Satellite imagery

We have all heard stories of spy satellites that can read a number plate (or even a newspaper headline!) whilst orbiting 200km above our heads. Given this apparent optical resolution there is a perception that satellite imagery should be used to replace or at least augment aerial photography. At the time that English Heritage last looked seriously at satellite imagery the truth was rather different in that most easily available images had a ground resolution of more than 5m. In satellite imagery this is referred to as the GSD (Ground Sample Distance), which is the distance on the ground represented by each pixel. What this means is that for a GSD of 5m one pixel covers an area of 5m from top to bottom and left to right, which means that anything smaller than 5m probably won’t show up and many features that are considerably larger will be so pixellated that it is nearly impossible to identify them.

Since that date some in the archaeological community have examined the usefulness of satellite imagery with Dr Martin JF Fowler in the forefront of such activity and improvements in resolution have meant that smaller features could be seen. Within English Heritage we are aware that imagery has moved on even further in the last few years and that much higher resolution imagery is now available. There are still issues of resolution and cost that need to be resolved, but it is appropriate for English Heritage to look again at the practicalities of using satellite imagery.

To this end English Heritage are involved in a number of groups including the cross-Government Earth Observation (EO) Forum, which includes representatives from the Department for Environment, Food and Rural Affairs (Defra), Natural England, the Highways Agency and many others with the aim to discuss "where common interest in EO lies and what activities should be undertaken to make the availability and use of EO more efficient for all." They are also involved in the UK Forum of Global Monitoring for Environment and Security (GMES). The GMES programme seeks to bring together the needs of society associated with the issue of environment and security with the advanced technical and operational capability offered by terrestrial, airborne and space-born observation systems and includes both end users of such data as well as the service providers. Whilst the general resolution of data to be provided by services under GMES is likely to be insufficient for the recording and interpretation of archaeological remains the overview of land use is likely to be helpful in setting sites in their landscape context and in assessing potential threat.

As more high-resolution satellites come on-line over the next decade English Heritage will continue to monitor their potential as an additional source for recording sites in the landscape.