Category: Archaeology

DroneML

Year: 2024-2025

Supervision: Jitte Waagen

Execution and research: Jitte Waagen, Wouter Verschoof-van der Vaart, Agnes Schneider, eScience engineers: Christiaan Meijer, Maurice de Kleijn

Project description: 

The DroneML project aims to develop machine learning-based software that can rapidly screen multiple feature types (e.g., regularly shaped features that contrast with natural soil and grassland surroundings) and multiple multimodal input layers simultaneously, to enable rapid processing of large datasets for subsequent manual assessment of identified features. After initial identification, an ensemble learning approach such as ‘stacking’ may provide a machine learning supported metamodel for interpretation. The optimal solution would be if such a tool could be realized as, for example, a plugin in QGIS, a FOSS geographic information system, that can be run on a decent laptop, or could make use of cloud processing.

eScience Research Software Directory page

Publications:

Blogpost project announcement

DroneML project announcement

Posted on by Jitte Waagen

 

Jitte Waagen

Jitte Waagen has been granted a project in the 2023 Open eScience Call with DroneML, a project that seeks to improve the analysis of drone remote sensing data using machine learning approaches. In this project, the 4DRL team will closely collaborate with machine learning experts and software engineers from the eScience center, as well as two other researchers in the field (Wouter Verschoof-van der Vaart of the Nederlands Forensisch Instituut https://www.forensischinstituut.nl/ and Agnes Schneider, PhD at Universiteit Leiden https://www.universiteitleiden.nl/en/staffmembers/agnes-schneider#tab-1). This blog post is intended as a project announcement in which we will share the basic ideas of the project and what we aim to achieve.

Introduction

In the context of contemporary developments in climate change and human impact on the landscape, archaeological prospection techniques have become increasingly important. Due to decreasing groundwater levels, intensification of agricultural impact on the landscape and ongoing landscape development, important archaeological information is under continuous threat of being destroyed (e.g., Willemse 2021). Timely detection of buried archaeology is thus of essential value to its protection. Within archaeology, and heritage as a broader disciplinary context, remote sensing is part of an important set of non-invasive field techniques allowing archaeologists to search, evaluate and monitor archaeology in the landscape.

Drones in the Remote Sensing spectrum

In the field of archaeological prospection, drone/UAS (unmanned aerial systems)/RPA (remotely piloted aircraft) sensor techniques are very rapidly developing. Because of their operational flexibility they can very efficiently be deployed. In comparison with e.g., geophysical prospection techniques drones have a wider reach and can cover very irregular or inaccessible terrain. In comparison with remote sensing data derived from airplanes and satellites, their relative proximity to the ground is hugely beneficial because it allows for unprecedented high resolution and decreases distortions caused by atmospheric attenuation (e.g., Waagen et al, 2022). A technique such as thermal infrared imaging was of limited value for archaeological prospection until the arrival of drones. As such, the discipline has seen a swift rise in the deployment of drones using optical, thermal infrared, multispectral and LiDAR sensors, as an invaluable approach to detect, monitor, and evaluate archaeology.

Fast analysis required!

Due to the ongoing time-pressure, there is an urgent research challenge as to the analysis of the information captured by the drones. The deployment of state-of-the-art multisensor drone research strategies creates large multi-resolution and multimodal datasets of significant potential for archaeological sites and landscapes (e.g., enhanced visualisations of ground morphology, reflectance maps, vegetation indices). These layers add to existing satellite, aerial and terrestrial remote sensing data. The analysis of such datasets is a time-consuming process of inspecting every individual data layer one by one, marking potential anomalies, and interpreting them in a comparative analysis. The field of drone remote sensing data analysis would therefore advance fundamentally by the application of computer-aided inspection of these complex datasets.

DroneML, Machine Learning for multimodal and multilayered data

Therefore, DroneML aims to develop machine learning-based software that can rapidly screen multiple feature types (e.g., regularly shaped features that contrast with natural soil and grassland surroundings) and multiple multimodal input layers simultaneously, to enable rapid processing of large datasets for subsequent manual assessment of identified features. After initial identification, an ensemble learning approach such as ‘stacking’ (e.g., Wang et al, 2023) may provide a machine learning supported metamodel for interpretation. The optimal solution would be if such a tool could be realized as, for example, a plugin in QGIS, a FOSS geographic information system, that can be run on a decent laptop, or could make use of cloud processing.

