Introduction The Discovery Programme’s Medieval Rural Settlement Project is undertaking an excavation at an earthwork mound in the village of Tulsk, Co. Roscommon, Ireland. The excavation is revealing at least three distinct phases of activity on site, including the remains of a large masonry tower. Work began in 2005 with the task of excavating through the large amount of rubble that filled the tower's interior. The tower measures some 20 m long and 10 m wide, and had rounded corners and a battered external wall profile. It seems to have been destroyed by the late 1500s at which time the mound was reoccupied. The refortification of the mound might be attributed to the presence of Sir Richard Bingham, Queen Elizabeth’s Governor in Tulsk in the 1590s.

Instruments and software In advance of excavation a digital elevation model (DEM) of the site was created by DGPS survey of over 20,000 height points (approx. 1 m spacing) referenced to the Irish Grid. GPS processing was undertaken using Trimble Geomatic Office software, with the DEM created using the ESRI’s 3D Analyst software. A Mensi GS101 laser scanner, controlled by Pointscape 3.1 software hosted on an Itronix pen computer was used to scan the excavation surfaces. Each surface was recorded by between four and six scans, depending on the size and complexity of the surface to be scanned, with the objective being to minimize shadow areas on the scans. Scan resolutions were generally 5 mm (at 10 m), giving scan times of approx 20 minutes, generating data sets of approx 200 MB. A portable electrical generator was used to provide a constant reliable power source for the digital equipment. A fixed network of seven control spheres was established around the excavation site, positioned to allow at least four spheres to be seen from any scanner set up. Surrounding this, a series of reflectorless survey targets were used to place all scanning within a preknown georeferenced framework. Registration of scans was done in Realworks Survey 5.1, using the automatic registration function. Georeferencing of the registered scans was also done in Realworks. Orthometric views of the RGB point cloud were generated and output as high detail tiff images. The resulting orthometric images were adjusted in Adobe photoshop to enhance image contrast and brightness. The images were then converted to GeoTIFF images using GeoTiff Examiner software, by applying the pixel scale factors and world X & Y tie points as provided by Realworks as an associated text files. The GeoTiff images were opened in ArcView 9.1 GIS software and displayed with all other relevant site survey data, e.g. trench grids. Resulting scaled plots from ArcView were printed and laminated to allow field completion and interpretation by the site supervisor to be marked before the excavation proceeded to the next level.

Why was scanning selected? The first two seasons of excavation used conventional plan and section drawing to record the excavation, processes which rely on pencil drawing – a time consuming, highly subjective method that has a low level of accuracy and a high level of error. The excavation was revealing large elements of complex stone work which the graphic survey; using tapes and planning frames, was not satisfactorily recording. We believed that scanning would be able to provide a much improved quality of record in a shorter time. Additional benefits of the scanning process would be the 3D nature of the data which would replace the need for conventionally surveyed spot heights, and which would aid visualization and interpretation in the post-excavation phase.

What problems were encountered? The Mensi scanner has a limited vertical field of view so some of the scan set ups required tilting of the instrument to view down into the trenches. Care was needed to maintain the instruments balance, and that it could still scan the target spheres. It was originally planned to operate through a wireless connection between scanner and control software, but this proved unreliable and was replaced by a fixed 5m network cable. Problems also persisted with network connection cables displaying irregular interruptions between the scanner - computer connection. In the field the problem of commonality of control spheres was solved by using a template job which contained a station set up with all seven spheres observed. In this way any 4 could be observed from each subsequent station set up. Weather restrictions were a problem with the laser scanner not being useable in the rain (due to droplets on the window, and interference of the beam).

What were the final deliverables? The georeferenced orthometric tiff images of plans and sections are considered our basic deliverables during the field phase of the excavation. These can be produced ready for field verification within 30 minutes of the final scan being completed. This rapid turn around is a vital part of the process as it minimizes the down time of the excavation team, and allows the record to be completed in the field before further excavation takes place. Initial scepticism from the site supervisors was overcome by carrying out a comparison between the laser scanning and conventional hand drawn planning methods on the same surface. The improved speed and quality was immediately obvious and the excavation team was excited and supportive about the implementation of the laser scanning method. Beyond the immediate field deliverable the challenge is to merge the point cloud records with the finds positioned by total station survey into a useable GIS ‘project’ Whist seeing immediate benefit of ‘flattening’ our data to produce plans and sections it is important that we generate the added value from the 3 dimensional data of our record. Assessment of the various point cloud meshing software is currently underway to enable the creation of correct and detailed surfaces.

Related Links

• The Discovery Programme