Reflectance Transformation Imaging (RTI) and Photogrammetry at Mount Auburn

November 23, 2015

One of the most thrilling moments for anyone working with historic monuments is the moment of discovery. The moment when a partially illegible inscription can be deciphered, a maker’s signature found, or a forgotten detail rediscovered.

At Mount Auburn, thousands of marble monuments are slowly disintegrating in some way, having suffered from over a century of exposure to the harsh New England climate. The surfaces have been etched by acid precipitation and have eroded, resulting in a loss of once finely carved surface detail. The inscriptions and symbolism, telling relics of the people buried and commemorated here, are in danger of being lost forever. Mount Auburn’s Monument Inscription Project has worked to address these losses since 1993 by training staff and volunteers to document the details and inscriptions on our 19th century monuments. However, although much can be deciphered using mirrors and other special techniques, there is a limit to what we can see with the naked eye.

In 2014, with funding from a grant from the Preservation Services Fund for Eastern Massachusetts of the National Trust for Historic Preservation., Mount Auburn contracted with Cultural Heritage Imaging (CHI) to document several representative monuments using two innovative computational photographic techniques, Reflectance Transformation Imaging (RTI) and Photogrammetry. First, we wanted to explore the use of computational photography to document historic monuments and headstones with a greater degree of accuracy while maintaining a high level of archival permanence. Second, in the case of inscriptions on headstones that have deteriorated to the point that they are illegible, we hoped to use software to enhance any traces of the inscriptions and improve our ability to read the eroded inscriptions.

In June 2015, a team of three consultants from CHI traveled from San Francisco and spent most of three days on site. Over the course of the three days, twelve memorials were photographed using the RTI technique and four more using the Photogrammetry technique.

RTI is a two step process involving first taking multiple digital images of an object from a fixed location using a camera flash set at different angles, and then processing those images through RTI software. The software measures the light reflecting off the surface of the stone to “map” in detail the irregularities in the surface.   The resulting image is viewable through a program that allows manipulation of a virtual light source on the image, permitting the viewer to place “raking” light from any direction across the image and potentially revealing details that might not be clearly visible in natural light. While set-up to capture the digital images is relatively straightforward, certain rules must be followed in order to get a complete set of usable images. CHI provided all of the photographic and computer equipment needed, including a good quality SLR digital camera with multiple lenses, several tripods, and a laptop computer with adequate memory to store digital images saved in their RAW format.

 

Photogrammetry also involves multiple digital images that are then processed through computer software. However, in Photogrammetry the images are taken from many points relative to the object in order to create overlapping images that can be used to render the object in three dimensions. The initial digital images can be taken by a single experienced photographer knowledgeable about the sequence of images required and the processing technique. Processing times are much greater and require significantly greater computing power to achieve a high level of detail, especially when working on larger, life-size sculpture. CHI was able to complete the initial processing of several objects during the three-day visit to Mount Auburn, and then completed their work back at their computer lab.

 

There is great potential for archival documentation of headstones and funerary art using the digital imaging processes pioneered by CHI. RTI is strongest for capturing inscriptions or carving in light relief. Typical techniques for capturing inscriptions may involve multiple digital images as well as written transcriptions of inscriptions gathered using portable raking light sources. Much can be lost in the written transcription, however, and if there is a question regarding the accuracy, a second trip to the site is necessary. RTI captures the inscriptions as they exist, documenting accurately details such as the size and the font of the lettering, and spacing and location on the stone. Also captured is the current condition of the lettering. All of these details can be measured with extreme precision. These images can be used as a baseline when monitoring an object’s condition over the course of years or decades.

Photogrammetry also allows extreme precision in capturing the small details, although as mentioned, modeling larger objects or sculpture require greater and greater computing power and data storage to achieve a high level of detail. While there are multiple competing photogrammetry software programs available that offer varying degrees of accuracy and detail, the strength of this technique is that once the proper sequence of photographs is taken with a digital SLR camera, the images can be saved and reprocessed and 3D models created in the future as software changes and computers evolve. This is different from most laser scanning processes, which require proprietary software based on the scanning device. The resulting data may not even be available to the end user, and if it is it cannot be reprocessed in the future without the proprietary software that created it.

While we will still continue to rely on the staff and dedicated volunteers working on our Monument Inscription Project, these two digital imaging processes offer exciting opportunities for using the data to monitor changes and to better enable the preservation of Mount Auburn’s monuments over time.

 

 

About the Author: Gus Fraser

Vice President of Preservation & Facilities

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