The 4DRL has a lot of concrete use cases that can use as input and testing ground for the development of the tool; for example recent investigations of sites such as Weesp and Siegerswoude in The Netherlands (Waagen & van der Heiden 2021, Waagen 2023), but also abroad (e.g., Halos in Greece: LINK https://4dresearchlab.nl/drone-based-remote-sensing-at-halos-greece/).

Hopefully, the technology that would be the result of DroneML will hugely facilitate the work of archaeologists, as well as widen research possibilities, both within the field of heritage as well as any other discipline making use of remote sensing.

  • Waagen, J., & van der Heiden, M. (2021). Casestudy Siegerswoude-Middenwei. Thermisch infrarood remote sensing van een laatmiddeleeuwse nederzetting. In Archeologische prospectie vanuit de lucht.: Remote sensing in de Nederlandse archeologie (landbodems). (pp. 76-78). Rijksdienst voor het Cultureel Erfgoed.
  • Waagen, J., García Sánchez, J., van der Heiden, M., Kuiters, A., & Lulof, P. (2022). In the Heat of the Night: Comparative Assessment of Drone Thermography at the Archaeological Sites of Acquarossa, Italy, and Siegerswoude, The Netherlands. Drones, 6(7), [165].
  • Waagen, J. (2023). In search of a castle: multisensor UAS research at the Medieval site of ‘t Huijs ten Bosch, Weesp. 4DRL Report Series, 4. https://doi.org/10.21942/uva.23375486.v1
  • Wang, S., Wu, Y., Li, R., & Wang, X. (2023). Remote sensing-based retrieval of soil moisture content using stacking ensemble learning models. Land Degradation & Development, 34(3), 911– 925. https://doi.org/10.1002/ldr.4505
  • Willemse, N.W. (2021). Fragile monuments of the past, physical threats and countermeasures. (Nederlandse Archeologische Rapporten (NAR); No. 067). Rijksdienst voor het Cultureel Erfgoed.

 

4DRL project page

eScience Research Software Directory page

 

UAS Remote Sensing in a Medieval Castle Landscape in Alkmaar

UAS Remote Sensing on a Medieval Castle Site in Oudorperpolder, Alkmaar

Year: 2023

Supervision: Jitte Waagen

Execution and research: Jitte Waagen, Tijm Lanjouw, Alicia Walsh, Markus Stoffer, Mason Scholte

Project description: The drone remote sensing operations were commissioned by Nancy de Jong-Lambregts, municipal archaeologist of Alkmaar. In the Oudorperpolder, two castles have been built by count Floris V in the 13th century AD, the Middelburg and the Nieuwburg, and subsequently destroyed in the context of ongoing conflicts with the West-Frisians.

The drone operations were aimed at both complementing the existing state of the research based on historical sources, old excavation documentation, and Geophysical Prospection data collection, as well as at the methodological research into the potential of drone sensor archaeology in the context of buried stone remains in the Dutch landscape. An important archaeological and historical question is to what degree the landscape directly surrounding the castles could still present traces from that period of use, such as roads and connected buildings.

The analyses are still ongoing, but preliminary visualisations of the collected sensor data already generated some interesting clues as to both the castle areas themselves as well as the landscape directly surrounding the castles.

Publications:

Forthcoming

DJI M300 RTK ready for takeoff in the Oudorperpolder
Drone-generated thermal orthomosaic of the castle Middelburg area

Large-scale drone remote sensing at Rijnenburg, Utrecht

Large-scale drone remote sensing at Rijnenburg, Utrecht

Year: 2023-2024

Supervision: Jitte Waagen

Execution and research: Jitte Waagen, Mikko Kriek, Alicia Walsh, Mason Scholte

Project description: 

The drone remote sensing operations were commissioned by Erik Graafstal, municipal archaeologist of Utrecht. Rijnenburg is a development location of the municipality that in time will develop into a new urban district. People were already living on these higher river levees around the beginning of the common era. They lived in small hamlets of sometimes only two or three farms. The remains are presumably well preserved in the soil of Rijnenburg.

The aim of the overarching project is to save the most important archaeological sites as much as possible. In large developments such as Rijnenburg, archeology often only comes into focus once the master plans have been drawn, development positions have been negotiated and site preparation is about to start. Plan adjustment is then usually no longer an issue. In Rijnenburg, an innovative approach was chosen by bringing the inventory phase of the archaeological research to the forefront of the planning process.

