Cultural Heritage Spotlight: Swedish Rock Art Research Archive

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About the Swedish Rock Art Research Archive (SHFA)

The Swedish Rock Art Research Archive, or SHFA for short, was founded as a research infrastructure of the University of Gothenburg in 2007 with the aim to collect, digitize, archive, and publish all Swedish rock art documentation on an open access web portal. The purpose is to stimulate research by providing easy access to rock art and to engage the public by raising awareness of the dangers to the rock art cultural heritage and its preservation. Here Sketchfab has been a tremendous tool to publish 3D models and to make them widely available. Currently, we have uploaded 567 models all available for download under a Creative Commons license.

What are Petroglyphs?

The rock art in Sweden is part of a wider southern Scandinavian tradition of engraving images into rock outcrops or on boulders. Such images are called petroglyphs. The majority of the petroglyphs date to the Bronze Age (1800-550 BC) and are between 3800 and 2600 years old. Although some painted rock art exists, the motifs are mostly negative reliefs that have been produced by pecking.

The rock art provides information about Bronze Age ideology, religion, society, long distance connectivity, means of transportation, and warfare by depicting boats, wagons, weapons, warriors, and various ritual and abstract features such as cupmarks. The panels can depict highly dynamic and exciting scenes such as combat, hunting, intercourse, etc. (see embedded 3D models).

The largest concentration of petroglyphs can be found in Tanum, West Sweden which was designated a UNESCO World Heritage Site in 1994. The SHFA works from here and helps to fulfil UNESCO’s requirement to document and preserve world heritage. However, since rock art can be found in many other parts of Scandinavia and throughout Europe the SHFA has extended significantly to include rock art from all of Sweden, Denmark, and Italy. Soon we will also have data available from Norway and the Iberian Peninsula. This work is made possible by funding from the Department for Historical Studies at the University of Gothenburg, the Swedish Research Council (project: “Rock art, Atlantic Europe, Warriors” (RAW)), and the Swedish National Bank’s Jubilee Fund Infrastructure for Research scheme (project: “Rock art in three dimensions”; Grant number: IN18-0557:1).

Fighting scene in Bro utmark (Tanum 192); laser scan model

How the SHFA Started with 3D

Apart from archiving rock art documentation, the SHFA was founded as a research and method development institution. The task today, as it was then, is to find ways to document petroglyphs as holistically and as bias-free as possible. Older documentation techniques, like rubbing, produced good documentation, but they relied heavily on the perception and experience of the documenter. Techniques like these also condensed the images into two dimensions on paper sheets, which means that the third dimension of rock art—its depth—is lost for later research.

Attempts to computationally record Scandinavian rock art already existed in the 1990s. Around 2000, ATOS scanner technology was developed to document the open-air rock art at Tanum for the Swedish National Heritage Board’s Rock Care Project under the guidance of the SHFA’s founding member and first director, Ulf Bertilsson. Under the tutelage of current director Johan Ling, the SHFA expanded the use of laser scanning. Today, recording rock art in 3D has become a standard method in Scandinavia.

Johan Ling (SHFA director) scanning a panel in Askim (Gothenburg)

Chris Sevara (now University of Vienna), Johan Ling (SHFA), Catarina Bertilsson (SHFA), Ellen Meijer (SHFA), Rich Potter (University of Gothenburg), and Christian Horn (SHFA) introduced photogrammetric methods, i.e., Structure from Motion (SfM) and Reflectance Transformation Imaging (RTI) starting in 2010. The SHFA’s current staff is mostly self-trained; only Ylber Qallaki received formal training in digital archaeology at the University of Lund.

Catarina Bertilsson (SHFA) conducting photogrammetry in Nämforsen.

How 3D Documentation Advances Petroglyph Research

There are several advantages to documenting rock art with 3D models, but one of the major concerns is that UNESCO demands the uniform documentation of rock art designated as world heritage. The “fingerprint” of the individual documenter is less distinct in 3D documentation than it is in more traditional documentation methods. Thus, 3D documentation provides a more uniform record. The fact that we can capture the depth of petroglyphs makes it a more complete documentation. We utilize the availability of depth information for the investigation of rock art and in our ongoing research projects.

