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Surveying our past along the HS2 route

Surveyor Richard Bath and principal archaeologist and works package manager Caroline Raynor, both of the Costain-Skanska Joint Venture (CSJV), talk us through some of the tech being used on the archaeological digs along the planned HS2 route.

The complexities of the HS2 enabling works contract represent an exciting challenge for the CSJV team, particularly at Euston, where all works are taking place adjacent to the live station. Of particular note is the huge programme of archaeological works currently underway at St James’s Gardens to the west of the station. This former post-medieval cemetery site was in use between 1789-1853 and closed as a result of the Metropolitan Burial Act 1853.

Although only in use as an extra-parochial burial ground for St James’s, Piccadilly for 64 years, the site is believed to have received around 61,000 burials. An extensive programme of archaeological investigation, the largest in the UK to date, will generate a large number of artefacts and human skeletons which will be assessed and studied by a specialist team of osteologists. In order to complete the works efficiently and in line with HS2’s agreement with the Archbishops’ Council of the Church of England, the site has been enclosed by an 11,000m³ bespoke encapsulation structure.

Commercial archaeology in the UK is inextricably linked to construction and developer-led programmes of work. The recent commencement of works on HS2, where archaeology is taking place on an unprecedented scale, has given cause to reassess the use of technologies and evaluate collaborative processes between the two groups. Uniquely, this approach has been led by the construction team.

Archaeologists, particularly those working within commercial archaeology, have often been swift to adopt new forms of technology. Laser scanning, 3D-photogrammetry, and unmanned aerial vehicles (UAVs) have all been added to the archaeological toolkit over the past decade, even if they’re still used in tandem with a trowel. The benefits of UAV technology for archaeological survey and recording were recognised over a decade ago, however this has generally been restricted to more basic photographic techniques. CSJV is seeking to maximise the opportunities afforded by UAV technology in a way which supports both the archaeological works and feeds significant data into ʻLeanʼ processes to ensure continuous improvement.

A key requirement is to monitor productivity and removal of spoil within the archaeological worksite in order to achieve key dates and maximise efficiency. UAVs have been deployed by Costain-Skanska’s dedicated UAV team to generate detailed images of the site using photogrammetry. The resulting ortho-mosaic of overlapping digital images provides exceptional high-quality images of the archaeological excavation while generating measurable data showing progress on site.

Due to the vast area being excavated, CSJV tested multiple innovative survey techniques in order to identify the best method to accurately and quickly survey the daily change in volume and track the progress of the archaeological team. Early trials showed that utilising UAV technology was the most effective method. The UAV is flown about six to seven metres above the site to capture images covering the excavation. These images are then combined with ground control points to produce a photogrammetric model of the site. Survey software package LSS, augmented for point cloud data, is then used to compare the difference in volumes from one survey to the next.

The team identified the DJI Mavic Air as a suitable UAV for the task given its industry-leading safety features, such as the in-built obstacle avoidance sensors, stability sensors, and level of versatility and responsiveness within an enclosed space that was not readily available using other models. The obstacle avoidance provides a secondary level of protection (after the trained drone pilot) against the UAV coming into contact with hazards during the flight. The Mavic Air also has advanced downward facing stability sensors, which are crucial as there is an increased potential for variability in the GNSS signal to the drone when it passes through the roof of the encapsulation structure. Without the stability system, flying a UAV inside the structure would potentially be a less reliable and more complex undertaking.

The accuracy of the surveys is of paramount importance over a survey area as large as St James’s Gardens, where an error of a few percent would quickly evolve into an error of tens of metres cubed when measuring volumetrics. The main factors that affect the accuracy of the survey are:

• Overlap between the images;
• Quantity and location of ground control points;
• Lighting conditions and picture quality;
• Obstructions and objects within the survey area, in particular items which are replicated shapes such as fence panels, traffic cones, etc.

To ensure a sufficient overlap (60-80%) between adjacent images is maintained, the UAV is flown at a constant height throughout the survey. The UAV is set to capture an image every two seconds and a maximum horizontal speed of 0.5m per second is set throughout the survey. This ensures a significant overlap and the direction of travel is maintained throughout the process.

The team use specialised QR code control points which tally automatically with the given named survey control points. The centre cross is surveyed after the control points have been placed surrounding the survey area. The XYZ coordinates for the control points are then imported into the photogrammetry software and the targets are automatically recognised from the images. This mitigates the need for user input during the processing stage.

The team has found that accurate shutter speed and ISO setting for the 4K camera on the Mavic Air are important when flying under artificial LED lighting conditions. If the shutter speed is too fast, then there won’t be enough light entering the aperture and the images will be too dark. Conversely, if the shutter speed is set too slow, the images will become distorted and out of focus, thus affecting the quality of the survey.

To date, the outputs have been excellent and an efficient and quick way of monitoring the progress on site while supporting a suite of Lean tools put in place to ensure transparency between Costain-Skanska and the client, HS2 Ltd. The added benefits come in the form of enhanced images of the archaeological record, but the leading factor is demonstrating a commitment to delivering the project on time and to budget.

As well as providing a numerical volume difference, the photogrammetry surveys have been useful to record areas of site capturing features of archaeological significance.

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