Computing

The December issue of the flagship magazine Engineering and Technology features the uses of science and technology in archaeology using Wessex Archaeology as a case study.

Watch our experts explain how the techniques are used and how archaeology works in the supporting video.

Wessex Archaeology have been using digital survey instruments including Total Station Theodolites (TST) and Geographic Positioning System (GPS) survey instruments for many years now and they have allowed us to radically improve the efficiency with which we can set up and survey archaeological sites, monuments, landscapes and buildings.

Here is a brief history of survey instrumentation at WA from the very beginning to this latest development.

In the beginning...

There were tapes, theodolites, plane tables and dumpy levels. These tools and instruments have been around for centuries and whilst being tried and tested, they rely on the surveyors skill to record appropriately and process the measurements being taken. All in all, a time consuming process.

The Total Station Theodolite (TST or just TS) has been commonplace on archaeological projects for many years now and provides a robust, reliable, and importantly very accurate and precise way of recording spatial information in three-dimensions. It is basically a traditional theodolite for measuring angles combined with an electronic distance measure (EDM) and a computer/data logger for processing and storing the measurements. We use these for many tasks where accuracy and precision is of prime importance and also for recording upstanding structures and buildings.

Total Stations typically require two people to operate them: one person controlling the instrument itself and another holding a prism as a target for the laser used to measure distance. Our Leica 1200 series instruments which we have been using for a number of years now are fully robotic with remote control via radio; this means a single person can operate the instrument with the controller mounted on the prism staff and the instrument following the prism wherever the operator goes. They are also capable of operating in reflectorless mode, removing the need for a prism and allowing us to record measurements in places where it would otherwise not be possible such as high up on the elevations of buildings. They can also interface directly with Computer Aided Design (CAD) systems running on portable tablet PCs so that we can record straight into digital 3D models.

Global Positioning Systems (GPS)

Our first GPS instruments were Leica 500 series GPS units which could be used to set up survey stations ready to survey with a TST. This meant survey data could be captured using the British National Grid coordinate system rather than by using a local grid coordinate system and converting coordinates back in the office by means of a surveyed ground marker of known location such as a benchmark or trig point. We still use this approach where we need to set up TS stations where there is no other form of control.

Another way of the using the instruments is to use two in a setup called differential GPS or dGPS where one instrument is set up in a fixed location (the base station) which then provides live correction signals via radio to another instrument (the rover). This allows the rover to survey with improved positional accuracy and precision than if it was used on its own without the correction signals. To improve quality of measurement still further, the survey data can be processed back in the office against data recorded by one or more fixed base-stations maintained by the Ordnance Survey. Importantly, using dGPS in this way is far more efficient for many survey tasks as, unlike with a TS, there is no need for lines of sight across the site; as long as the instruments can see enough satellites and are within radio range, the surveyor can survey anywhere they like.

More recently we have been operating a fleet of Leica 1200 series instruments which have a number of technological advances over the older 500 series. The 1200 series instruments are smaller and lighter, use less power (so have smaller batteries), have improved antennae and are generally easier to use in the field. One version of the 1200 series which we used was entirely pole mounted, equivalent in use to the prism pole used with a TS but without the TS.

The smart solution

The biggest change to date came with the advent of SmartNet. This is a collaboration between the Ordnance Survey and equipment manufacturers including Leica. SmartNet uses the mobile phone network to provide real-time correction signals from OSNet, the Ordnance Survey's correction signal network. This uses the location information from fixed base-stations located around the country. In other words, the need for a physical on-site base-station is removed; instead, a correction signal is fed straight to the rover instrument across the internet giving differential GPS with only one instrument. Using this system it is possible to go out to site and be surveying within minutes; simply set up the rover device, connect to the internet and go!

From GPS to GNSS

Another major change is the move from the US operated Global Positioning System (GPS) to the more generic Global Navigation Satellite System (GNSS) platform. These newer GNSS instruments can recieve signals not only from the US GPS satellite constellation but also the Russian GLONASS constellation, the Chinese COMPASS constellation and soon the European Galileo constellation.

This means more satellites visible to the device, improving the quality of the location measurement. Also,  these newer satellites broadcast stronger and improved signals, allowing GNSS instruments to operate reliably in places the older GPS only instruments will not work effectively such as near to buildings or trees.

Find out about Wessex Archaeology's new fleet of Leica Viva GNSS/SmartNet survey equipment.

Web mapping has now been made even easier by the advent of platforms such as Google My Maps where it is possible to create layers from scratch or easily overlay any KML (Keyhole Markup Language) file or GeoRSS feed over Google Maps, all done through an easy to use web interface.

 A map of crime on the UK produced using the Maker! application at Geocommons.com A new site, Geocommons.com from FortiusOne, takes the concept of web mapping a step further by not only providing an online mapping application called Maker but also a spatial data library or warehouse complete with search tools called Finder. Together, Finder and Maker allow non-specialist users to quickly and easily load their own spatial data or find relevant data from the warehouse and then produce cartographically appealing maps in minutes. Data can be uploaded by registered users in simple table format for points or as shapefiles or KML for more complex geometry.

