CONSERVATION PERSPECTIVES
SPRING 2017
IMAGING IN CONSERVATION
Imaging in Service to Built Heritage
A Canadian Perspective
By Christian Ouimet
An elevation of the West Memorial Building in Ottawa captured with approximately thirty individual thermography images
 

Heritage Conservation Services (HCS), a unit within the Canadian federal government’s department of Public Services and Procurement Canada, plays a significant role in assisting all federal government departments in the conservation of historic places. HCS’s task includes providing technical expertise for the protection and management of designated heritage buildings, landscapes, and engineering works. Among these are about two hundred national historic sites under the purview of Parks Canada and more than one thousand federally owned heritage buildings across Canada, as well as Canadian war memorials in Europe.

A fundamental aspect of the technical services HCS provides toward the protection and management of these sites is recording and documentation. Of the various recording methods that HCS applies to built heritage, imaging techniques are the ones most commonly used and most accessible: architectural photography or record photography, photogrammetry, and rectified photography. Photographic-based imaging techniques have evolved significantly in recent decades, providing conservation professionals with a range of tools that facilitate documentation, investigation, diagnosis, and monitoring. These tools are critical to HCS’s work, which adheres to the pan-Canadian Standards and Guidelines for the Conservation of Historic Places in Canada. These standards guide conservation decision-making, with the ultimate aim of protecting heritage values.1 This conservation decision-making process follows a sequence of actions that includes understanding the historic place, planning for its conservation, and intervening. Imaging is also an important tool for communication and dissemination.

Imaging built heritage can be challenging and very different from working in a studio or museum environment. There are many factors that must be taken into account. In Canada, one is frequently faced with harsh conditions and electricity constraints when working at remote sites. Selecting the most appropriate equipment is important in ensuring successful imaging processes, keeping in mind the required information, level of detail needed, and purpose of the work to be undertaken. Lighting is a major issue, as the photographer must work with lighting conditions present during the allocated time period, with limited options for augmentation. Available lighting must be considered, as well as the possibility and advisability of supplementation with flash photography. Since inclement weather is common, it is imperative to determine the optimal day and time to undertake the work. For challenging high-contrast conditions, images can be bracketed—identically positioning images taken at different exposures to ensure that the details in both shadowed and bright areas are captured. Using software, these images can then be combined into a single photograph by creating a High Dynamic Range (HDR) image to show all details. (HDR images often can have a surreal visual quality, however.) Accessibility is another issue, given the scale and location of some sites. HCS employs long poles, aerial work platforms, cranes, kite aerial photography, and unmanned aerial vehicles to capture the desired point of view.

Understanding the Historic Place

The first step in the conservation decision-making process as identified in the standards and guidelines is to understand the historic place. This includes—through research and investigation—identifying and describing character-defining elements that contribute to the overall heritage value of the place. Record photography is generally used to capture and graphically convey a sense of the site or space and its character-defining elements and heritage values. Such images help to itemize, explain, and communicate the important elements in heritage values assessment reports and conservation guidelines. The resulting photographs are normally contextual in nature, assisting in the understanding of a site and in planning for future documentation work. A complete posterity record is also typically produced during the first phase of a project to ensure that information is available to future generations in the event of catastrophic loss, demolition, or substantial alteration.

Record photography, which captures a specific moment in time and perspective, is the simplest form and most widely used of technical photography tools for documentation and communication purposes. It requires only basic camera skills—proper focus, exposure, and composition. Advanced photographic techniques can be utilized to augment or enhance record photography. These techniques include perspective control—using shift lenses or digital correction for the removal of perspective through rectification—and image stitching to produce high-resolution panoramic photos.

Various accessibility tools for photography used by Canada's Heritage Conservation Services
 

Another important aspect of the understanding phase is the process of investigating and documenting current conditions and changes over time. Rectified photos of building elevations are often used as a visual basis for recording conditions and then referred to for analyzing potential deterioration mechanisms. Systematically replicating an image from the same viewpoint over time can provide information on a site’s evolution and rate of physical change. This is a simple and effective method for detecting change, using a baseline record against which comparisons can be made.

Photogrammetry (see ahead) can be used to produce orthographic scaled images, from which accurate measurements can be obtained, and it can provide the basis for producing scaled architectural drawings, such as elevations and sections. The resulting images can be used for on-site inspections or condition assessments. A popular and effective use of photogrammetry is to periodically capture a building or site to monitor for movement and to determine whether conservation interventions are required.

Thermography is another imaging tool available to investigate the performance and condition of historic places. This technique can help to determine the wall composition or assembly of a building or to detect performance issues or deficiencies, such as air leakage or water damage. These images are usually presented in different colors, representing variances of temperature. For this use of thermography to be effective, there must be sufficient temperature difference between the interior and exterior of the building (winter months usually are best). This technique is normally limited to the building exterior, since temperature differentials are less pronounced for interior floor and wall partitions.

Planning

The second phase of the conservation decision-making process is planning for a historic site’s conservation. This phase, which includes selecting appropriate and sustainable uses for a heritage place, often entails carrying out impact assessments and determining the primary conservation approach. Having comprehensive record photos as a reference is a tremendous aid to the planning process. For example, HCS produced rectified images of the exterior masonry walls of the Fort Henry National Historic Site of Canada—an 1830s fortification located along the St. Lawrence River—as a basis for stone conservators to identify ashlar blocks in need of repair or replacement.