As one of the applied methods, large-scale multisensory drone surveys are carried out by the 4D Research Lab over an area of ca. 350 hectares. Using LiDAR, optical, multispectral, and thermal sensors, the resulting data will be compared to already existing desk research and archaeological surveys, as well as new geophysical prospection measurements. The drone operations and analysis of the results is still ongoing.

For more information please check the project website.

Publications:

Forthcoming

Rijnenburg drone remote sensing area
Point cloud of the research area generated by thousands of drone photographs

Acquarossa Drone Remote Sensing

Optical and Thermal UAS research at the Etruscan site of Acquarossa, Italy

Years: 2018-2019

Supervision: Jitte Waagen

Execution and research: Jitte Waagen, Mikko Kriek

Project description: The drone remote sensing operations were part of an ongoing ACASA investigation directed by P. Lulof. Acquarossa is the name of an archaeological site consisting of the remains of an Etruscan settlement on the tuff plateau Colle San Francesco, in the province of Lazio and ca. six kilometers north of Viterbo, Italy. Previous excavations of the Swedish Institute in Rome (1966–1978) uncovered substantial urban remains throughout the area. They revealed various zones with Etruscan houses and public buildings which were inhabited from the late eighth century BC until shortly after the middle of the sixth century BC, when it was abandoned. With remains of foundations, walls, decorated roofs, and thousands of household utensils, it is one of the scarce examples of an intact Etruscan townscape.

Excavations in the outermost northeast tip of the plateau brought to light remains of Archaic habitation, with a structure interpreted as a domestic building dating from the last half of the seventh to the sixth century BC. The drone survey was oriented here, because of the attested remains of structures at a probably shallow depth. The drone operations were accompanied by a GPR survey, to generate a complementary dataset for the analysis of the efficacy of the remote sensing efforts.

Based on the combined data projections of both the thermal recordings and the GPR measurements, it could be shown that both techniques traced similar features, for example one that can be found as an anomaly northeast of the research area, which shows as an angular structure on the GPR data and has a clear thermal signature as well.

Publications:

Waagen, J.; Sánchez, J.G.; van der Heiden, M.; Kuiters, A.; Lulof, P. In the Heat of the Night: Comparative Assessment of Drone Thermography at the Archaeological Sites of Acquarossa, Italy, and Siegerswoude, The Netherlands. Drones 2022, 6, 165. https://doi.org/10.3390/drones6070165

Lionserpolder drone Remote Sensing

Lionserpolder (NL)

Year: 2021

Supervision: Jitte Waagen

Execution and Research: Jitte Waagen, Mikko Kriek, Rik Feiken

Project Description: The drone remote sensing operations at Lionserpolder, Friesland, were commissioned by the Rijksdienst voor het Cultureel Erfgoed (RCE), by archaeologist drs. Menno van der Heiden. The project research and reporting on the RCE side has subsequently been taken over by archaeologist dr. Rik Feiken. The area under investigation is an Iron Age/Roman period landscape surrounding an unexcavated site, probably a late Iron Age/Roman period (LIA/R) farm. There are clear patterns of LIA/R habitation, observed through ditches that are likely of LIA/R origin, and LIA/R pottery retrieved from test corings and test trenches.

The surrounding landscape may still be a largely intact late Iron Age landscape with old watercourses and salt marshes (‘kwelders’) still visible in the terrain morphology, and possible offsite archaeological remains. Therefore, a drone remote sensing operation was considered to be an effective method to map potentially present remains of the local LIA/R past. Indeed, the drone remote sensing operations have produced clear visualisations of Iron
Age/Roman period natural and cultural landscape features.

Publications:

Waagen, J.; Feiken, Rik (2024). Drone remote sensing over a late Iron Age/Roman period landscape in Lionserpolder, Friesland. University of Amsterdam / Amsterdam University of Applied Sciences. Journal contribution. https://doi.org/10.21942/uva.25008602.v1

ARCfieldLAB: Where are we now?

Posted on by Mason Scholte

Mason Scholte and Jitte Waagen

Introduction

In a previous blogpost we introduced ARCfieldLAB, a research project aiming at creating an inventory of the most important technological innovations of the last ten years in the field of archaeological remote sensing, and disseminating this knowledge to improve the quality of archaeological fieldwork and research in the Netherlands. In this blogpost, we provide an update on the achievements of the past few months, the current state of the ARCfieldLAB project and the progress over these past few months.

