GIS for Coastal Zone Management - Chapter 15

CHAPTER FIFTEEN High-Resolution Elevation and Image Data Within the Bay of Fundy Coastal Zone, Nova Scotia, Canada Tim Webster, Montfield Christian, Charles Sangster, and Dennis Kingston 15.1 INTRODUCTION The Applied Geomatics Research Group (AGRG) is a component of the Centre of Geographic Sciences (COGS) located in the Annapolis Valley, Nova Scotia. Its mandate is the application of geomatics technology for environmental research within Maritime Canada. In the fall of 1999 and summer of 2000 a large data acquisition campaign was initiated to collect high-resolution elevation and other remotely sensed image datasets along the Bay of Fundy coastal zone (Figure 15.1 and colour insert following page 164). The purpose of the research was to evaluate their effectiveness in obtaining critical information about the coastal zone, particularly data to be used to assess flood-risk potential associated with storm surge events. Global mean sea level has been increasing between 0.1 and 0.2 meters per century. With increasing greenhouse gases, sea level rise is expected to accelerate and the Intergovernmental Panel on Climate Change predicts that global average sea level may increase by 0.09 to 0.88 meters by 2100, placing the lives and property of an estimated 46 million people at risk (Houghton et al., 2001). The Bay of Fundy is no exception: relative sea-level is rising in this region by an estimated rate of 2.5 cm per century and many coastal areas are becoming more susceptible to flooding from storm events (Stea, Forbes, and Mott, 1992). In addition to sea level rise, storm surge and ocean waves are also factors at the coastline and are carried to higher levels on rising mean sea level. Storm surge in general is defined as the algebraic difference between the observed water level and the predicted astronomical level as one would find in tide tables. With possible increased storminess associated with climate change, the next 100 years will probably see more frequent flooding of coastal zones, and an increase in erosion of coastal features. With the recent increase in the spatial resolution of geomatics data available, both multispectral imagery (Ikonos, Quickbird, CASI) and high accuracy elevation data, landuse planners and policy makers now have access to the information required to manage the coastal zone. © 2005 by CRC Press LLC Figure 15.1 Location map, Bay of Fundy and Annapolis Valley (between Bay of Fundy and Nova Scotia label), Nova Scotia, Canada. This image is made up of a Radarsat S-7 mosaic for Nova Scotia, merged with a colour shaded relief map for the rest of Maritime Canada. Radarsat data © 1996, Canadian Space Agency. LIDAR (Light Detection and Ranging) technology has been employed for a number of years in atmospheric studies (e.g. Post et al., 1996; Mayor & Eloranta, 2001) and as an airborne technique for shallow bathymetric charting (e.g. Guenther et al., 2000), although cost was initially an impediment to widespread acceptance for the latter purpose. The technology can also be used to image the land and water surface (Hwang et al., 2000), as was done in the present study. Terrestrial LIDAR applications have been demonstrated in forestry (Maclean & Krabill, 1986), sea-ice studies (Wadhams et al., 1992), and glacier mass balance investigations (Krabill et al., 1995, 2000; Abdalati & Krabill, 1999). A general overview of airborne laser scanning technology and principles is provided by Wehr and Lohr (1999). Applications to coastal process studies in the USA have been reported by Sallenger et al. (1999), Krabill et al. (1999), and Stockdon et al. (2002), among others. Preliminary trials in Atlantic Canada were reported by O’Reilly (2000) and subsequent experience was described by Webster et al. (2001, 2002, 2003) and McCullough et al. (2002). Most of the coast of the conterminous USA has now been mapped using this technology (Brock et al., 2002). A comprehensive review of the theory and applications of Digital Elevation Models (DEM) covers both terrestrial and marine LIDAR as well as other technologies © 2005 by CRC Press LLC used for DEM construction such as IFSAR – interferometric airborne synthetic aperture radar – is given by Maune et al. (2001). Two different data providers were contracted to acquire LIDAR for the region. This chapter will discuss the details of the LIDAR systems, mission planning, data validation, and processing. The LIDAR DEM was then used in generating flood-risk maps associated with storm surge events along the coastal zone. This information has been passed to the local planning commissions to aid in management of the coastal zone. Another outcome of the project is that one of the data providers has significantly improved their acquisition system and approach to quality assurance when collecting LIDAR data. 15.2 DATA ACQUISITION AND PLANNING Data that were acquired during the fall of 1999 were airborne polarimetric synthetic aperture radar (PSAR) data from the Convair 580 aircraft operated by Environment Canada, and satellite imagery from Radarsat-1, and Landsat-7. The