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Low-Cost Very High Resolution Intertidal Vegetation Monitoring Enabled by Near- Infrared Kite Aerial Photography

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Low-Cost Very High Resolution Intertidal Vegetation Monitoring Enabled by Near- Infrared Kite Aerial Photography CHAPTER 7 LOW-COST VERY HIGH RESOLUTION INTERTIDAL VEGETATION MONITORING ENABLED BY NEAR- INFRARED KITE AERIAL PHOTOGRAPHY Klaas Pauly & Olivier De Clerck Unsubmitted manuscript Chapter 7 ABSTRACT With ecosystem services of intertidal habitats under rising pressure of human disturbance and climate change, monitoring habitat diversity is increasingly required. However, field-based surveys are time and resource- intensive and often do not provide spatially explicit information. While airborne (multi-spectral) photography and LIDAR (Laser Imaging Detecting And Ranging) offer an efficient, very high resolution and high-quality solution, the costs for skilled crew and equipment often preclude their use in remote areas, for small reserves and in developing countries. We present a simple yet robust, low-cost, low-altitude aerial photography solution using a kite and off-the-shelf camera equipment, resulting in photos covering the near-infrared part of the spectrum for vegetation monitoring. Photos can be mosaiced to generate 3D models, orthophotomosaics, vegetation indices and supervised classifications using low-cost computer vision and remote sensing software. We demonstrate the utility of kite aerial photography for intertidal monitoring in a case study in Northern France and discuss strengths and weaknesses of kite aerial photography. 170 Low-cost intertidal monitoring using kite aerial photography INTRODUCTION Rocky intertidal coasts offer important habitats supporting biodiversity by providing food and shelter. However, these habitats have been observed to decline globally over the past decades, affecting the ecosystem services that they provide (Ambrose & Smith, 2004). Links to human disturbances such as collecting, trampling and turning of rocks have emerged, and these effects may be worsened by climate change in the coming decades. Many countries now require monitoring schemes for these vulnerable habitats (Chust et al., 2008). Changes to benthic communities have often been recorded qualitatively using field surveys in the past. However, inconsistent timing, detail and extent of surveys have hampered establishment of a baseline map and quantitative spatially explicit change detections (Alexander, 2008). Advanced technologies such as remote sensing have been shown to lower the cost in monitoring schemes and increase mapping accuracy significantly (Lengyel et al., 2008). However, spatial resolution of spaceborne imagery precludes capturing the typically high intertidal rocky habitat variability. By contrast, aerial color or multispectral photography or airborne LIDAR have been shown to be effective in intertidal mapping efforts (Chust et al., 2008). Unfortunately, since many factors such as weather and remoteness are involved, the elevated costs for an aircraft together with highly trained staff and special camera equipment often rule out regular monitoring campaigns. Recent years have seen the development of low-cost alternatives, such as the use of small unmanned aerial vehicles (UAV; see Laliberte et al. (2010), although the cost for a professional UAV system still amounts to approximately $60,000) or tethered low-altitude balloon (Planer-Friedrich et al., 2007), helikite (Verhoeven et al., 2009) or kite aerial photography using consumer-grade cameras. Additionally, recent advances in computing power and software availability have enabled low- cost processing of consumer-grade photos, including advanced classifications algorithms and image-based 3D reconstruction. From these systems, kites provide arguably the cheapest and most simple yet robust solution. Kite aerial photography (KAP) has been around since 1887 (Archibald, 1897), but was only much later used in mapping and monitoring studies in the coastal (Scoffin, 1982) and the terrestrial 171 Chapter 7 environment. Since, applications have covered archaeology (Dvorak & Dvorak, 1998), geomorphology (Marzolff & Poesen, 2009), agriculture (Oberthur et al., 2007) and vegetation monitoring (Wundram & Löffler, 2008). As several of these applications require patterns in vegetation health to be detected, imaging the near-infrared (NIR) part of the spectrum became essential to discern different vegetation types and stress factors (Lebourgeois et al., 2008). CCD and CMOS sensors found in digital cameras are inherently sensitive to NIR light, and modified cameras (see Verhoeven, 2008); obtained by removal of the internal NIR-blocking filter in front of the sensor, used by manufacturers to simulate human eye color perception) mounted for KAP have been demonstrated to yield information otherwise not achievable with a digital compact