I recently read Sarah Parcak’s “Archaeology from Space: How the Future Shapes Our Past ” and enjoyed it. It inspired me to take a closer look at remote sensing in archaeology, mentioned previously in this blog, but not really expanded upon. I thought it would be good to summarise a few learnings from the book itself, as well as from my personal digging into the topic.
Space archaeology, in this context, refers to the use of satellite imaging for applications in archaeology. These applications include, but are not limited to, identifying new sites, identifying looting pits, change analysis of a site (e.g. before and after looting), etc. You may also hear of it referred to as remote sensing in archaeology.
How to get the data
We have a variety of satellites orbiting the earth, each providing a variety of measurements. The most useful kind of satellite data for archaeologists is high-definition multi-spectral data. High-definition refers to how small an area each pixel in the image represents (e.g. 50cm), whereas multi-spectral refers to it containing different frequency bands. The high resolution imagery allows archaeologists analyse visible differences in vegetation (crop marks), soil colour (soil marks) and elevation (through shadows), as well as pin point sites on the surface from above. The additional bands allow archaeologists to highlight certain elements in the environment, such as vegetation health, soil composition, moisture, heat, etc, which can in turn bring to light the existence of a site that is not so easily identifiable by the naked eye.
High-resolution satellites are generally part of the private domain. Therefore, gaining access to such data can be expensive. Some high resolution satellites include:
- Skysat [80cm resolution] – A commercial satellite. There are, however, some freely available areas accessible via Google Earth Engine for research and educational purposes. You can contact Planet to purchase access to your area of interest.
- Pleiades [50cm resolution] – A commercial satellite which you can easily access using the Up42 platform (which includes a limited set of free credits to get you started). Alternatively, you can purchase data outside of the platform by visiting this link.
- WorldView-3 [31cm resolution] – A commercial satellite. You can contact DigitalGlobe/Maxar to purchase access.
- Geoeye[46cm resolution] – A commercial satellite. You can contact DigitalGlobe/Maxar to purchase access.
- Quickbird-2 [65cm resolution] – A commercial satellite. You can contact DigitalGlobe/Maxar to purchase access.
There are various free satellite image sources which are not high resolution. Should your use-case be covered by a lower resolution, you can check out:
- Google earth engine – free for research and educational purposes
- Open topography – not just satellite, but also aerial (from airplanes/drones)
- USGS earth explorer – not just satellite, but also aerial (from airplanes/drones)
- Copernicus open access hub
- Nasa – just a warning, the data dumps are huge as there is very limited filtering available. I played with some of their LIDAR data and there was a lot of waiting for downloads involved.
Looking for archaeological/looted sites
This is something that takes years of study, but for those wanting to get started, like me, this section aims to summarise some useful tips found along the way. First, take a look at GlobalXplorer. This is a platform that teaches the public how to identify loot pits and archaeological sites in order to crowd source the labelling of satellite images. As sites will look differently based on the environment (vegetation, soil, moisture, etc) and materials used to build the sites, there is no “one fits all” tutorial. Each campaign will provide a tutorial, as well as feedback on your selections, based on what others have selected for an image. It is a great way to satisfy your inner explorer whilst making an actual difference, by providing information needed to protect sites from looting.
Next are a couple of points worth considering when trying to analyse satellite imagery for archaeological sites. Note that these will be stated in rather general terms, as there are no globally applicable rules. Different environments require different approaches.
- Time of day matters when analysing satellite imagery. For example images taken in the morning could reveal more features than those in the afternoon, due to additional moisture.
- The season matters when analysing satellite imagery. For example, images from summer or early fall could show maximum differences in vegetation health.
- Understanding the landscape, its geology, and the building materials available in that environment helps select the appropriate analysis tools (e.g. infrared bands, temperature, etc) in order to identify features.
- Crop marks are differences in vegetation visible from above. Vegetation will grow faster, slower or not at all depending on what is beneath the ground. These can be visible in high definition images, but can be enhanced by looking at the different bands, such as infrared.
- Soil marks are like crop marks, but on soil. Soil will have a different composition depending on what it contains. For example, soil that contains organic materials is often darker, which can point to things like burial grounds.
- What works well in one part of your image might not work well across the entire thing. Therefore, you may need to combine different techniques side by side in order to get a complete picture of the features of the area.
- I’ve mentioned multi-spectral data above. However, Hyper-spectral data can also be used, which is just like multi-spectral data but with a lot more bands. Such data allows archaeologists to identify distinct activity areas on archaeological sites, because it provides information about chemical composition, mineral content, etc.
- To identify looting, you need to look for multiple 2-5 meter rounded pits grouped together. You can also look for disturbances in the landscape, such as evidence of machinery (e.g. bulldozers) or exposed archaeological features (e.g. a wall)
Here are various tools that can help you in your satellite analysis journey:
- ER Mapper – This is the paid tool mentioned in Parcak’s book.
- QGIS– This is a popular free tool.
- ArcGIS – ArcGIS for personal use is a free version of ArcGIS pro. So if you are playing around, this may be an option for you.
- Lastools – If you are specifically looking at aerial topograhical LIDAR data, this tool is more appropriate for you. It is mainly command line based, but it does have a very simple UI available. I’ll be going into LIDAR data in a bit more detail in a future post, as it was not really the focus of this one, so don’t worry if this seems out of place for now 🙂
Brief outline of Parcak’s book
This section is for those interested in reading the book from which most of the learnings in the “looking for archaeological/looted sites” section above came from. The author is an archaeologist, and founder of GlobalXplorer, who has used satellite imaging on multiple sites in order to find buildings or evidence of looting. On a high level, the book is structured as follows:
- Explanation of what space archaeology is and the history behind it.
- Stories of the author’s various archaeological expeditions and the role that satellite imagery investigations have had in those. One thing that I really liked about these, other than the stories themselves, is that Parcak discusses successful cases as well as those which didn’t go so well.
- Descriptions and references to other technologies currently used in archaeology (ROVs, photogrammetry, etc). If you have read this blog a few times before, you will see a lot of familiar names.
- Mentions of some tooling available for archaeologists (satellites, processing software, open datasets, etc). These are spread across the book, rather than there being a specific section on them.
- A discussion about looting and how satellite imagery has been used to identify looted sites
- Some short fictional stories, as a fun way to get the reader into a particular context.
- An introduction to crowd sourcing space archaeology and the story of GlobalXplorer.