The Finder and data warehouse are an exciting development. For the first time, layers of geographic information can be uploaded and published using Creative Commons licensing. Tags are used to describe and find resources. A bit like Flickr, Scribd or YouTube but for spatial data rather than photos, documents or videos. Any data which is in the public domain is allowed to be uploaded with the onus firmly on the uploader to ensure any copyright or intellectual property conditions are met. With only this small caveat, any layer uploaded is instantly available to any registered user to create a map from. There are also enterprise options for business users to allow them to keep their data private, the aim here presumably being to make inroads into what has till now been the preserve of heavyweight desktop GIS applications.

The map Maker side of things provides the same kind of functionality as other online map creation tools but with the benefit of seamless integration with the Finder and everything being controlled through an intuitive graphical interface in the web browser. So any layer in the Finder can easily be added to a map and styled with only a few clicks with a choice of basemaps from Google Maps. In addition to simple maps showing locations of points of interest, lines or polygons with fairly basic symbology, Maker provides users with more advanced cartographic tools for producing thematic maps more akin to the sorts of thematic maps produced using desktop GIS but without the need for any in-depth background knowledge of GIS applications, programming, markup or anything complicated at all, everything being accomplished in the web browser interface. I managed to create a map showing levels of crime in the UK and one of antimony mining sites in only a few minutes (and the first thirty seconds of that was creating a user account!).

There are a couple of drawbacks however. The main problem in the UK which detracts from the ease of use is the lack of support for coordinate systems. As with the majority of online mapping tools, positions have to be recorded using global lat/lon coordinates. Whilst this makes it ideal for uploading data captured using GPS, many datasets here in the UK use a British National Grid projection, so publishing them using the Finder/Maker application requires that the coordinate system for the data must be transformed before the data is uploaded. As any GIS professional will tell you, this can be fraught with danger for novices and may require specialist software.

The fact that data in the Finder is only accessible to the Maker application or as a download is a bit restrictive; it would be nice to be able to access the layers as web-services to build into other applications. Or bring layers into Maker from other sources. Trying to do both the warehousing and mapping aspects may be great for performance but is not necessarily the best way forward; layer separation and keeping data separate from interface is way more flexible but I imagine completely impractical with todays infrastructure. Roll on web 3.0!

Of course, as with any such site based around user generated content, there will be some poor quality content and it will be necessary to be a bit discerning when choosing layers. I wasn't aware for example that the London borough of Brent has moved to the south-west, as indicated on the UK Crime map I produced using one of the layers from Finder. But in so far as providing a great set of tools for producing maps online quickly and easily and a very big warehouse in which we can put collections of geodata to be shared, it has to be a big well done to the folks at Geocommons.

Wessex Archaeology were asked to contribute an animation to the new touring version of last year's successful Making History exhibition, organised by the Society of Antiquaries of London. Over the next year, Making History will visit Salisbury, Stoke on Trent, Sunderland, and Lincoln. The exhibition will change at each venue to incorporate aspects of each region's own unique heritage.

Our animation, on show in Salisbury and South Wiltshire Museum until 3rd January 2009 (and at the end of this post!), gave us the opportunity to show some more of our work with the wonderful Stonehenge LiDAR dataset, as well as 3D laser scans of the Amesbury Archer's bones, and some new data captured with the University of Southampton's Archaeology department of WWI and WWII graffiti carved into trees on Salisbury Plain.

Lasers and Light from Wessex Archaeology on Vimeo.

LiDAR uses laser survey equipment mounted in an aeroplane to record the surface of the land below in three dimensions. The animation focuses on a field system in the Stonehenge World Heritage Site. Barely visible on the ground and in aerial photography, the features of the field system are revealed when a low level light is applied to the virtual landscape, throwing the virtual landscape into relief. The light source circles the earthworks, so their extent can be seen from all angles

Lasers can also be used on a smaller scale to study objects in greater detail. Here the skull and some of the long bones from the Amesbury Archer have been scanned. The 3D model has sub-millimetre accuracy, and can be used to study and measure their physical aspects without the need to touch the original.

3D laser scanning has also been used to record graffiti on a tree trunk on Salisbury plain on which the names of soldiers stationed there during training for both World Wars. Since they were carved the tree has grown, the bark expanded and the names have become harder to read. This visualisation shows how 3D data may be able to enhance the carvings and read the names more clearly, preserving them for the future. It may be possible to correlate the information on some of the trees with military records including dates of deployment on Salisbury Plain and the fate of the soldiers who carved their names.

We are very grateful to the Environment Agency for permission to use the LiDAR dataset from Stonehenge, and to Gareth Beale and Graeme Earl from the Archaeological Computing Research Group at the University of Southampton for processing the tree graffiti data during a hectic run-up to a season of excavations in Italy.

Find out more about our 3D laser scanning services.