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Thermography

Remote sensing through the use of thermography is a relatively new technique in the toolbox of archaeological sensor technologies. Within the Dutch AMZ cycle, thermography can be applied during the exploratory and mapping phases of preliminary field research. Under ideal conditions, thermography can map both surface and subsurface archaeological features on land.

What?

How does it work?

Thermography measures the thermal infrared radiation (heat) emitted by the Earth’s surface. Solar radiation is reflected and absorbed by materials on and beneath the Earth’s surface (soil, vegetation, buildings, water, etc.). These materials emit absorbed thermal infrared radiation to varying degrees, depending on their thermodynamic properties. In the absence of direct sunlight, thermal infrared radiation can still be measured with thermal cameras. The recorded values are captured in thermogramsimages that visualize the recorded temperatures using a color scale (figure 1). Archaeological features can also emit varying levels of thermal infrared radiation. Such features may appear as anomalies in a thermogram.

Figure 1: An example of a thermogram, captured at Huis ten Bosch, Weesp (Waagen 2023, 24). Lighter colors indicate warmer areas, while darker colors indicate cooler areas. In the northwestern part of the field next to the farmhouse, castle walls are visible.
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In-depth explanation X

Figure 2: Overview of wavelengths in the electromagnetic spectrum. The range measured by thermography is indicated with diagonal stripes (adapted from Wikimedia Commons).

Thermography operates within the thermal infrared spectrum, detecting wavelengths between 8 and 14 μm (figure 2). This range enables the identification of temperature differences linked to the varying physical properties of materials and soil (e.g., Cool 2023, Waagen et al. 2022).

An archaeological feature or material can have a higher volumetric heat capacity, allowing it to absorb more heat and stay warmer for longer after sunset. For example, a ditch dug and filled in the past may retain more moisture than the surrounding soil. During the day, such a feature could remain cooler than the surrounding ground, which heats up more quickly in direct sunlight. After sunset, this temperature difference may appear as a distinct anomaly in a thermogram. Under favorable conditions, this effect can even be detected through a layer of soil, indicating features both on and below the surface (see figure 3).

In addition to volumetric heat capacity, three other thermodynamic properties are key:

  • Thermal conductivity: This property determines how much heat a material can transfer. Higher thermal conductivity of the soil increases the likelihood that a thermogram can detect buried warmer objects from the surface.
  • Thermal diffusivity: This property dictates the speed at which heat is transferred. It is equally important as thermal conductivity.
  • Thermal inertia: This property defines how quickly a material’s temperature changes. It depends on both thermal conductivity and volumetric heat capacity. Significant differences in thermal inertia often drive temperature variations, and therefore radiation differences, between materials.
Figure 3: The interaction between the natural substrate and archaeological features affecting thermographic visibility (Casana et al. 2017, 312).

What do you need?

To capture thermograms, specific sensors equipped with diodes capable of effectively detecting radiation in the thermal infrared spectrum are required. These thermal cameras are often offered as part of an integrated UAS (drone) system, which provides a direct live feed displayed on the screen. While thermograms can be recorded as individual images, the relatively large diodes typically result in limited resolution (e.g., 640 x 512 pixels). For archaeological purposes, such as later analyzing the measurements using Geographic Information Systems (GIS), it is essential to capture a series of photos. Photogrammetric techniques, also used for optical data, can then be employed to create mosaicked thermographic reflection maps.

Additionally, handheld thermal cameras can be used to study the physical properties of standing architecture, such as detecting moisture in walls (Brooke 2018), and their potential application in archaeological field surveys has also been explored (Thomas 2019).

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In-depth explanation X

Basic thermal cameras produce a thermogram in which the measured range of values is distributed over 256 bits. This means that the pixel values in each thermogram can represent different measured radiation levels. Therefore, it is highly recommended to use a radiometric thermal camera, which can accurately record the actual measured radiation. Well-known photogrammetric software packages, such as Agisoft Metashape or Pix4D, can work with radiometric JPGs.

Can be used with..

Using a handheld camera or a camera mounted on a drone allows for the effective capture of thermal infrared radiation at short distances (figure 4). The rise of UAS technology has provided a new and effective approach to archaeological remote sensing. However, there are limitations in terms of geographical range, which are influenced by regulations and battery life.

Figure 4: Left: Multirotor UAS (DJI M210) in Siegerswoude, ready for thermal imaging; right: the radiometric thermal camera used (Zenmuse XT2).
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Thermograms can also be collected using sensors on satellites and airplanes; however, their suitability for archaeological applications is quite limited. This is primarily due to the relatively coarse resolution of the images and the atmospheric distortion affecting thermal infrared radiation, which have a significant impact when the sensor is positioned far from the Earth’s surface. Furthermore, the exact timing of the image capture is crucial in thermography. Without control over this aspect, it becomes challenging to accurately assess the value of the images.

Archaeological Applications

Place in the Dutch archaeological heritage management process

Thermography, like most other archaeological remote sensing methods, can be applied during the exploratory and mapping phases of archaeological surveys. The collected and analyzed images yield a set of thermal/spectral anomalies (figure 5), accompanied by descriptions and interpretations. These findings can enhance the understanding of both subsurface and surface archaeological features and may inform further prospecting or excavation research.