Achievements and Progress

Filling the knowledge base

The first of the two main components of the ARCfieldLAB project is the creation of an online knowledge base on innovative sensor technologies. In the course of the project, an extensive literature review has provided an overview of many ground-based, water-based, aerial, and satellite-based sensor technologies being used at the cutting-edge of international archaeological research. To highlight the value and potential of these sensor technologies for their application in the field of Dutch archaeology, as well as to provide an overview of resources and best-practices, several of these sensor technologies will be described in detail by ARCfieldLAB knowledge partners in the future knowledge base.

In the same vein, several case studies will take place to apply these innovative sensor technologies in Dutch contexts, both on land or in the water. The focus of these case studies is on ‘benchmark sites’: archaeological sites where the focus is explicitly on the application of various sensor technologies in combination. This will provide comparative opportunities that can provide validation of results, as well as a frame of reference to compare the strengths and limitations of each individual sensor. Further importance is placed on how these novel methods fit into the current AMZ-cyclus (the Dutch archaeological heritage management cycle).

For both the description of the innovative sensor technologies and the case studies, a call for projects was distributed to come into contact with potential knowledge partners. From the applications, a selection was made by the core consortium. The following knowledge partners were selected:

  • GAIA Prospection will be comparing the strengths of several different types of ground-based magnetometers at the site of a burial mound alignment in the province of Gelderland where various other geophysical techniques have taken place.
  • As part of a larger survey, Periplus Archeomare will be conducting a novel drone magnetometry survey on a sunken village in the province of Zeeland. Periplus Archeomare will also be responsible for the description of several maritime techniques: the sub-bottom profiler, side scan sonar, and multi-beam sonar.
  • Together, GAIA prospection and Periplus Archeomare will create the sensor technology description of magnetometry for both ground and water contexts.
  • ArcheoPro will return to one of their long-standing test locations for geophysical techniques in the south of Limburg at the site of a late medieval farm and conduct surveys using a wide array of sensors.
  • ArcheoPro is also performing a geophysical investigation in the center of a modern city to identify the medieval urban context underneath. This unique location provides a perfect opportunity to experiment with geophysical sensors in a highly complex sitecontext.

Several of the sensor descriptions and case studies are to be covered by knowledge partners from within the core consortium.

  • The 4D Research Lab will be continuing their multi-sensor drone-based investigation into the site of a 13th c. AD castle in Noord-Holland and a Late Medieval farmstead in Friesland  to further identify the environmental factors which influence the visibility of archaeological features. Given their expertise in applying thermography, the 4DRL will produce that sensor technology description.
  • Wouter Verschoof-van der Vaart will be advancing the application of the Dutch national LiDAR dataset in archaeological heritage protection through the use of change detection at various known sites in the Netherlands. The sensor technology description of LiDAR will similarly be produced by him.
  • Saxion will provide the technique descriptions for drone photogrammetry and multispectral research, as well as contribute to a yet-to-be-determined case study.
GAIA Prospection working on the magnetometry survey in the heath of the Veluwe. (Image credits: GAIA Prospection)
The 4DRL will continue with their drone-based analysis of a castle site in Noord-Holland (Image credits: Monumenten en Archeologie Amsterdam)
The first expertmeeting of sensor specialists within Dutch archaeology took place at the University of Amsterdam. (Image credits: 4DRL)

First expertmeeting

The second of the two main components of ARCfieldLAB is the organization of a series of expert meetings. The goal of these meetings is to promote the formation of multi-disciplinary networks and knowledge exchange collaboration between various sectors: archaeological professionals and academics, both from Dutch and international contexts, as well as remote sensing experts outside of the archaeological field.

On the 25th of April, the first of these expert meetings took place at the Bushuis at the University of Amsterdam. The goal of this meeting was to identify the needs and requirements for the online knowledge base, identify currently underexposed remote sensing techniques or sectors of untapped knowledge partners, and gain feedback on the direction of the project as a whole. All in all, the meeting was a great success and provided the opportunity for many interesting discussions. Various points were raised on the need for a decision support tool for sensor technologies, the importance of FAIR-data, and how sustainability of the project could be ensured after the project end date.

Looking forward

Second expertmeeting

On September 29th the second ARCfieldLAB expert meeting will take place at the Rijksdienst voor het Cultureel Erfgoed (RCE). The theme of this meeting will be ‘sustainable archaeological information structures’. The importance of this topic was highlighted in the first expert meeting. As ARCfieldLAB is involved in such a rapidly evolving field, it is important that the information provided is kept up-to-date throughout the project runtime. Furthermore, it is essential that there are plans in place for the maintenance and availability of the knowledge base after the project's duration ends. The focus of this meeting is therefore on how the information generated by the project can be made sustainable, and in what way this can be achieved through effective collaboration with the various E-RIHS projects. Stakeholders, experts on data management and sustainability, and members of related E-RIHS projects have been invited to discuss these issues.

