Castle Sween

Project Profile

Project Title :

Rectified Photographic Surveys

Client :

Greenhatch Group

Type of Work :

Historic Buildings Recording, Survey and CAD

Project Gallery 

Eileach-an-Naoimh grave   InchKenneth chapel graveslabs  
InchKenneth Chapel   Iona nunnery vaulted ceiling  
Iona nunnery architecture   Urquhart castle at night  


Rectified Photographic Surveys Historic Scotland Sites

Using Advanced Technologies to Record the Past

NAA are pleased to be working with the Greenhatch Group on an extensive programme of building recording at a number of Historic Scotland sites across the country. Using a combination of state-of-the-art laser scanning and rectified photographic technologies we will be producing a comprehensive, highly accurate record of these sites in a fraction of the time taken using more traditional survey techniques.

We have been commissioned by Greenhatch to work on several sites across Scotland including the beautiful Urquhart Castle on the north shores of Loch Ness, Castle Sween on the stunning west coast and the haunting Iona Nunnery. The surveys will provide an accurate record of the buildings as they stand and will be used by Historic Scotland to inform the future conservation, management and maintenance of these nationally important sites. NAA are proud to be involved in such a tremendous project but the task has not been easy! Not least, the range and variety of location, and nature of the buildings, has meant that each site has had its own set of challenges - representing round towers accurately in 2D (not an easy task); poor lines of sight, multi-faceted surfaces and issues of physical access (two sides of Urquhart Castle overlook the loch) - to name but a few. There are also the logistical problems of moving equipment between islands, co-ordinating the Greenhatch and NAA survey teams, short daylight hours not to mention the weather! Our survey manager, Damien Ronan was the first to face gale force winds when he ventured across the waves to Eileach-an-Naoimh in October. Not much fun for someone who is a self confessed land lubber.

To address some of the difficulties of recording, NAA have worked together with Greenhatch to develop a rectified photographic recording methodology which will allow us to produce highly accurate photographic montages which can be a record in themselves, or serve as a base for stone-by-stone digitisation. To achieve this, we have combined laser scanning, pole-cam photography and powerful rectification software.

Laser scanning is basically a means of capturing a large amount of data accurately, rapidly and in 3 dimensional space. The laser moves across the face of a building taking millions of millimetre accurate 3D recordings, creating what is known as a 'point cloud' of the scanned area. Each of the scans are tied together using a series of control points which, when tied together, create an immediate 3D model of the building. A 'slice' through the structure is then taken and a 2D plot created to provide elevations and sections. NAA use this as the framework for the next stage of the process, photographic rectification.

Rectified photography is often used as a quick method of recording relatively flat building elevations and surfaces. It involves a number of photographs being taken of a building with the image plane of the camera parallel to that of the elevation being recorded. The image is then 'rectified' using special software which stretches it to a series of surveyed control points. However, there are a number of problems with photographic rectification particularly when recording complex buildings like medieval castles. In the first instance, some of the structures were too tall to record from the ground without distortion. To combat this NAA have a series of solutions. If the building is not too tall, we have a Canon 45mm F2.8 tilt and shift perspective correction lens which will reduce much of the parallax in the photograph. If this is still not enough, then we use a pole-cam which allows us to extend the height of the camera. This proved essential given the height of some of the buildings. More traditional solutions like scaffolding or cherry pickers were out of the question given access issues and space limitations. There are also a range of other controls employed when the photographs are taken, as well as using the right camera for the job - ours is a Canon EOS 5D MkII full sensor 21 megapixel camera.

Alongside the technical solutions, we have devised a stringent field methodology in order to ensure accuracy, and make certain that the point cloud data and rectified images can be combined together to produce a good baseline record of the site. Keep watching our websites for updates, including examples of the finished product.

In the meantime, do contact Damien Ronan or Penny Middleton if you would like further information on any aspect of the project. If you have a building which needs to be recorded give us a call to discuss how we might be able to help you speed things along and cut costs by employing some of the techniques discussed above.

We are hoping to continue onsite recording in the New Year when the days get a little longer and the weather improves, although the latter might be wishful thinking; Damien and Giles, you better pack your sou'westers!