrimination between archaeological regions of crop stress independent of prior sources of bias (Bennett et al, ; Cerra et al, ; Traviglia, Cowley, & Lambers, ). Whilst the change detected in both the multispectral and RGB results from this study was partially defined by visual analysis, the separability found shows promise that as analytical techniques develop (including methods such as edge detection, unsupervised classification and object‐based image analysis), change regions may be automatically rather than manually classified.…”

“…More recent studies have looked at how wider ranges of spectral information captured by both airborne and spaceborne multispectral sensors may be most effectively, and subjectively, exploited for archaeological prospection particularly in comparison to aerial photography (Agapiou et al, ; Bennett et al, ; Doneus et al, ; Traviglia, ; Verhoeven & Sevara, ). Different vegetation indices (VIs), mathematical combinations of observed wavelength bands, have been widely studied in the search for improved visual enhancements in processed images (Agapiou et al, ; Bennett et al, ; Bennett, Welham, Hill, & Ford, ; Cerra, Agapiou, Cavalli, & Sarris, ; Challis, Kincey, & Howard, ; De Guio, ; Traviglia, ; Verhoeven & Doneus, ). Over 150 VIs have been utilised across different studies (Bennett et al, ) although their comparative effectiveness for crop mark detection has been found to vary depending on observing conditions (Agapiou et al, ; Aqdus et al, ; Aqdus, Drummond, & Hanson, ; Bennett et al, ; Cerra et al, ; Challis et al, ).…”

“…Different vegetation indices (VIs), mathematical combinations of observed wavelength bands, have been widely studied in the search for improved visual enhancements in processed images (Agapiou et al, ; Bennett et al, ; Bennett, Welham, Hill, & Ford, ; Cerra, Agapiou, Cavalli, & Sarris, ; Challis, Kincey, & Howard, ; De Guio, ; Traviglia, ; Verhoeven & Doneus, ). Over 150 VIs have been utilised across different studies (Bennett et al, ) although their comparative effectiveness for crop mark detection has been found to vary depending on observing conditions (Agapiou et al, ; Aqdus et al, ; Aqdus, Drummond, & Hanson, ; Bennett et al, ; Cerra et al, ; Challis et al, ). Whilst there has been criticism that VIs are often applied haphazardly, without appropriate consideration of the vegetation properties they have been formulated to highlight, they are still commonly used for their simplicity and effectiveness in highlighting contrasts in vegetation health (Agapiou, Lysandrou, Lasaponara, Masini, & Hadjimitsis, ; De Guio, ).…”

“…Whilst there has been criticism that VIs are often applied haphazardly, without appropriate consideration of the vegetation properties they have been formulated to highlight, they are still commonly used for their simplicity and effectiveness in highlighting contrasts in vegetation health (Agapiou, Lysandrou, Lasaponara, Masini, & Hadjimitsis, ; De Guio, ). Indeed, recent work (Cerra et al, ) has evaluated a range of VIs to provide objective means of comparing information return through the application of information theory. Alternative approaches have looked at information extraction techniques beyond the use of simple band algebra, such as Principal Component Analysis (PCA) or Tasselled Cap Transformations (TCTs) to transform spectral data into new information components that minimise redundancy, or the work by Doneus et al () on visualising the red‐edge inflection point and distribution of a crop's reflectance curve (Agapiou et al, ; Agapiou, Alexakis, Sarris, & Hadjimitsis, ; Aqdus et al, ; Bennett et al, ; Cavalli et al, ; Challis et al, ; Doneus et al, ; Lasaponara & Masini, ; Traviglia, ).…”

Hyperspectral Imaging Techniques for the Study, Conservation and Management of Rock Art

“…Such subjectivity must be avoided, and hyperspectral remote sensing is an optimal tool for this.A plethora of techniques, analyses and applications for hyperspectral remote sensing have been developed in the last few decades [3]. It has been used for the documentation of murals [4], detection of buried archaeological relics [5], and exploration of their geologic [6] or mineralogic composition. It has a number of advantages [7]: It is considered a safe detection technique because it is non-contact and non-destructive.…”