Figure 5: Thermal anomalies observable on a mosaicked thermographic reflection map at Siegerswoude, interpretable as ancient ditch or drainage systems (Waagen et al. 2022).
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In-depth explanation X

Due to current legal restrictions (such as maximum distance from the pilot and limited visibility after sunset) and technical limitations (e.g., battery life), deploying drones equipped with thermal sensors over large areas can be challenging. For this reason, such research typically takes place in locations where there is a strong indication of the presence of archaeological remains.

Given their limited range, handheld cameras are mainly used at known archaeological sites and are usually tied to very specific research questions (e.g., moisture distribution in a wall).

What types of archaeological materials/landscapes

Thermography can be employed in any situation where it is plausible that differences in thermal infrared radiation between archaeological features and the natural landscape, including the soil, can be detected. Whether these differences are observable depends heavily on local conditions.

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In-depth explanation X

Although Dutch soil types differ in relative thermal conductivity, soil moisture content and vegetation condition are often much more critical factors. It is essential to identify conditions where thermography is unlikely to be effective. This is especially true for soils with high moisture levels or waterlogged soils, which lead to an equalization of the thermal signals. Additionally, the thermal radiation from objects closest to the sensor will dominate the readings. For instance, a surface layer of asphalt, stones, or dense vegetation will mainly reflect their own temperatures.

The detectability of archaeological materials depends heavily on local conditions. Stones may provide a clear contrast against clayey soil, but this contrast fades if they are buried deep within a poorly conductive soil matrix. It is also important to note that thermal variations in vegetation can reveal crop marks, indicating the presence of buried archaeological remains if different plant species retain heat differently (Seyfried 2020).

Thermography is sensitive to a wide range of factors. Previously, the importance of material properties, soil characteristics, and vegetation was mentioned. In addition, temporal factors also play a role, such as the temperature difference between day and night and atmospheric conditions like relative humidity. There are also different thermal cycles to consider: the daily cycle of day and night, the seasonal cycle, and longer cycles of periodic heating and cooling. All of these can affect the thermal infrared energy emitted by materials (see e.g., Casana 2017). Regarding the Dutch landscape, several studies have been published examining these local and temporal factors (Van Doesburg et al. 2022, Rensink et al. 2022, Waagen 2023).

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In-depth explanation X

Due to the wide variety of landscapes and climates worldwide, and even within the Netherlands, research into the relative effectiveness of thermography is still ongoing. As a result, the use of thermography, especially as a UAS-based remote sensing method, remains largely experimental. A key aspect of thermographic research is therefore recording the situational parameters during drone surveys and employing complementary and potentially validating prospecting techniques (Waagen 2023).

Casestudies

Curious to learn how thermography has already been successfully applied in Dutch archaeological field research? Click on the tiles below to explore case studies showcasing the use of this innovative sensing technique!

References/further reading

Brooke, C. (2018). Thermal imaging for the archaeological investigation of historic buildings. Remote Sensing, 10(9). https://doi.org/10.3390/rs10091401

Casana, J., Wiewel, A., Cool, A. C., Hill, A. C., Fisher, K. D., & Laugier, E. J. (2017). Archaeological Aerial Thermography in Theory and Practice. Advances in Archaeological Practice, 5(4), 310–327. https://doi.org/10.1017/aap.2017.23

Cool, A.C. (2023). Thermography. In The Encyclopedia of Archaeological Sciences, S.L. López Varela (Ed.). https://doi.org/10.1002/9781119188230.saseas0579

Van Doesburg, J., van der Heiden, M., Waagen, J., van Os, B., & van der Meer, W. (2022). Op zoek naar lijnen: De waarde van elektromagnetische inductie en optische en thermische infraroodbeelden in Siegerswoude (Friesland). Rapportage Archeologische Monumentenzorg 273.

Rensink, E., Theunissen, L., Feiken, R., Bourgeois, J., Deforce, K., van Doesburg, J., Emaus, R., van der Heiden, M., de Jong-Lambregts, N., Karagiannis, N., de Kort, J. W., Liagre, E., van Londen, H., Meylemans, E., Orbons, J., Stichelbaut, B., Terlouw, B., Timmermans, G., Waagen, J., & van Zijverden, W. (2022). Vanuit de lucht zie je meer. Remote sensing in de Nederlandse archeologie. Nederlandse Archeologische Rapporten (NAR) 80.

Thomas, H., & Williams, E. (2019). High resolution terrestrial thermography of archaeological sites. Archaeological Prospection, 26(3), 189–198. https://doi.org/10.1002/arp.1733

Seyfried, Simon. (2020). Thermal and Multispectral Monitoring of cropmarks by UAV AARGnews, 61.

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. https://doi.org/10.3390/drones6070165

Waagen, J. (2023). In search of a castle: Multisensor UAS research at the Medieval site of ‘t Huijs ten Bosch, Weesp. 4D Research Lab report series, 4. https://doi.org/10.21942/uva.23375486.v2