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Surveying a Grade 1 Listed Grotto
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By Tony Rogers BSc FInstCES


View of the grotto from across the lake

Laser Scanning has become a relatively well established technique for surveying over the past few years but in some cases there are no tangible benefits to using scanning over traditional survey techniques because the additional deliverables such as the scan data, scan images and 3d drawings/models are not required by or have not been offered to the end client. This survey is an example of a project that was not only specified by the client as needing scanning, but that the scan data would constitute the main deliverable. It also shows how it is important to consider the project and be prepared to combine using different scanners and traditional survey to achieve the best survey result.
The Background

Scan cloud of the grotto exterior

The grotto is situated within the grounds of Ascot Place in Berkshire. As you look at the rock formation from across the lake it is difficult to believe that it is a manmade structure and that it is over two hundred years old. The grotto built circa 1770 is in fact a grade 1 listed building and once you get inside it you soon see why. The grotto has a series of tunnels leading into domed caverns. The walls are lined with flints and foundry slag and in the caverns false wooden stalactites covered in lime plaster and gypsum hang in geometric patterns from the ceiling. Although the grotto is complete it is in need of fairly extensive restoration and remedial works to stabilise the structure. HGP Conservation had no existing drawings or records to work from and a detailed survey was essential before any works could commence.
The Project

False wooden stalactites

The grotto was required to be fully surveyed to produce a full plan of the interior and exterior and also its immediate surroundings. Potentially this was possible using conventional survey techniques but the client required sections to be drawn through the structure so that necessary remedial works could be completed. However, as the construction, both in terms of layout and materials was virtually unknown it was necessary to be able to cut sections through the structure in any direction. By Laser scanning the structure both internally and externally it would allow the architects and their engineers to cut ‘live’ sections as required to determine the structure. The scanning also provides a comprehensive permanent archive of the structure.


The cramped working conditions in the interior of the grotto

The Survey
The survey was tied by GPS to Ordnance Survey Grid and a number of additional stations were established to allow other parts of the site to be tied to the same grid system. There are a number of other separate listed structures within the grounds such as ‘The Gothic Seat’, which was also surveyed at this time.

A basic topographic survey of the main outline of the structure was carried out whilst creating a looped traverse around, over and through the inside of the grotto to provide control stations from which the scanner targets could be observed. Prior to surveying it was necessary to get the exterior of the grotto cleared of the majority of the weeds and light foliage covering the structure and laurel bushes were trimmed at the bottom to give 0.5m of clear space for scanning of the ground surface.

Interior of the grotto

The exterior of the grotto from the rear appears as a series of stone steps spiraling up the grassy mound. The front of the grotto is a series of rocky cave entrances about a metre back from the lake edge. To be able to scan the exterior it was necessary to scan from a large number of locations from both short range but also from across the lake at 50-60m away.

The exterior was scanned with a Long Range Laser Scanner from a number of locations (including across the Lake) to provide coverage over the whole of the exterior of the grotto and adjoining waterfall. The interior was a different story as the size of the chambers and the passages meant that the scanning distance was between 0.5-5m. The smallest brick chamber is in fact only approx 1.5m in diameter.

We decided to use a high speed phase based scanner to complete the interior, as its characteristics are best suited to these short ranges.

Section through point cloud

The interior was scanned using 17 full 360-degree scans. All scans both internally and externally were targeted and tied into the control grid so that the whole cloud could be combined together. Despite the number of scans taken there were a number of small voids or holes in the coverage which was known to be a problem from the start but the data recorded was far more than adequate to facilitate drawing plans.

The Drawings
The survey of the exterior was processed traditionally and augmented with detail extracted out of the external scan data using A pointcloud ‘plug in’ to  AutoCAD to create a detailed topographic survey of the exterior of the grotto. Internally a horizontal section was cut through the scan data at a height of approx 1m above ground to give a wall line. All detail including the floor patterns, water features and seats below this point were drawn to create a floor plan of the grotto. Main ceiling features were added including all openings, arches and stalactites.

Floor plan of grotto

The client was then provided with a provisional copy of the plans so that a number of vertical sections could be decided on through the structure as this was the first time that anyone had an accurate plan showing the layout of the grotto. Five primary sections were then drawn through the grotto internally and externally. Using the survey and careful analysis on site, it has been possible for the exact layout of the brick structure hidden behind its covering to be fully determined. The Architects and Engineers have
analysed the cloud data themselves (and are continuing to do) so that a suitable renovation programme can be established.

Laser Scanning was the ideal technique to survey this extraordinary organic structure. It has provided a unique way for the people that need to work with the data to work with the survey directly and also to really get a true 3 dimensional understanding of its construction. The renovation of the grotto will take a long time and the scan data will undoubtedly be invaluable throughout this process as well as providing a permanent archive.  

References: Thanks is given to Michael Underwood of HGP Conservation for his help with this article. Images created using Pointools View.

With 16 years market experience, our expert team enables us to deliver an exceptional level of service to customers. Our reputation for dealing with difficult and unusual surveys has been maintained using cutting edge technology, namely Laser Scanning for complex, architectural, engineering and heritage projects. With our Leica, Mensi, Riegl and Faro Laser Scanners, we can now tackle a wider variety of projects.

Land Surveying: Highways, Site and Boundary surveys, Engineering surveys and Structural Monitoring.

Measured Building Surveys: Floor plans, Elevations, Sections, Rights of Light.

3D Laser Scanning: Scanner Hire or Service provision to production of 2D or 3D drawings. Reseller of Pointools software.

3D Modelling and Visualisations: 3D Models, Animations, Walkthroughs, Photomontages.
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