polarimetric SAR was a simulation of Radarsat-2, Canada’s second earth observation satellite planned for launch in 2005. Data acquired in the summer of 2000 campaign included high-resolution airborne data from the Compact Airborne Spectrographic Imager (CASI), and LIDAR systems, and more satellite imagery from Ikonos, Radarsat-1, and Landsat-7 (Table 15.1). Table 15.1 Types of data collected during 1999, 2000 summers. Data Type/Sensor PSAR Radarsat Landsat Ikonos CASI LIDAR Resolution (m) 6 12 (variable) 15 pan, 30 mss 1 pan, 4 mss 2 (variable) 2 (variable) Attribute Polarimetric SAR signal, C-Band 5.6 cm, HH, VV, HV polarizations C-Band 5.6 cm, HH polarization, variable incidence angle Visible, near and mid-infrared imagery Visible and near infrared imagery Visible and near infrared imagery Elevation (ground and non-ground) Polarizations: HH – horizontal transmit, horizontal receive, VV – vertical transmit, vertical receive, HV – horizontal transmit, vertical receive. The study area consists of the Annapolis Valley region of Nova Scotia, located on the southeast shore of the Bay of Fundy (Figure 15.1). LIDAR and CASI coverage consisted of the entire length of the valley and coastal zone. The satellite coverage was concentrated in the Annapolis and Minas Basins (Figure 15.2, see colour insert). © 2005 by CRC Press LLC 15.2.1 Acquisition Planning Issues The Bay of Fundy is famous for its great tidal range, up to 13 m in this area. For such a large study area the timing of the tides vary by approximately 1 hour between Digby, within the Annapolis Basin, and the Minas Basin. Because of the variability of tide times within the study area, three locations were used to predict tide times and sites: Digby, Margaretsville, and the Minas Basin/Cape Blomidon. Acquisition of remotely sensed data at low tide has several applications including: 1. validation of tidal models; 2. determination of inter-tidal slope from derived elevations from the “waterline method” i.e. knowing the water depth at the time of image acquisition allows the land/water line to be used as a topographic isoline; 3. morphological and biological classification of the inter-tidal zone. Tide predictions were acquired for each port via the Internet (http://tbone.biol.sc.edu/tide/sitesel.html). Figure 15.2 Location map of Annapolis and Minas Basin showing LIDAR, and airborne CASI coverage. Minas Basin is located in the upper right of the study area, and Annapolis Basin is located in the lower left of the study area. Radarsat data © 1996, Canadian Space Agency. Radarsat-1 standard mode 2 scenes and a Landsat 7 scene were acquired near low tide conditions. The Ikonos satellite, which is owned and operated by Space Imaging, is capable of acquiring 1 m panchromatic and 4 m multispectral (3 visible and 1 near infrared band) imagery. Ikonos imagery were acquired near low tide by determining the date of low tide near 11:30 am local time (sun synchronous orbit © 2005 by CRC Press LLC pass time) and requesting image acquisition from 2 days prior to the low tide’s date to 2 days after the low tide’s date. This allowed for variable weather conditions (clouds or overcast) and the fact that tide times advance approximately 45 minutes each day. Ikonos orders were sent through a local distributor. In addition to the satellite coverage, the coastal areas were imaged twice with the CASI sensor, once at high tide (3 m resolution) and once at low tide (1 m resolution) (Figure 15.3). The LIDAR survey area was divided into three regions, two flown by Vendor A, and one by Vendor B (Figure 15.2). The coverage for each company overlapped to allow a comparative analysis of the data from each company. All coastal areas were flown near low tide in order to acquire detailed inter-tidal topography. The in-land vegetation state in mid-July is at maximum leaf cover. Both LIDAR providers assured the AGRG that canopy penetration was still possible and that a significant number of laser hits would make it through the canopy to the ground. The data accuracy and data specifications for the LIDAR can be found in Appendix 15.1. The accuracy specifications are discussed in more detail in section 15.4. Details on each LIDAR system are presented in sections 15.3.1 and 15.3.2. Figure 15.3 Mosaic of 1 m CASI at low tide (left image), 2 m LIDAR DSM at low tide (centre image), and 3 m CASI at high tide (right image) for Port Lorne along the Bay of Fundy. Overall image is approximately 4 km across. 15.2.2 Data Acquisition Issues LIDAR is an active system providing its own pulse of near-infrared laser radiation and recording the reflected signal; thus it is not dependent on cloud free weather conditions as is the case for traditional aerial photography. However, rain or fog would cause the LIDAR survey to be delayed because the radiation cannot penetrate dense cloud or fog and therefore could not hit the ground. As a safety measure to ensure the laser is at a significant distance from the target, thus the power levels of the laser are not harmful to human eyes, the system will © 2005 by CRC Press LLC ... - tailieumienphi.vn