Satellite Imagery, Multi and Hyper-Spectral Data
However, in England, where aerial photography is readily available, the limitations of cost and resolution coupled with poor weather conditions that limit the number of cloud-free scenes had meant that satellite imagery was rarely used.
Advances in the resolution of imagery over recent years have meant that this is an area that deserves further investigation. The related technology of multi-spectral (M/S) data captured from airborne systems has shown considerable potential. Historic England continues to monitor developments and support research in this area.
Resolution is the key
Online tools such as GoogleEarth and Bing Maps are providing imagery to a wider audience, however, the most detailed views available on these systems are actually derived from aerial photographs.There are a number of satellites currently in operation which function at a resolution that may be suitable for archaeological research.
These include IKONOS, Quickbird, WorldView and GeoEye. Historic England is involved with wider partnerships that monitor developments including the Forum for Earth Observation Applications and GMES.
Another potential area of interest are constellation systems which do not have the highest spatial resolution, but do have a very high revisit rate allowing a site to be monitored on a regular basis.
Historic England is investigating the possible use of these high temporal resolution satellites to help predict the best time to carry out aerial reconnaissance.
Multi and hyper-spectral data
One particularly useful factor that satellite imagery has over conventional photography is that most provide multi-spectral data., Rather than just the standard visible spectrum, Near infra-red (NIR) and several other wavelengths are recorded.
By manipulating the different wavelengths it is possible to reveal features not otherwise easily discernible.. This is particularly valuable for environmental monitoring, but also has some potential for archaeological survey.
Multi-spectral data can also be captured from airborne platforms, and the higher resolution data available from systems such as CASI is more useful for archaeological purposes than satellite sourced data.
As well as multispectral (M/S) data airborne platforms (and indeed a small range of satellites) also have the capacity to collect hyperspectral (H/S) data. A multispectral sensor may have a number of discrete bands within the spectrum from the visible to the longwave infrared.
A hyperspectral sensor, in contrast, will deal with numerous narrow bands over a continuous spectral range. This allows the sensor to produce the spectra for all pixels in a given scene and allows much more subtle and varied analysis.
Increased resolution in both M/S and H/S sensors over the past few years have again made this an area that is of interest to HE.
Historic Places Investigation