Large warrior from Litsleby (Tanum 75); Photogrammetric model

In recent years the study of superimpositions of lines and motifs has revealed that there was an ongoing process of transformation, adding, and updating, not only of the rock art panels but also of the individual motifs. In Litselby (Tanum 75), a large human figure was engraved positioned to hold a spear that had been carved up to 500 years earlier. In Finntorp (Tanum 89), in an ongoing process, two simple circular features on the rock (cupmarks) were transformed into a fully articulated warrior with sword, shield, and spear over the course of perhaps 700 or 800 years. Thus, we were able to discover that rock art was a highly active component in the spiritual life of humans living during the Bronze Age. The petroglyphs allowed people to engage with their ancestors directly on the rock. However, the motifs and their meaning were, so to speak, not carved in stone.

Warrior figure from Finntorp (Tanum 89); Laser scan model

In Johan Ling’s project “Rock art, Atlantic Europe, Warriors,” funded by the Swedish Research Council, we use rock art to investigate motifs, compositions, and engraving techniques to compare the rock art of Scandinavia and the Iberian Peninsula. Recent research from other archaeological fields has shown that Scandinavia and Iberia were in contact during the Bronze Age for the trade of raw copper. The rock art tradition in both regions can help to investigate whether that also led to the exchange of ideas and ideologies. 3D models have helped to make comparison widely available. In our collaboration with Marta Diaz-Guardamino (Durham University) we were already able to show that there are many shared motifs between the two areas. A recent laser scan, on a standing stone (stele) in Halland (Sweden) revealed a warrior with a shield. This means that the practice of putting warrior figures on stelae, which is especially strong in the ore bearing regions on the Iberian Peninsula, may have resonated in Scandinavia.

Hunting scene in Finntorp (Tanum 90); laser scan model

In the project “Rock art in Three Dimensions” led by Christian Horn and granted by the Swedish Central Bank’s Jubilee Fund scheme Infrastructure for Research, we are currently pushing the boundaries of the use of 3D data in rock art research. An Artificial Intelligence algorithm is currently trained to recognize different general motif categories in visualizations derived from 3D data. These visualizations have been developed within the project to address some of the problems we are faced with when using 3D models in publications and for public outreach. One of these methods has recently been published in Open Access in the Journal of Archaeological Sciences: Reports.

Visualization of rock art from a laser scan of a panel in Finntorp (Tanum 90). Original laser scan provided by our collaboration partner, the County Administration Board in Västra Götaland (Henrik Zedig).

Tools and Workflow for Capturing Petroglyphs in 3D

We employ a multi-method approach that combines laser scanning and photogrammetry. Today we include in our photogrammetric documentation images taken with a macro-lens. The laser scanner is a Handyscan 700 purchased in collaboration with the Department for Historical Studies at the University of Gothenburg. This scanner sends out about 480,000 measurement points per second, which reproduce the panels with a resolution of 0.05 mm. Thus, the main objective with the Handyscan is to detect “mechanical” impact at 0.05 mm. Our collaboration partner, the County Administration Board in Västra Götaland (Henrik Zedig), uses the same scanner in a big scanning project which provided us with data for publication and research.

Rich Potter (Research engineer, University of Gothenburg) and Christian Horn (Research coordinator, SHFA) discuss the results of a laser scan on site of a standing stone in Hagbard’s Gallow (Halland).

For photogrammetry, we use various dSLR from Nikon and Canon, like the Canon 7D dSLR. The process is rather straightforward. After cleaning the panel of loose branches, leaves, and potential other vegetation, target points for either the laser scanner or SfM are set out. The distance between several targets for SfM is then measured to allow us to precisely scale the model later. When the necessary equipment is available, the target points are also measured using GPS. Rock art can be very shallow and imperceptible to the naked eye and touch. For this reason, we have used a cheap low-resolution scanner (3D Systems Sense) to quickly survey the surface to discover the position of the images and mark the area. We are often working in teams of two so that one can clean the rock while the other sets out the target points in the already cleaned area, or places new points while the other has already started scanning. Every documentation is done at the highest possible resolutions.

For processing SfM models, we are using Agisoft Metashape and, for the post-processing of the models, Meshlab. To visualize we use different forms of engraved lines and depth differences filters and renderers such as curvature colorization and ambient occlusion. To study the engravings, we make use of the opportunity to shift lighting angles and view models from directions not possible in the field. It is simply not possible for the researcher to go from a close-up position to hovering 10 m above the panel within a matter of seconds out in the field. The models we use are sometimes much larger than 1GB. We use Sketchfab, therefore, as a repository to upload decimated meshes to showcase our models to the public and professionals. These models are free to download from Sketchfab and should higher resolutions be required we provide them.

