Multispectral Survey

UAS Multispectral Surveys have the capacity to detect sub-surface features with archaeological potential, even on sites hidden beneath crops.

Multispectral surveys involve a UAS equipped with a 5-band sensor array which collects images at different wavelengths using reflected light. These bands are green, red, red-edge, near-infrared, and RGB, the latter of which can be processed to extract the blue band.

For a survey area covered in vegetation, the reflectance represents the amount of chlorophyll present in the plants; an indicator of the plants’ health. Since plant health can be influenced by sub-surface features, multispectral surveys can show where these features are by the appearance of the plants in the processed image.

For stripped or bare earth survey areas, the reflectance can represent soil organic carbon, soil moisture, or soil texture/density.

This method can be used to complement geophysical surveys, and even serve as an alternative option when crops inhibit access on to site.

Above : Dense crop preventing geophysics ahead of a multispectral survey.

Raster Transformations

The multispectral orthorectified photo can undergo a series of raster transformations, according to different Vegetation Indexes (VIs).

VI formulae combine surface reflectance using two or more wavelengths that are designed to highlight a particular property of vegetation, which may then indicate subsurface variation. These formulae are applied to multispectral images to produce a variety of indices. We typically carry out 9 transformations including: Normalized Difference Vegetation Index (NDVI) which quantifies the presence of living green vegetation using reflected light in the visible and near-infrared bands; and Red Edge Normalized Difference Vegetation Index (RENDVI) which uses reflectance measurements in the Red Edge (RE) to establish changes in vegetation health.

The various transformations provide a detailed account of plant health and soil composition, allowing for greater understanding of sub-surface potential.

Above : Various vegetation indexes.

Thorough Approach

We have the capacity to capture RGB and multispectral data simultaneously, enabling us to conduct an earthwork and subsurface survey at the same time. With the ability to record upwards of 100 hectares in a single day, we can very quickly and efficiently provide detailed knowledge of an historic landscape.

Both datasets will be georeferenced to OS coordinates, generally achieving an error of <3cm and a ground sampling distance of 1 – 3cm / pixel.

The RGB survey will produce an orthorectified photo, Digital Elevation Model (DEM), and 3D model of the site, to complement the various transformations from the multispectral survey.

Above : Orthorectified photo of a large landscape.

The various multispectral and RGB outputs are analysed to produce an interpretation figure, highlighting areas of potential archaeology. This forms the basis for a comprehensive discussion, outlined in an interpretive report.