Conservation professionals also require accurate and detailed metric information, including building elevations, sections, and detailed drawings of character-defining elements, to assist in preparing drawings for architectural interventions and estimates of materials required for conservation work—such as of the stones, windows, rafters, and other architectural elements, as in the case of the rehabilitation of Fort Henry. Photogrammetry, a primary imaging technique to capture metric information, has been used since the invention of photography. Photogrammetry in its various forms relies on the manipulation of photographs using mathematical principles combined with known or calculable variables of the subject and camera to derive metric information. It involves determining the exact location of the camera in relation to the subject when the photo was taken. The process removes distortion, applies known scalar control, and matches common points from one photograph to another through triangulation to orient the images.

In recent years the field of photogrammetry has grown significantly, mainly because of advancements in photogrammetric software that provide automated identification and matching of common points in multiple images, and the refinement of a technique known as Structure from Motion (SfM) photogrammetry. SfM enables the estimation of the dimensions of three-dimensional structures from two-dimensional image sequences combined with metric information captured through a series of camera movements. Photogrammetry offers significant advantages when contrasted with other comparable techniques (such as terrestrial laser scanning) because of the simplicity of the equipment required—one needs only a camera and photogrammetric software—and its relatively low cost. Advanced photogrammetric software packages, however, can run into tens of thousands of dollars.

Intervening

The third and ultimate phase of the conservation decision-making process is intervening. It includes undertaking project work—either through preservation, rehabilitation, or restoration activities—and carrying out regular maintenance. When physical changes to character-defining elements are undertaken, they must respect and protect the place’s heritage value. The documentation work done during the understanding phase helps conservation professionals clearly recognize the elements that contribute to the heritage value, thereby helping to ensure sensitive interventions.

A fortification wall at Lévis Forts National Historic Site of Canada, Fort No. 1
 

A single photogrammetric model shows that the wall is out of plane by 0.7 meters
 

The various outputs of metric records that result from HCS imaging activities—which include architectural plans, sections, and elevations—are essential for carrying out physical interventions. For example, three-dimensional models produced by HCS primarily through photogrammetry have been used with robotic digital fabrication technologies for replacement of deteriorated stone bas-reliefs on the facade of the East Block building of the Parliament Buildings National Historic Site of Canada, in Ottawa. Documentation records produced by HCS (including those produced through imaging) were also essential to implementing conservation interventions in the stabilization of masonry walls at Fort Henry. The physical changes produced through interventions are typically recorded using various imaging techniques before, during, and after their implementation.

Dissemination

In addition to their application within the various phases of the conservation decision-making process, a range of digital imaging techniques also contribute to communicating the heritage value of historic places to a variety of stakeholders, including researchers and the general public. This use is particularly important when access to a historic place is limited or when its location is remote.

Panoramic images are an effective communication tool that can be employed to help overcome some limitations inherent in photographic equipment, namely focal length and resolution. This technique typically consists of stitching together multiple images to facilitate ultrahigh resolution and extremely wide-angle perspectives. Images can also be processed together to produce interactive 360° photo spheres, which the user can zoom into and pan around from the perspective of the camera position. More advanced communication tools, such as animations of photogrammetric models or virtual tours with the use of panoramic images, can also be produced. This information can be supplemented with descriptions or historic information, either in audio or text format. In addition, photogrammetric models can be used to demonstrate how a site is used. The animation of a heritage lock system can show how it assists boats to ascend or descend a river system, for example.

Supporting Conservation

Recording and documentation activities support each phase of the conservation decision-making process. Of those activities carried out by HCS, imaging techniques are the most commonly used and most accessible in their application to built heritage conservation. Outdoor recording of built heritage often requires special tools to provide appropriate vantage points for image capture as well as careful planning, given the often extreme and unpredictable nature of the Canadian climate. Advancements in imaging techniques, including increasingly accurate metric data and improved thermographic imaging techniques, are improving the ability of HCS to document and analyze sites and monuments.

Image-based documentation also greatly assists in providing clarity within conservation projects. Visual and metric information plays a key role in ensuring that various stakeholders gain a clear understanding of the heritage value and character-defining elements of historic places and that they speak the same language when communicating about it. This in turn plays a positive role in scheduling and project budgeting, often resulting in overall cost savings.

While technical innovations in digital imaging are collectively supporting better conservation outcomes, there are issues to address. The increasing ease of digital image capture and the proliferation of an ever-expanding range of digital image data types have brought to the forefront the task of managing an ever increasing volume of digital image data. The benefits of innovation in digital imaging are many, but accompanying those benefits are the challenges of accessing and preserving the data it produces in the future.

Christian Ouimet is a conservation technologist with Public Services and Procurement Canada’s Heritage Conservation Services. He was assisted in the preparation of this article by John Gregg and Shawn Kretz, also with Heritage Conservation Services.

1. Standards and Guidelines for the Conservation of Historic Places in Canada, 2nd ed. (2010): www.historicplaces.ca/media/18072/81468-parks-s+g-eng-web2.pdf