Charles Fort

The survey utilised a calibrated Leica camera to photogrammetrically record the details of the cannons and the fort walls as viewed from the sea. An interchangeable black and white target system was developed to function with both laser scan and photogrammetric surveys. Targets were located onsite in a position where the laser scanner could capture them safely and without risk to the equipment. A substantial overlap of the two data-sets would ensure that the targets would be present in both the photogrammetry and laser scan data-sets, linking the two surveys to the same Cartesian grid. The photogrammetric survey utilised over 500 images of the cannon site and the sea wall and it took just under an hour to survey the entire site.

This survey is part of a larger initiative to record important structures within the UNESCO World Heritage Site of the Bridgetown Garrison, Barbados. The project was set up as a partnership between the York Archaeological Trust and The Barbados Garrison Historical Consortium. In May 2013 David Strange-Walker and I travelled to Bridgetown with a Leica HDS7000 to record three heritage sites.

Below the sea wall of the fort is a small beach which is rich in archaeology. Decommissioned cannons from the fort can be seen buried in the sand. The cannons are also abundant offshore, visible on the sea bed lying in shallow water. This unique archaeological site lies on the tidal interface between the beach and the surf. At low tide it is possible to gain access to the beach and to use the laser scanner to record the cannons; however the tide did not recede enough to allow the use of a laser scanner to survey the entire site, to do so would have involved setting up the tripod in the surf, which was not an option. Apart from the obvious risk of damage to the scanner from salt water, the motion of even the smallest of waves on the tripod and the sand it was bedded into would have a dramatic effect on the ability of the scanner to collect data from a level position. The constantly changing level position would render the data inaccurate and fairly useless.

Photogrammetry can provide a solution to this problem.

I have used photogrammetry in combination with laser scanning on several projects and the results have been encouraging. With the correct survey control it was possible for a person with a camera to wade into the shallow waters of the surf and conduct a photogrammetric survey of the fort’s sea wall and the partially submerged cannon site. I had some trepidation about the shifting reflections of the water and the movement of the waves which would be different in every image. Discrepancies between image sets can have unpredictable and undesirable results in the final data-set, however if there are enough strong geometric anchor points in the survey then this exercise would produce some accurate 3D data

The objective of this survey project is to analyse the structural evolution of the fort using the 3D survey and historical documentation to determine which elements of the structure relate to specific periods of development. Where possible the structural remains of this coastal fort were recorded using the Leica HDS7000 laser scanner. The laser scan survey methodology also included HDR imaging for the texture mapping of the pointcloud data, adding real world RGB colours to the data-set.

A major benefit of this technique is the speed at which complex archaeological sites can be recorded. This rapid technique proved to be a useful factor in recording the cannon site. The returning tide would cover the site completely and shifting sands buried the cannons so that they were hidden from view even at the next low tide. These events made it imperative that all visible traces of the site were recorded when they were seen. There was no guarantee that they would be visible at the next low tide or even if they would be visible again in the near future.

A system of groyns/breakwaters designed to create sandy beaches for tourists could be a contributing factor to the build up of beach deposits on the site and may eventually result in the archaeology being completely buried.

The results of this photogrammetric survey were processed in Agisoft Photoscan pro. Coordinates from the laser scan were extracted for each of the targets and were identified in Photoscan as ground control points. This process ensured that I had 3D location compliance between the two data-sets and that the point clouds generated from the photogrammetry would fit seamlessly with the laser scan data

The 3D pointcloud of the cannon site had some noise created by the moving water and reflections, however as predicted the geometry of the features which remained above water was sufficient enough to produce an accurate data-set. Aggressively filtering the data removed points which were not part of a rigid surface and points which were poorly referenced in the image set. Consequently the automatic filtering process reduced the noise in the data-set to an acceptable level. A 3D wireframe model was created from the remaining points. An RGB texture map was made by creating an average RGB value for each point dependent on the average value of the points corresponding pixels spread throughout the image set. Averaging the texture helps reduce the colour distortion created by the movement of the water and reflections.

Some of the cannon which lay on the site have been lifted from the sea and conserved. These cannon now serve as ornamental features on the Savannah, the former parade ground of the garrison. Cannon which share similar characteristics to those on the seabed were identified on the Savannah and 3D recorded using photogrammetry. Unusually these cannons are set on original carriages and are an excellent example of what the guns of the fort looked like when these cannon were in place.

This project is an excellent example of how photogrammetry can be used to create detailed and accurate data-sets that complement those obtained through laser scanning. When incorporated into a sound survey strategy, photogrammetry offers a powerful and accessible way of 3D recording archaeological sites that are not suitable for laser scanning.