NSCC Topo-Bathymetric Lidar Research

NSCC Topo-Bathymetric Lidar Research in support of Sustainable Coastal Development

Thank you to the team at Nova Scotia Community College – Tim Webster, with support from Kate Collins, Nathan Crowell, Kevin McGuigan, Candace MacDonald, and many students and research assistants for the research contributing to this article.

Introduction and Background
The Applied Geomatics Research Group (AGRG) was formed in 2000 in Middleton, Nova Scotia, Canada, as part of the Nova Scotia Community College’s Applied Research Department. In 2012 AGRG acquired a Chiroptera II integrated topographic-bathymetric (topobathy) lidar sensor equipped with a 60-megapixel multispectral camera, and since then research has been focused on mapping the coastal zone. The Chiroptera II employs a 1064 nm red laser for surveying the land and a 515 nm green laser that penetrates the water to reach the seabed. The lasers scan in an elliptical pattern which enables coverage from many different angles on vertical faces, causes less shadow effects in the data, and is less sensitive to wave interaction. The bathymetric laser is limited by depth and clarity, and has a depth penetration rating of roughly 1.5 x the Secchi depth (a measure of turbidity or water clarity using a black and white disk). The Leica RCD30 camera collects co-aligned RGB+NIR motion compensated photographs which can be mosaicked into a single image in post-processing, or analyzed frame by frame for maximum information extraction.

The resulting primary data products include seamless land-sea digital elevation models (DEMs), orthomosaic photographs, and intensity maps. For the past three years, AGRG has been collaborating with government agencies and industry to research various applications of the lidar data in the coastal zone such as sustainable coastal development and aquaculture, oil spill preparedness, and coastal erosion. AGRG has conducted three missions with the Chiroptera II between 2014 and 2016, and surveyed coastal environments such as the deep, clear Atlantic Ocean; the shallow productive waters of the Northumberland Strait; the dynamic tidal region of the Bay of Fundy; lakes, and rivers. Leading Edge Geomatics (LEG), based out of Lincoln, New Brunswick, partners with AGRG to provide the aircraft for the surveys each year.

Merigomish Harbour Survey
Merigomish Harbour is located along the Northumberland Strait in Nova Scotia (Figure 1) and was surveyed in July 2016 as part of a multi-year, multi-stakeholder study to collect baseline information in coastal zones to support sustainable coastal development. The lidar study area was 87 km2, and took 5.5 hours to complete. Merigomish Harbour opens to the ocean through a narrow opening with a deep channel, and is bounded at the seaward side by a strip of land 2 km at its widest point and 0.050 km at its narrowest point. The harbour is popular with recreational users such as cottagers, swimmers and boaters; and is also home to shellfish aquaculture and a small craft harbour. During the lidar survey field teams collected ground truth data including hard surface validation and depth measurements to validate the lidar, Secchi depth measurements for information on water clarity, and underwater photographs to obtain information on bottom type and vegetation. Due to its large size, the Merigomish survey was completed over the course of two days. Meteorological conditions on both days were favourable (<15 km/hr wind, good visibility), and the majority of the survey was completed at or near low tide.

Figure 1: The Merigomish study area on Nova Scotia’s north shore, on the Northumberland Strait in the southern Gulf of St. Lawrence.

Data Processing
The aircraft trajectory was processed using the GPS base station data, and the navigation data were georeferenced and linked to the laser returns. The lidar returns from both the topo and the bathy lasers were then classified as discrete land, water surface, or bathymetry points. The classified point data were examined using a variety of attributes (flight line, elevation, intensity) and the 5 MPIX quality control air photos for quality control. Custom processing codes were developed at AGRG to further refine the data and reduce noise, and to improve the separation of bathymetric points and noise.

The RCD30 60 MPIX camera imagery was processed using the aircraft trajectory and direct georeferencing. The aircraft trajectory was linked to the laser shots and photo events by GPS-based time tags and was used to define the Exterior Orientation (EO) for each of the RCD30 aerial photos that were acquired. The EO and lidar DEM were coupled with the aerial photos to produce digital orthophotos.

Ground truth measurements with RTK GPS showed the lidar to be accurate to within -0.07 m (± 0.04 m, n=2079) for topographic measurements and within -0.08 m (± 0.39 m, n=26) for bathymetric measurements.

Data Products
Several data products were derived from the classified lidar point cloud data including the Digital Elevation Model (DEM), Colour Shaded Relief model (CSR) and Depth-Normalized Intensity Model (DNIM). Intensity is a measurement of how much light is reflected by the seabed, and can provide information on bottom type.

The lidar penetrated to approximately -7 m depth at the seaward edge of the Merigomish study area, and the majority of elevations deeper than -4 m occurred outside of Merigomish Harbour (Figure 2a). Within the harbour the bathymetry was largely shallow and flat bottomed away from the main channel, but also exhibited fine-scale features such as sand bars and ripples (Figure 3). The lidar did not penetrate to the bottom of the channels, which ground truth measurements revealed were up to 12 m deep. The photographs revealed a mix of land uses ranging from agricultural to residential, and in shallow water the photographs revealed information on bottom type, which was dominated by sand and submerged vegetation (Figure 2b, Figure 3b). The dark blue colour of the bathymetry in Figure 2c and 3c likely indicates submerged vegetation, while the lighter blue areas typically represent a sandy bottom.

Figure 2: (a) DEM bathymetry (m CGVD28) binned to highlight depth contours in Merigomish Harbour, draped over a 5x hillshade; (b) 5 cm resolution RGB orthophoto mosaic; (c) Merigomish CSR draped over depth-normalized intensity image. The red box indicates the extent of Figure 3.

Figure 3: (a) DEM bathymetry (m CGVD28) binned to highlight depth contours in Merigomish Harbour, draped over a 5x hillshade; (b) 5 cm resolution RGB orthophoto mosaic; (c) Merigomish CSR draped over depth-normalized intensity image.

Future Work
Future work on this dataset will focus on deriving a bottom classification map that differentiates between submerged vegetation types, such as rockweed, ulva, or eelgrass; and between other non-vegetated bottom types, such as mud, fine-grained sediment, or rock. The research will be directed towards deriving these comprehensive bottom classification maps from the primary lidar products (DEM, photos, and intensity). A hydrodynamic model is being constructed for Merigomish to help understand ocean current dynamics and tidal flushing metrics. The bottom classification maps and model results promise to be of interest to research partners for planning future development of coastal resources such as aquaculture, and to help in achieving a sustainable balance between recreational and industrial users of Merigomish Harbour.

For more information, please visit: https://www.nscc.ca/About_NSCC/applied_research/index.asp

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