camera (Gerard et al., 1997; Siebert et al., 2004). The aim of the present paper is to explore the utility of NIR-enabled KAP as a tool for monitoring intertidal rocky shore habitats in the Wimereux area (northern France), with a focus on seaweed communities. We assess best baseline mapping practices and show the potential for change detection using two imagery series acquired over 1 year. The rationale is to keep the design of the kite, the camera suspension and operation as well as the subsequent image analysis as simple and low-cost as possible, while using the latest technologies. MATERIAL & METHODS STUDY AREA The study area comprises a rocky intertidal stretch running south-north between the coastal towns of Boulogne-sur-Mer and Wimereux (Nord-Pas- de-Calais, France), known as Pointe de la Crêche, located between N50.750 and N50.756. The area is known to have supported extensive and dense intertidal brown algal communities dominated by Fucus spp. which collapsed between 1990 and 2000 (Coppejans, pers. comm.). Since 2000, wave- exposed rocks are either bare (upper zones), dominated by limpet/barnacle communities (Patella/Balanus) or mussel communities (Mytilus; mid-tidal zones) or spionid worm reefs causing heavy siltation on rock platforms (lower zones). Intertidal seaweed communities dominated by dense Fucus, green algal Ulva spp. and red algal Porphyra stands (mid to upper zones) are 172 Low-cost intertidal monitoring using kite aerial photography still found mostly on the edges of rocky platforms and vertical surfaces. Scattered mixed assemblages with mainly red algae can be found in the lower zones. KITE AERIAL PHOTOGRAPHY Kite aerial photographs were acquired on 16 April 2010 and on 7 April 2011. Depending on wind conditions, either a Rokkaku 7' or FlowForm 32' (figure 1a and 1b) were launched to a height between approximately 80m (in 2011) to 160m (in 2010). The camera was mounted on Brooxes Basic Frames tethered to a Picavet suspension system attached to the kite line approximately 20m below the kite (figure 1b). The camera rig was set to look straight down and an intervalometer was programmed on the camera's SD card using Canon Hacker Development Kit (CHDK, freely available at http://chdk.wikia.com) which triggered the camera every 5 seconds. Hence, no external electronic parts or remote control were used on the camera rig and all settings were made prior to the KAP session, enabling the kite pilot to walk around freely for terrain acquisition. In 2010, photos were taken under overcast conditions with an unmodified (true-color) 12MP Canon Powershot SX200 IS set at ISO 200, 5mm (28mm equivalent) focal length and variable shutter speed and aperture. A shutter speed of at least 1/500th is needed to prevent motion blur. In 2011, both a true-color (RGB) and a false-color series were subsequently acquired. The former used the same SX200 camera, while the false-color camera was a full-spectrum modified 10MP Canon Ixus 870 IS with a red-blocking Lee 172 Lagoon Blue film filter fitted to the lens, hence capturing blue, green and NIR light (Hunt & Linden, 2009). Both cameras were set to 5mm focal length, ISO100 (because of intense direct sunlight) and variable shutter speed and aperture in the latter session. Individual image extent and inherent resolution were calculated before the KAP sessions as a guideline. Photo coverage can be calculated based on the relation between focal length (f), acquisition height (H) and sensor width (d), from which the image width (D) can be calculated as d ⋅ H D = (1) f 173 Chapter 7 A B Figure 1: Equipment for kite aerial photography: a Rokkaku 7' framed (A) or FlowForm 32' frameless kite (B), used depending on wind speed and variability. The camera is suspended from the kite line using a Picavet suspension (1), a Picavet cross (2) and two pivoting Brooxes Basic KAP frames (3) The spatial resolution or ground sampling distance (GSD) can be calculated based on pixel size, acquisition height and focal length as H ⋅ D P(d) GSD = (2) f where P(d) is the number of pixels at the long side of the sensor. For a 12MP camera at 140m flying height and a 10MP camera at 60m flying height at minimal focal length of 5mm, this results in expected coverage and resolution of 173m by 130m at 4cm GSD and 74m by 66m at 2cm GSD per photo for a 1/2.3” camera sensor, respectively. GROUND TRUTHING Two days after the first acquisition date and coinciding with the second date, two separate transects measuring 50m by 2m were delineated including all major habitat types covered by the KAP, for which the outlines were drawn 174 Low-cost intertidal monitoring using kite aerial photography together with field identity codes. The drawings were subsequently digitized an overlaid on the image mosaics to provide classification training and test data, as well as to visualize the difference between aerial photography-based and traditional intertidal
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