Reuvensdagen

The sharing of knowledge is a vital part of the ARCfieldLAB project. For this reason, ARCfieldLAB will be showcased at the Reuvensdagen, the annual Dutch archaeological congress which brings together professionals, researchers, and other individuals interested in archaeology, within the context of a broader session on the various E-RIHS projects. The goal of this showcase is to increase the visibility of this project and obtain feedback to better integrate the final results with the current (and future) KNA (Knowledge Infrastructure in Dutch archaeology). The Reuvensdagen take place on the 16th and 17th of November in Hoorn. Interested? Find more information and tickets on the Reuvensdagen website!

ARCfieldLAB project announcement

Posted on by Mason Scholte

Mason Scholte and Jitte Waagen

ARCfieldLAB. Innovative sensor technologies and methodologies for archaeological fieldwork: network, knowledgebase, and dissemination

BACKGROUND

The field of archaeological remote sensing has in the past decade seen significant developments in terms of novel sensor technologies and applications. These innovations can be applied to improve and expedite the archaeological fieldwork process in terms of the documentation, visualisation, and monitoring of archaeological features in a non-invasive manner, both on land as well as underwater.

With this blogpost, the 4D Research Lab presents ARCfieldLAB, a brand-new research project with the aim of creating an inventory of the most important technological innovations of the last ten years in the field of archaeological remote sensing, and disseminating this knowledge to improve the quality of archaeological research in the Netherlands. The project concerns a wide-ranging audience, including academic researchers, students, professional archaeologists and other specialists in this field (i.e. commercial companies or municipal and governmental archaeological services), and volunteers in archaeology.

This project is set to run for two years, and is funded by E-RIHS. E-RIHS is the European Research Infrastructure for Heritage Science which supports research on heritage interpretation, preservation, documentation and management. The mission of E-RIHS is to deliver integrated access to expertise, data and technologies through a permanent scientific infrastructure for heritage research, to which ARCfieldLAB will add a national digital platform for innovative methods and techniques and a collaborative network aimed at sharing experiences and best practices.

A core consortium led by the 4DRL of institutions firmly embedded in the Dutch archaeological sector or in the field of archaeological remote sensing has been appointed and acts as a steering committee this project. It consists of representatives of the Rijksdienst voor Cultureel Erfgoed (RCE), Stichting Infrastructuur Kwaliteitsborging Bodembeheer (SIKB), as well as the private sector (as represented by the Vereniging Ondernemers in Archeologie (VOiA)) and experts from Leiden University (LEI), the Free University of Amsterdam (VU), and University of Amsterdam (UvA).

An example of a recent innovation in archaeological remote sensing: drones are increasingly being utilized as remote sensing platforms.

THE PROJECT

There are two main components which constitute ARCfieldLAB:

The first component is the collection and dissemination of knowledge on innovative sensor technologies which can be applied to archaeological fieldwork by a) creating an overview of these developments in the last decade and sharing this knowledge through a publicly-accessible online knowledge base of resources and best practices, and b) providing examples of successful applications of the novel technologies and methods by which their value and potential for the archaeological fieldwork process is illustrated.

The second component is the organisation of a number of expert meetings, in which the possibilities and added value of innovative sensor technologies are elucidated and space is provided for experience in the application of these techniques to be exchanged. To promote multi-disciplinary collaboration, participants in these meetings will come from various sectors: archaeological professionals and academics, both from Dutch and international contexts, as well as remote sensing outside of the archaeological field. Additionally, workshops will be hosted for the promotion and education of these techniques.

As part of this project, various case studies will take place. These case studies serve to expose and fill in existing gaps in the knowledge of archaeological remote sensing in The Netherlands and aid in the development of best practices. The potential of the technological innovations which have so far not seen wide application in Dutch archaeology (but possibly have seen use in other countries or other sectors) as well as the efficacy of combining multiple remote sensing data sources in one site will be tested.

A graphical abstract of the ARCfieldLAB project.

EXAMPLE CASE STUDY: SIEGERSWOUDE

An example of a case study assessing the potential of a novel sensor technology in the context of Dutch archaeology is the use of drone-based thermal infrared remote sensing at the late medieval site of Siegerswoude, Friesland.

