Thermography in volunteer aerial archaeology
In August 2022, the "Archäologische Verein Erding" (Archaeological Association of Erding, AVE e.V.) started a new project on thermography in the field of voluntary aerial archaeology. This project, initially planned for 2 years, is supported by the Bavarian State Office for the Preservation of Historical Monuments (Bayerische Landesamt für Denkmalpflege), Volunteer Section, and by the "Verein für Archäologie Stadt und Landkreis Landshut e.V." (Association for Archaeology of Landshut, ARLAN).
Objective of the project
In the course of the project, it will be clarified whether high-resolution, drone-supported thermography is another method for the documentation of ground monuments that complements conventional aerial archaeology, and whether there are areas of application in which thermography has clear advantages over the prospection methods already in use today. Furthermore, it will be examined whether certain (climatic) conditions or characteristics of a ground monument (soil condition, type of monument, plant cover) favour the gain of knowledge in the thermographic image or are a necessary prerequisite. The focus of the investigation will be the southern Bavarian region, especially the districts of Erding and Landshut.
The following topics and questions are to be investigated within the framework of the project:
Type of ground monument
- When can underground building structures (walls, floors) be recognised as thermal features and what are the differences to conventional aerial photographs?
- How do trenches, pits and postholes relate to thermography?
- Are there additional findings on old roads and streets that are no longer visible above ground when they are flown over with a thermal camera?
- Which soils and soil conditions favour the development of thermal characteristics?
- Are there certain climatic conditions (dryness, wetness, coldness) that support a certain soil in the formation of thermal characteristics?
- Which plants form thermal features and when?
- Are there plants that are not yet known in conventional aerial archaeology to form crop marks, but which do show thermal features (e.g. grass)?
- Under what circumstances or at what stages of development do plants show thermal features?
- At which times of the year/weather conditions is it worthwhile to fly with a thermal camera?
- To what extent does wind have a negative influence on the formation/visibility of thermal features?
- Does it make sense to fly with a thermal camera in direct sunlight?
Time of day
- At what time of day does it make sense to fly with the thermal camera?
- How long do thermal features remain after dusk/sunset?
- Does it make sense to fly at night?
- Can thermography also be used usefully in the field of excavations, e.g. for better detection of soil discolouration after the removal of an examination area?
- How big is the difference in terms of knowledge gained between images from a low-resolution and a high-resolution thermal imaging camera?
The author previously had his own drone with a thermal camera (FLIR Duo), with which the first successful flights were carried out. However, since this thermal camera only has a low resolution of 160px x 120px, a drone specialised in thermography (DJI Mavic 2 Enterprise Advanced) was acquired for this project, whose built-in thermal camera provides a high resolution of 640px x 512px.
Procedure and implementation
First of all, it will be a task to collect and review the experiences already made and published on thermography in (aerial) archaeology and to check whether they can be transferred to this project.
At the same time, it is important to gain experience with both the technology and the recording of thermal features at the beginning of this project. For this purpose, known ground monuments are to be flown over more frequently at all times of the year and day. Priority will be given to ground monuments that have already become visible due to snow features. These flights should be precisely documented, so that air and ground temperature, wind speed, current solar radiation, weather patterns in the previous days, ground conditions, vegetation, flight altitude, etc. are also recorded as accurately as possible.
In a later step, these flights are systematically evaluated. Attempts should then be made to identify certain rules and correlations, which are then checked during further targeted flights under previously defined conditions.