Mapping Medieval Heerlen

Goal/research question

Medieval Heerlen was a small, roughly round, fortified settlement known as the “Landsfort Herle” (figure 1). It featured a large central church with cemeteries to the north and south. These cemeteries may have been enclosed by a wall, with a road or path running around them. Inside the city walls was a single ring of buildings. There were several entrances, and outside the walls lay a moat. The entire settlement had a diameter of approximately 140 meters.

In the post-medieval period, Heerlen gradually expanded. The most significant growth occurred in the early 20th century with the rise of coal mining in the region. Over time, the church was enlarged, the buildings inside the city walls were demolished, the city walls were torn down, and the moat was filled in. Parts of the area were redeveloped with modern buildings, now forming the heart of Heerlen’s shopping district. The current Pancratiusplein and De Bongerd square represent the open spaces within this old Landsfort.

Multiple reconstructions have been made based on historical maps. These reconstructions differ at certain points due to uncertainties in the maps. The goal of the research was to use non-destructive methods to gain insight into the actual location of the moat, city wall, building zone, and the wall surrounding the cemetery. Some key reference points could be helpful in determining the location of the intended structures. Based on this preliminary information, a map of expected structures was created. Figure 2 shows the archaeological expectation map, along with the area that was surveyed.

Methodology

The preparation for this study drew on the desk-based research conducted by RAAP (Keunen, 2020), which used historical maps, prints, and excavation data to create an archaeological model.

Given the presence of cables, pipes, sewer systems, street furniture, and other modern infrastructure, a comparative multi-sensor approach using different geophysical techniques was selected (figure 3).

It was decided to survey the open areas within the former Landsfort Herle and its immediate surroundings using a 1 m grid, covering the entire area with both Ground-Penetrating Radar (GPR) (figure 4, left) and Electromagnetic Induction (EMI) (Figures 4, right). For the EMI, both shallow measurements at depths of 50, 100, and 150 cm below ground level and deeper measurements at 200, 400, and 600 cm were chosen.

Ground-Penetrating Radar

The results clearly illustrate the challenges of conducting research in an urban context. The measurements complement each other: where the EMI reveals something, the GPR does not, and vice versa. For example, the northern Bongerd is clearly visible in the EMI results, showing the location of the moat. However, in the Pancratiusplein, the EMI reveals little to nothing due to interference from metal structures such as cables, pipes, grates, and fences. In contrast, the GPR was able to identify several structures in this area.

The EMI investigation has provided clarity regarding the location of the former northern moat in the Bongerd. Meanwhile, the GPR survey revealed a structure with angular, straight patterns in the Pancratiusplein, which aligns with the expected location of medieval buildings.

Lessons Learned

In an urban context, it is crucial to combine as much research data as possible. This includes both historical data and taphonomic data, such as the presence of cables and pipes in the study area. By applying both EMI and GPR within the research area, as well as combining these methods with known information, valuable archaeological insights can be extracted from the data. However, the following considerations must be taken into account:

• EMI measurements are heavily affected by environmental metal and underground metal. In a city center, this is often extensive, with cables, pipes, benches, street furniture, etc., which cannot be removed for the research.
• GPR measurements are often affected by shallow disturbances in the study area. This interference limits the radar signal’s ability to penetrate the ground, making it difficult to detect deeper structures.
• The combination of GPR and EMI proved crucial for uncovering archaeological findings in specific areas of Heerlen’s city centre.
• The use of historical map material is invaluable. Without this data, the research would not be properly interpretable.
• Site managers often don’t fully grasp the research methods or how environmental factors affect measurements. Phrases like “If the chairs are moved, it should be fine, right?” are well-meant but create obstacles for the research. Despite thorough pre-planning, such obstacles continue to complicate urban research.

Geophysical research in urban areas can be a valuable piece of the larger archaeological puzzle, especially when combined with other methods. While it may not yield miraculous results, asking the right questions can certainly help uncover additional archaeological data and insights!

References to reports

• Keunen L.J., M. Kocken, 2020, Een slapende schoonheid, Een cultuurhistorische studie van het middeleeuwse landsfort in Heerlen, met handreikingen voor de toekomst, RAAP Rapport 4244
• Orbons, J. 2024, Landsfort Herle, Gemeente Heerlen, Inventariserend Veldonderzoek (IVO-O); Geofysisch onderzoek, ArcheoPro rapport 23057

Colophon

This research was only made possible due to the excellent collaboration between ArcheoPro, the municipality of Heerlen, and Saricon.