The theory behind thermography has previously been described in a previous blogpost, where it was used at the site of Halos in Greece. This pilot at Siegerswoude adds to a body of case studies which can be systematically compared to determine to what extent certain variables (e.g. soil composition, time of day, soil humidity, thermal properties of archaeological features) influence the outcome of an archaeological survey using drone-based thermography.

Historical sources associate the village of Siegerswoude, currently located on the meadow of a dairy farm, with a late-medieval grange from a regional Benedictine monastery situated approximately one kilometre west of the site. The site itself consists of at least five rectangular plots, evenly spaced along an axis and encircled by ditches.

Thermal imagery taken at the site revealed multiple traces which contrast with the background (marked A-E on the image) that have been identified as being archaeological in origin. The clearest of these is the rectangular ditch encircling the westernmost plot of land (A), visible on both the orthophoto as well as LiDAR data, which has a distinct thermal signature. On the northside of this feature, a double line is visible which is not present on the non-thermal data sources. Other traces included a rectangular trace in the centre of the western plot (B), long lines in SW-NE direction (C), and part of a ditch encircling the eastern plot (D) which continues into a similar double line feature as near (A). Test trenches have further validated these results, and provided insights into the use of this area: the ditches were used for draining the surrounding peat landscape, as well as for the extraction of loam.

One of the main takeaways from this survey, is the fact that thermography is capable of identifying archaeological features which are not visible on both orthophotos and LiDAR data of Siegerswoude. Furthermore, the noticeable differences in visibility of thermal signatures on the thermal imagery taken at different points throughout the day at Siegerswoude serves as a prime example of the importance of understanding the influence of variables on the results of these surveys.

The site of Siegerswoude as it is located in the Netherlands, together with optical imagery and AHN3 height data.
Thermal imagery from Siegerswoude showcasing various traces (indicated A-E).
References:
Waagen, J. & van der Heiden, M. (2021). Casestudy Siegerswoude-Middenwei. Thermisch infrarood remote sensing van een laatmiddeleeuwse nederzetting. In Archeologische prospectie vanuit de lucht.: Remote sensing in de Nederlandse archeologie (landbodems). Rijksdienst voor het Cultureel Erfgoed, p. 76-78.
Waagen, J., Sánchez, J.G., van der Heiden, M., Kuiters, A., & P. Lulof (2022). In the Heat of the Night: Comparative Assessment of Drone Thermography at the Archaeological Sites of Acquarossa, Italy, and Siegerswoude, The Netherlands. Drones, 6, 165. https://doi.org/10.3390/drones6070165
Rensink, E., Theunissen, L. & H. Feiken (eds.) (2022). Vanuit de lucht zie je meer. Remote sensing in de Nederlandse Archeologie, Amersfoort (Nederlandse Archeologische Rapporten 80).

Drone-based remote sensing at Halos, Greece

Posted on by Jitte Waagen

Elon Heymans and Jitte Waagen

Drones are gaining ground as a versatile tool for archaeological prospection. Cheaper and more efficient in use than conventional airplanes, high-end drones available on the consumer market can be mounted with different cameras and sensors, thereby creating what is essentially a flying modular remote sensing lab.

The advantages of drone-based prospection versus traditional methods in terms of logistic- and cost-efficiency, make it an attractive tool for exploring archaeological landscapes. Moreover, a drone can fly at different altitudes, unhampered by vegetation or relief, and because all it really needs is a charged set of batteries, it can carry out many survey flights collecting a large amount of data. These advantages make drones particularly suitable for experimental research setups that that facilitate a steep methodological learning curve and allow for the development of best practices, while at the same time contributing to site-specific research questions.

Testing Ground: Ancient Halos

The archaeological landscape of ancient Halos in central Greece provides a good testing ground for the possibilities that drone-based prospection has to offer. Dutch archaeologists from Groningen (RUG) and later Amsterdam (UvA), in collaboration with colleagues of the Greek Archaeological Service (the Ephorate of Volos), have been active in the region for over 40 years; and also work carried out by the Archaeological Service around the construction of the Athens-Thessaloniki highway has contributed much to our knowledge of the area.

