Can you tell us what photogrammetry is?
Photogrammetry is at its most simple a method of measuring distances by comparing
how features change between multiple images of the same object or area of space. It is, in
effect, an application of trigonometry. Today, with the mind-bogglingly advanced processing
power of even the average home computer, software exists which is able to run many millions of these calculations across thousands of images, and use the network of values to compose an interwoven network of point values, each representing a minuscule element of the photographed subject distanced in relation to all of the other minuscule elements the software can identify.
Today, photogrammetry as a term almost always refers to photogrammetric modelling,
whereby this network of distances is converted into a digital 3D geometry, creating in effect a digital reconstruction of the volume of a photographed subject. To this geometry a texture
derived from the colours in the photographs may be applied, mapped across the surface,
capable of creating a visually identical digital replica of the photographed subject. These replicas - sometimes referred to as digital surrogates or simulacra – open up a whole new world of potentials to archaeology, museum studies, and the global distribution of heritage.
How is photogrammetry expanding the field of archaeology and is it paving the way for further innovation?
Slowly but surely, photogrammetry is beginning to succeed sketching, planning, and
photography as the primary mode of visual representations of archaeological sites and finds.
Photogrammetry allows for the recipient of the model to inspect, peruse, and analyze the
depicted materials in a way not previously possible with other methods of representation. A plan is necessarily the interpretation of the planner, and although maximal accuracy is always the goal, is inevitably reliant on the acuity of their eyes and the skill of their hands in representing what is before them, and requires the planner to make on the spot decisions on what to include and what to ignore. Photography, though in theory less subjective than planning, necessarily represents only one face a subject, selected by the photographer, and open to tricks of perspective warping by the camera.
Photogrammetry is able to avoid some of these pitfalls. It is, when executed correctly,
capable of a degree of precision down to the millimeter, and represents directly what was
photographed, rather than selective depiction of a planner. Like photography, it theoretically
shows what is there, free of interpretation – although in practice this is of course idealistic. But unlike the photograph, the photogrammetry model is capable of inspection from whichever distance, angle, or even distortion the viewer desires.
The geometric nature of 3D models permits a degree of analysis not possible from
photography, such as the analysis of volume. It is possible to precisely calculate the volume
excavated from an excavation trench, for instance, even if the trench slopes, has an undulating surface, or contains architectural features which would render manual calculations highly impractical. Excitingly, by creating a developing series of photogrammetry models of an excavation, geo-referencing them and layering them on top of one another, some researchers are experimenting with a kind of ‘virtual re-excavation’ where every step of the excavation process can be reviewed and considered by interested scholars, and the decision making process behind the analysis of the site is made clear for all to see.
To what extent is photogrammetry currently used within the field of archaeology and museums?
Photogrammetry is becoming every day more common in field archaeology. Five to ten
years ago it was the remit of well-funded, renowned, and large-scale research excavations.
Today, at least within Australia, any moderate commercial excavation of an historical site will be using photogrammetric modelling as part of their research, recording, and public outreach.
Museums are beginning to bring the method into the core of their online presence, with
major collections – most notable the British Museum – storming ahead with modelling their
collections and uploading them to platforms such as SketchFab as an integral component of
their public outreach, making their collections accessible to people across the globe.
The creation of such models holds great potential for facilitating inter-institutional
collaboration and research, with the possibility of sending the schematics of their scanned
objects to collaborating schools, universities, or museums across the globe, with the potential for these replicas to be 3D printed – and perhaps even included in the displays, enriching the visitors understanding of the context of the collection.
What are the shortcomings or challenges in working with this method?
Photogrammetry is often presented as a less subjective, more concrete representation
of an object – indeed, I’ve presented it that way just now! But like photography before it, the
veneer of authenticity can perniciously mislead, intentionally or no. What is selected to be
modelled remains a subjective decision of the researching, introducing an element of bias into digital collections, but also the process of creating digital models is a much more involved process of sculpting that we may like to admit.
With the current software packages, a degree of ‘cleaning’ is always required in the
creative process. Computers make errors, and elements around the subject are sometimes
erroneously incorporated into modelling, and must be removed. The degree to which these are removed is a decision made by the modeler, and the final product can be edited in 3D modelling software at will. Colour can be changed, perhaps damaged sections removed, repaired, or replaced – indeed, anything that is available to 3D modelling can be applied to photogrammetry models. The lighting under which a subject is shot alters the ‘colour’ of the surface, at least as it appears to the camera, and how or whether to correct for this is a decision that must be made along the process.
None of these issues should be surprising or negate the value of photogrammetry
model, but it is important that they are understood. Photogrammetry can sometimes be
presented as a magical process, where by an object that was once only in a museum’s cabinet is now also within the computer. But in truth, the creation of this digital surrogate is the result of an extensive process of data accumulation, processing, editing, cleaning, and – significantly –decision making at every step along the way.
Where do you see photogrammetry going in the future?
The ever-increasing processing power of smaller computers is almost frightening. I saw
at the University of Melbourne the other day a handheld device (more of a gauntlet, but still)
fitted with several scanners taking dozens of impressions every second, with embedded
software which modelled the scanned object on the fly. In minutes, it was able to complete a
workflow which would take me – with my high resolution DSLR and high powered PC – a full
day (or longer) to complete. I was told this incredible device was commercially available for the very reasonable price of $40,000.
But inevitably this price will drop substantially, as has all the technological breakthroughs
before it. The most recent iPhone has a rudimentary LiDAR scanner capable of quick 3D
scanning (using a different workflow to photogrammetry, but accomplishing similar outcomes) which would have been inconceivable 10 years ago. With the increase in technology, and the progression of our understanding of it, I see photogrammetry as becoming standard in majority of excavations, and expected of even the most humble museum collection.