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From Low Cost UAV Survey to High Resolution Topographic Data: Developing our Understanding of a Medieval Outport of Bruges
JEROEN DE REU1*, JAN TRACHET1, PIETER LALOO2 AND WIM DE CLERCQ1
1 Department of Archaeology, Ghent University, Sint-Pietersnieuwstraat 35, B-9000 Ghent, Belgium 2 GATE bvba, Eindeken 18, B-9940 Evergem, Belgium
ABSTRACT This paper assesses the application of a consumer-grade unmanned aerial vehicle (UAV) solution for the image- based three-dimensional (3D) reconstruction of a buried Medieval landscape at Monnikerede, a deserted former outport of Bruges, with the archaeological aim to achieve a better characterization of the (micro)topography and ultimately a better understanding of the site. The UAV survey resulted in a highly detailed and accurate 3D model of the terrain, allowing a thorough topographic analysis of the complex archaeological landscape. Application of algorithms to produce an enhanced visualization of the topographic variability led to a sequence of derivatives each highlighting the topography in a different way. Additionally a decorrelation stretch was applied on the collected imagery, thereby generating enhanced orthophotos producing another view on the archaeological landscape. Integrating the (enhanced) orthophoto(s) with the digital surface model (DSM) and derivatives to conduct a thorough analysis of the archaeological landscape resulted in the detection and identification of new archaeological features and the formulation of new insights in the layout of this important Medieval outport. Copyright © 2016 John Wiley & Sons, Ltd.
Key words: UAV; image-based 3D reconstruction; structure from motion; DSM; topographic analysis; orthophoto; decorrelation stretch; Belgium
During the Middle Ages, the city of Bruges became a leading cultural and economic centre in Europe. The linear gateway that connected the town’s commercial centre with the North Sea played a crucial role in this development. From the twelfth century onwards, the natural tidal inlet of the Zwin was gradually adapted for seaborne trade through the construction of dikes, sluices and smaller ports of transhipment. One of these port sites is the deserted settlement of
Monnikerede. During the mid-nineteenth century, Janssen (1854) observed complex (micro)topographic features and remains of Medieval land use at the assumed location of the disappeared outport. Until today, the interpretation of these topographic features remained unclear and in need of discussion (e.g. Hillewaert 1986). Aiming at a more specific character- ization of the microtopography, and to achieve a better understanding of the buried archaeological landscape, we conducted an (low-cost) unmanned aerial vehicle (UAV) survey of the area. UAVs are extensively used in modern-day society
(e.g. delivery, medical applications and film industry) and research communities (e.g. Lucieer et al., 2013;
* Correspondence to: Jeroen De Reu, Department of Archaeology, Ghent University, Sint-Pietersnieuwstraat 35, B-9000 Ghent, Belgium. E-mail: firstname.lastname@example.org
Copyright © 2016 John Wiley & Sons, Ltd. Received 1 October 2015 Revised 19 February 2016
Accepted 25 May 2016
Archaeological Prospection Archaeol. Prospect. 23, 335–346 (2016) Published online 30 September 2016 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/arp.1547
Torres-Sánchez et al., 2013; Whitehead et al., 2014), and are used for image acquisition in photogramme- try and image-based three-dimensional (3D) recon- struction (e.g. Colomina and Molina, 2014; Whitehead and Hugenholtz, 2014). UAVs hold great potential for archaeology; to collect aerial images, to map archaeology or to protect and monitor endan- gered sites (e.g. Chiabrando et al., 2011; Eisenbeiss and Sauerbier, 2011; Ortiz et al., 2013; Nex and Remondino, 2014). UAVs come in different shapes and sizes, with
different payloads and user control, and at greatly different costs (Watts et al., 2012). Especially costs are a boundary for the UAV to be integrated in the archae- ologist’s’ toolkit. Therefore, besides the aimed land- scape characterization, it is our aim to assess the potential (and limitations) of a consumer grade, low cost UAV solution (DJI Phantom 2) for the image- based 3D reconstruction of this complex Medieval landscape.