Halos is mentioned already in Homer’s Iliad, yet research has not really been able to identify a settlement centre for the Iron Age (12th – 8th century BCE). As confirmed by later sources, it is likely that during the course of the Iron Age and archaic period Halos emerged as an independent political community, a Greek polis. Yet, how this community of citizens really came about is not completely clear. A large funerary plain measuring over 200 ha and used by people living in the surrounding area between the 12th and 6th century BCE can offer an important view on how this landscape was used through time and how people came together thereby shaping new and collective identities.

The Voulokaliva Funerary Landscape

While the area boasts virtually no substantial Iron Age settlement remains, this funerary plain, known as the Voulokaliva, is one of the largest Iron Age cemeteries in Greece. It is dotted with over thirty tumuli, some of which remain more or less intact (although their preservation is under pressure due to continued agriculture), and can often be recognised as a scatter of stones or elevated stone heaps, resulting from the fact that farmers plough out large stones, which are then deposited together. Other tumuli have been looted or excavated (Stissi et al. 2004). In a single instance, an excavated tumulus (site 36) contained 74 cremation burials, made over the course of several centuries, with the tumulus (eventually) being closed off with a cover of small stones (Lagia et al. 2013). Moreover, excavations carried out along the route of the highway suggest a spread of single burials in between the mounds.

Drone Remote Sensing
A Google Earth generated image overlooking the Almyros plain and the Pagasitic gulf to the northeast with the city of Volos in the centre on the other side of the gulf and the acropolis of Hellenistic Halos at the bottom. The Voulokaliva is located adjacent to the highway; Magoula Plataniotiki is located closer to the coast
A drone photo of the Voulokaliva looking in western direction with the nearby village of Neos Platanos beyond the highway, several arrows indicate visible tumulus sites

Drones aloft

While a better understanding of this extensive area is key to answering important historical questions, we also have a lot to learn from the process of researching it, not only in developing and finetuning our methods, but also in mapping the area and thereby complementing earlier survey and excavations. This is done by collecting large numbers of images with sufficient overlap and processing these into detailed 3D models that can be further analysed but also offer new visualisations. Using dGPS measured targets (mounted onto 1x1m aluminum plates) that can be easily identified in the images, these models have a high level of precision and can be placed in a GIS (Waagen 2019). In order to contribute to our understanding of the area as part of the study of the field survey data, we have focused our efforts on six areas or test sites, also included in the survey. Four of these (A–D) contain clusters of tumuli, giving the possibility to take a detailed look at these sites, while not losing sight of potential ‘off-site’ remains. The remaining two test sites are settlements: a small prehistoric settlement site located on the southern end of the Voulokaliva (site 35), and the Classical/Hellenistic settlement at the nearby tell-site of Magoula Plataniotiki, where excavations have been ongoing since 2013.

Thermography

Our approach uses thermal infrared imaging as a remote sensing technique. By measuring minute temperature differences on the surface, an advanced thermal camera can document material differences in the soil, including potential subsurface archaeological remains (Casana et al. 2017). Think of the fact that on a sunny day, a car can turn hot enough to fry an egg on its hood, but a grassy lawn would still stay cool. This is simply because some materials can easily heat up in the sun (due to their high thermal conductivity), and retain and emit a lot of heat (i.e. high heat capacity and thermal emissivity), while others (notably water) are more resistant to temperature fluctuations (something known as thermal inertia). Because different materials, due to variation in composition, density, moisture content, conductivity of the matrix etc, respond differently to temperature fluctuations, we can observe differences in the pace at which they heat up and cool down again. Such differences result from the diurnal flux (the difference between day and night), but are also affected by longer term (weeks) temperature fluctuations.

An Experimental Workflow

As the reflection of sunlight has a large impact on the measurable thermal radiation, the theory prescribes that it is best to fly between sunset and sunrise. Because the clarity of a thermal signal results from a variety of factors, our strategy is to collect data at different times during the night (after sunset, in the middle of the night and before sunrise) and in different times of the year (July, November and April). Collecting data under all these different circumstances allows us to systematically compare the images that we have. This is a versatile approach because comparing images recorded under different circumstances helps us to understand and interpret what we are seeing, but also because it enables us to understand what conditions are most favourable for recording clear thermal signals at Halos.

At the same time, a systematic approach of this sort is also labour and data intensive. In the summer, nights are already short, but when you go flying about with a drone, they become challengingly short. And when not out in the field, we do administration work – keeping records for each and every flight, diaries of different sorts, and storing and (if possible) processing images.

