Scanning in tight or constricted structures can be a challenge; fortunately, modern, mobile-mapping technology can help. This cutting-edge technology was recently put to good use by an MSt student and geospatial imaging analyst to survey the intricate and hidden spaces of one of the largest and most magnificent monastic gatehouses in Britain.

Mobile mapping

Thornton Abbey was one of the richest Augustinian monastic houses in Britain, founded by wealthy landowner William Le Gros in the 12th century. The surviving 14th century gatehouse is one of the earliest surviving brick buildings in the UK. Today, the gatehouse is in excellent condition and is a popular tourist destination, thanks to the guardianship of English Heritage.

In 2016, English Heritage and Historic England were approached by Rodolfo Acevedo Rodriguez, a student at the University of Cambridge who is undertaking an MSt in Building History. Rodolfo is conducting a historical and archaeological evaluation of Thornton Abbey gatehouse and its long-debated history. The focus of Rodolfo’s work is in studying the unique architectural design of the gatehouse and how the layout of the structure has evolved throughout its 600-year history. In particular, Rodolfo is keen to understand how the building was designed and how it was inhabited in the late-Middle Ages: how rooms were divided or connected, where floor levels changed over time and what services were incorporated into the building structure. The three-storey gatehouse structure contains a myriad of tight spaces, spiral staircases and rooms, some of which are only accessible via ladder.

The challenge

A traditional static scanner would require a large number of separate set-ups, increasing both time on site and project costs. It would also prove impossible to firmly set-up a tripod scanner on a spiral staircase or to scan inside smaller, more enclosed spaces. Due to this complexity, the decision was made to use a mobile laser scanner to collect the internal dimensions.

Rodolfo joined forces with Senior Geospatial Imaging Analyst at Historic England Jon Bedford in order to survey the structure. The team used a Leica P40 static terrestrial scanner to collect the external dimensions of the gatehouse, while a GeoSLAM ZEB-REVO and ZEB-CAM were used for the internals (the two datasets would be aligned post-processing in order to obtain a complete scan).

Tight spaces, spiral staircases and lots of high-level rooms (requiring ladder access) were among the reasons for going for the REVO, and, most importantly perhaps, time constraints.

Scan in processHandheld, mobile scanners are ideal for indoor environments

The survey

Having never previously used a mobile, handheld scanner, Rodolfo was keen to understand the practical use and workflow of the unit. Fortunately, Technical Support Engineer at GeoSLAM Michael Dutch was on-hand to provide Rodolfo with a "crash course".

Before the survey began, the team carried out a pre-scan assessment of the building in order to establish the optimum scan-route and ensure that no areas were missed out. The scanner and camera were initialised on the ground floor, a simple process taking 40 seconds or so that necessitates keeping the scanner static on a surface in order for the on-board inertial measurement unit (IMU) to establish a local co-ordinate system. Once given the green light, the scanner was picked up to begin the survey. The team walked through the building, following the pre-planned route, maintaining a steady walking pace throughout.

The beauty of mobile surveying is in the speed and flexibility of operation. In areas of less detail, the user can walk faster, whereas for highly-detailed stonework, the user can stand still, collecting a larger number of data points and producing a denser "point-cloud". In total, the time spent scanning the building’s internal structure was under one hour.

The results

3d mobile mapping captureThe internal and external scans were aligned together to form a complete cloud

The raw scan data was processed on-site using GeoSLAM Desktop processing software and was viewed immediately after scanning. Using post-processing software, the internal REVO data and external Leica data were aligned.

Having the internal and external scans together means we can understand the build-up between the inner and outer surfaces rapidly and in three dimensions. Since Thornton Abbey has so many chambers and passages, it would take months to measure and draw up plans and sections. We had no idea the first and second floor latrines shared the same chute — it was only when we analysed the internal scan data that we saw that these chutes were interconnected; this tells us that the building was very cleverly designed.

Rodolfo will be using the three-dimensional scan data to create architectural drawings of the building to include in his thesis. So, is Rodolfo a convert to mobile laser scanning? We let him have the last word: “I thought the scanner worked marvels. It was so brilliantly suited to this type of building — I could not believe my luck”.

All images are courtesy of Historic England, English Heritage, Rodolfo Acevedo-Rodriguez.

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