Materials and methods
Case study: the deserted Medieval town of Monnikerede
The first attestations of the site are found in early thirteenth century English documents that speak of merchants from Monnikerede who conducted trade with their ships in the Bay of Biscay and England (De Smet, 1937). Already by then, the small landing site and fishing village of Monnikerede seems to have been integrated in the international trade network of Bruges (Figure 1). Halfway through the thirteenth century, the village was granted city rights from the Flemish count and staple rights from the city of Bruges (Gilliodts-Van Severen, 1891). For nearly two centuries, Monnikerede profited from Bruges’ booming economy and trans- formed from a small landing site into a miniature city. However, the recession of Bruges’ economy and the regression and silting of the tidal inlet, demonstrated
Figure 1. Location of the study area in Belgium (bottom left), in relation with Bruges and the historic Zwin (Medieval situation) (top left). The current situation is visible on a recent orthophoto (AGIV, 2008) (right). The red contour line delineates the field under study.
336 J. De Reu et al.
Copyright © 2016 John Wiley & Sons, Ltd. Archaeol. Prospect. 23, 335–346 (2016) DOI: 10.1002/arp
that Monnikerede was not viable without the network in which it existed (Sosson, 1993). In an attempt to restore the navigable connection between Bruges and the sea, the canal ‘Verse Vaart’ was dug (1548–1557) (Ryckaert and Vandewalle, 1982). Although the course of this new waterway was dug into the former bed of the tidal inlet, it partly intersected the former fifteenth-century city-expansion constructed outside the dike on the alluvial bed. As the construction of the canal never brought the revival and with increas- ing military tensions of the Spanish–Dutch war being fought in the region, Monnikerede further declined and gradually shrank into a sparsely populated hamlet (Gilliodts-Van Severen, 1891). Eventually, it officially renounced its city rights to Damme in 1594. This slow decline was accelerated by the construction of the Damse Vaart, a Napoleonic canal that cuts straight through the former centre of Monnikerede. In particu- lar this last waterway profoundly changed the outlook of the site, dividing it in two parts, disfiguring the medieval landscape. West of the Napoleonic canal, the site transformed into arable land, while the eastern part became a pasture up until today. This meadow therefore still bears the unique topographical features of medieval habitation and exploitation (Figure 1). As the area comprises 3.5 ha (35,000m2), archaeologists are confronted with the methodological challenge to
assess and to map the vast topographical remains, in order to add to the understanding of this substantial part of the former Medieval town.
Available topographic data
Although Janssen (1854) already described the peculiar microtopography of the western part of the site in the mid-nineteenth century, it took until 1986 before the first proper archaeological and topographical study was conducted. The research consisted of an altimetric survey of more than 2000 measurements which were interpolated into a 2.5D model of the site. Unfortu- nately, the raw data were lost over the years. The only remaining record is a published 0.5m interval contour map of the terrain (Hillewaert, 1986). This contour map was digitized for interpretative and comparative means (Figure 2a). In January 1999, the site was moni- tored during one of the aerial surveys of the Depart- ment of Archaeology of Ghent University (Bourgeois et al., 2002). A set of five photographs clearly depict the topography of the field as shadowmarks. These aerial photographs were georeferenced using the methods described by Verhoeven et al. (2012). The result was adequate for georeferencing the shadowmarks, but it was however of insufficient qual- ity to be used for a topographic study (Figure 2b). In
Figure 2. Comparison between the available topographic data and the digital surface model (DSM) derived from the unmanned aerial vehicle (UAV) survey.
337From Low Cost UAV Survey to High Resolution Topographic Data
Copyright © 2016 John Wiley & Sons, Ltd. Archaeol. Prospect. 23, 335–346 (2016) DOI: 10.1002/arp
2003, a first generation airborne LiDAR (light detection and ranging) survey, with an average point density of 1 point per 2m2 and an altimetric accuracy ranging between 7 and 20 cm, was conducted in Flanders (AGIV, 2003; Werbrouck et al., 2011). From this dataset, a local digital elevation model (DEM) with 1m resolu- tion was generated (Figure 2c).
The consumer grade DJI Phantom 2 UAV, a light- weight quadcopter, equipped with a GoPro Hero 3+ (black edition) camera was applied to conduct the sur- vey. The UAV was manually controlled, from the ground, by the pilot, resulting in a slightly irregular flight pattern and flight altitude. The average flight altitude (above the terrain) was 8.9m. During the 24minutes flight, the camera, with rolling shutter, was set to take one image every 0.5 seconds, resulting in a dataset of more than 2000 12MP (4000×3000) images. A lens correction was applied before using the images for image-based 3D reconstruction – in the newer versions of the software Phot