Bad Weather

The preliminary processing of images is quite important, as we discovered last November. Having arrived to Greece after days of pouring rain, the clay-rich soil was completely saturated. With a limited diurnal temperature flux (between 10 – 14° C.) and no direct sunlight during the day, we soon discovered that the thermal imagery recorded on our first evening flights did not contain sufficiently distinctive information to enable proper photogrammetric processing. This forced us to adapt our strategy, but for the better. We decided to take high-altitude thermal overview photos, while focusing on specific test sites.

Multispectral Imaging

Luckily, we had other possibilities as well. Another sensor we use is a multispectral camera. More commonly used in agriculture, this camera captures images at different wavelengths of light, thereby enabling the visualisation of differences in vegetation, crop health and soil humidity. Think of the fact that subsurface archaeological features might impede or stimulate plant growth, or could prove favourable for certain plant species at the cost of others. The resulting patterns, known as crop marks, have long been studied through traditional aerial photography. Multispectral images are not only complementary to what can be observed in normal images, but provide a much more specific, diverse and detailed image. Paired with thermal imaging, this is already proving its worth as an effective prospection method (Agudo et al. 2018; Sagaldo Carmona et al. 2020).

In addition to a detailed 3D optical model of the whole Voulokaliva site, produced in November as well, the datasets we collected offer detailed remote sensing data of this extensive funerary site. One additional week of fieldwork is planned for early April, after which we can fully compare and analyse the different visualisations. So, to be continued.

 

 

A satellite image of the Almyros plain and the southern end of the Pagasitic gulf, showing our research area
A thermal image of us in the field

Credits

This project is part of the Halos Archaeological Project, which is a cooperation between the Universities of Amsterdam, Groningen and Thessaly, and the Ephorate of Antiquities of Magnesia. It is undertaken as part of the study towards the final publication of the 1990–2006 field survey. We thank dr. Vaso Rondiri, professor Reinder Reinders, and professor Vladimir Stissi for their kind support. The project was made possible through a grant from the Stichting Nederlands Museum voor Antropologie en Praehistorie (SNMAP), the support of professor Stissi/Amsterdam Center for Ancient Studies and Archaeology, the 4D Research Lab and the Stichting Thessalika Erga. Fieldwork is carried out by Elon Heymans, Jitte Waagen and Mikko Kriek.

 

A screenshot of photogrammetric processing software, showing the drone positions of captured imagery, and the 3D textured mesh of the Voulokaliva area
Left: an orthomosaic of test site A, showing an elongated stone heap on tumulus site 22; centre: a DEM of test site A clearly showing a large tumulus (site 22) and a smaller elevation (site 39) directly to its east – both based on optical images taken in November 2021; right: an NDVI (normalized differentiated vegetation index) image based on multispectral data, recorded in July 2021, also showing sites 22 and 39

References

Agudo, P., Pajas, J., Pérez-Cabello, F., Redón, J., and Lebrón, B. 2018. ‘The Potential of Drones and Sensors to Enhance Detection of Archaeological Cropmarks: A Comparative Study Between Multi-Spectral and Thermal Imagery.’ in Drones, 2(3), 29.

Casana et al., 2017, Archaeological Aerial Thermography in Theory and Practice, Advances in Archaeological Practice 5 (4) 310-329.

Lagia, A., Papathanasiou, A., Malakasioti, Z., and Tsiouka, F. 2013. ‘Cremations of the Early Iron Age from Mound 36 at Voulokalyva (ancient Halos) in Thessaly: a bioarchaeological appraisal,’ in Lochner, M., and Ruppenstein, F. (eds.) Cremation burials in the region between the Middle Danube and the Aegean, 1300–750 BC. Proceedings of the international symposium held at the Austrian Academy of Sciences at Vienna, February 11th–12th, 2010. Vienna, 197–219.

Salgado Carmona, J.A., Quirós, E., Mayoral, V., and Charro, C. 2020. ‘Assessing the potential of multispectral and thermal UAV imagery from archaeological sites. A case study from the Iron Age hillfort of Villasviejas del Tamuja (Cáceres, Spain),’ in Journal of Archaeological Science: Reports 31.

Stissi, V., Kwak, L., and de Winter, J., 2004. Early Iron Age. In: Reinders, R. (Ed.), Prehistoric Sites at the Almiros and Sourpi Plains (Thessaly, Greece). Assen, 94-98.

Waagen, J., 2019. New technology and archaeological practice. Improving the primary archaeological recording process in excavation by means of UAS photogrammetry. Journal of Archaeological Science, 101, 11-20.