Challenges in Rock Art Documentation

Documenting rock art is, in general, challenging. The Scandinavian petroglyphs are an outdoor cultural heritage, which makes any documentation susceptible to weather and climate conditions. Wet surfaces are detrimental to all kinds of documentation. Direct sunlight impacts the laser scanning. While sunny conditions can be accounted for using an umbrella or tarps, there is very little that can be done against the rain.

Another problem is the size of the heritage we document. A full panel can easily be larger than 5 x 5 m, which can make the file sizes very extensive, especially at the high resolutions the original recordings have. Such file sizes cannot reasonably be handled except by the most powerful computers.

The other problem is human vision itself. Humans can perceive 3D information only through movement. This is a problem when screenshots of 3D models are published in scientific or popular publications. We solved this problem by developing a visualization that uses the 3D surface models, removes their global curvature and then uses a colour scale to display local depth variations. Since the images are engraved in the stone, they are the local depth variation. The depth maps we use for this preserve real depth information, which means that the resulting visualizations are a compromise between the clarity of older documentations and the depth data preserved in 3D models.

Fight scene from Fossum. 3D models record the depth of rock art which helped us to see that the figure to the right (from the viewer) superimposes another fighters sword; photogrammetry model.

Advice for Getting Started with 3D

For anyone who wishes to start working with 3D, I would suggest that they start out with photogrammetry. The best thing about photogrammetry is the relatively low knowledge bar that is required to start with it. Furthermore, it is comparatively cheap. Someone just wanting to test it could just use their mobile phone camera. These cameras have developed significantly in the past decade and are now very good, using Zeiss or Leica components. There is also open source software available to calculate point clouds and construct meshes. Any knowledge and application that improves photogrammetry can be picked up on the way to becoming more professional. So, the best advice in my opinion is: Just start!

3D scanners are often a different matter because they are often rather expensive. The use of professional scanners also requires some knowledge, for example about the intricacies of the applications provided with the scanner, their calibration, etc. Scanners that use target points could also be viewed as problematic when they need to be stuck to potentially sensitive cultural heritage. However, even for a start in scanning, there are cheaper alternatives available on the market, for example infrared scanners that do not require target points for under $500. Of course, these tools provide only low resolutions (1 mm and more), but that may be enough to convince a dean or head of department that investing in 3D technology is a worthwhile undertaking.

The visualization method for 3D surfaces, we recently proposed, also has a low entry bar for anyone who wants to try it out. The method also works in open source software such as QGIS and does not require a lot of technological knowledge of these applications or the underlying calculations and algorithms.

Looking at the Future of 3D Heritage

When it comes to 3D data and models, rock art research is currently undergoing somewhat of an awakening. In many fields 3D models are used not just as a display or documentation but as a research tool, to reach out to the public, and for inclusion. I sincerely hope that the latter will be given a much wider platform because 3D prints are a wonderful tool to engage disadvantaged groups such as disabled children. Funding agencies should understand that it is perhaps worthwhile to not always look for the next revolution, but fund things even if they have become commonplace, because here 3D technology and cultural heritage could have a genuinely positive impact on the lives of individuals.

As much as any digital data, 3D is easy to share online, which makes it much easier—especially for rock art researchers and other willing specialists—to include these images in their research and to widely share comparanda. This is especially important since rock art is usually not, or not easily, transportable and often requires special methods to be visible (as described above). Data sharing requires open repositories using creative commons licenses so that such data can be discovered and used freely. In the future this approach will help cross-fertilize research and provide richer accounts of the past.

SHFA’s Favorite Models on Sketchfab

Currently, we are fascinated by the results published by Scotland’s Rock Art Project.

At a workshop we were recently introduced to the earliest writing on the British Isles that can be found on the so-called Ogham stones. These stones represent a unique documentation challenge that 3D methods could solve because their carvings go around the edges of the stones (Discovery Programme).

SHFA’s Website / Facebook / Instagram / Twitter / Open Access article about visualization from 3D data

 

 

About the author

Christian Horn

Research coordinator and project leader (Rock art in three dimensions), SHFA and University of Gothenburg


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