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Archaeological Survey at Wessex Archaeology - history and developments

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.

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