A 1.202Mb PDF of this article as it appeared in the magazine complete with images is available by clicking HERE
On the 21st of September 2014, Leading Edge Geomatics (LEG) working with staff from the Nova Scotia Community College (NSCC), Applied Geomatics Research Group (AGRG) installed the Airborne Hydrography AB (AHAB– http://www.airbornehydro.com) Chiroptera II system into a Beechcraft King Air 90C, to perform the team’s first ever acquisition with a topobathymetric system. Leading Edge Geomatics (LEG) is foremost a topographic LiDAR and aerial photo collection company with projects completed all over North America. LEG has decided to put the extensive operational and data processing expertise towards the exploitation of this new LiDAR scanning resource in hopes of opening new markets and research in the shallow water environment. It is hoped that this new sensor will lead the way in this area.
The state of the art Chiroptera II system combines a 500 KHz topographic scanner and a 35 KHz hydrographic scanner with a 60MP Leica RCD30 photogrammetric camera. The purpose of this system is to collect the littoral boundary and shallow water where it is difficult and expensive to capture with traditional side scan or multi-beam systems, due to the navigation difficulties in shallow waters and harbors. This system also has the potential to provide a wealth of data for inland waterways, where currently only physical measurements of the bottom of lakes and rivers are possible. The Chiroptera II is designed to penetrate to depths of up to 1.5 x Secchi depth. Depending on water clarity, it should be reasonable to expect that this system can penetrate to 15m. While this type of system does present initial challenges to a team accustomed to flying strictly topo sensors, with foresight and good planning (and good weather), it is possible to overcome these challenges.
The installation of the system began bright and early Monday morning with few issues. The team was impressed with the Chiroptera’s construction and system integration. Considering that the sensor head contains two laser scanners, an RCD30 camera, and a second quality control camera, it is extremely compact. The design and construction of the system is extremely efficient and robust. Installation of the system went very smoothly and after lunch a calibration flight was scheduled. A team from AHAB was on site to assist with the initial installation and to help troubleshoot any problems that might arise. The system had been calibrated after assembly in Sweden and these bore site values were used as the starting point in the calculations of the misalignment angles. Impressively, after being shipped across the Atlantic in a crate and sitting untouched for months, the original calibration was relatively unchanged. With only minor tweaks to the calibration, the system was pronounced ready to acquire data that afternoon.
Anyone who has ever attempted to fly aerial surveys in the Atlantic Region of Canada will realize that it is extremely rare to find two consecutive days of cloud-free conditions. Even given the low altitudes that the system is meant to acquire data at (400m), we were not expecting to move this initial project along as quickly as we did. Fortunately, the initial flights were launched during an uncommon consecutive series of sunny, clear days, some of the best flying weather in our collective memory. The favorable weather allowed us to collect all of our priority projects in less than one week. The Chiroptera system is very user friendly and easy to learn. The team was able to run several operators through training during the collection, and all reported an easy to learn and operate interface. A few areas that were planned had to be recollected due to turbidity in the water column. The system allows the operator to review waveforms sampled during acquisition to determine the strength of the returns in order to ensure that the submerged ground is being sampled. This enables operators to quickly make decisions on the possible success of a mission and save precious flight time and expenses.
The processing application provided by AHAB is called LiDAR Survey Studio (LSS). Having used several other vendors’ proprietary LiDAR data processing applications, the team found LSS to be easy to configure, process, and output final data. The application combines a trajectory (processed with any SPAN capable inertial suite) with the raw waveform, and presents it in an intuitive and well laid-out interface. Processing parameters are set via configuration files for system settings, processing settings (classification method), and calibration (sensor misalignment). The results of the processing provide positioned and fully classified LAS data. The user then examines the classification of the bottom hits and the derived water surface to determine if the classification was successful. If some modifications are required, changes are made and the data is run again until the classification represents the data appropriately. Producing calibrated, correctly positioned and classified datasets is a very straight forward process.
The full 3D view displays the points quickly and smoothly, allowing the user to examine the processing results easily and manipulate the data to represent it as required. The tools for display provide a myriad of perspectives for representing the results. Selecting an individual return will bring up the corresponding waveform data and image. These views aid in the determination of the accuracy of the data classification. The only limitation is the lack of manual classification tools. It would be useful to be able to change the classification of points based on the wealth of data contained in the full waveform and the rapidly available QC images. In practice, this may make it easier to determine if a return is in fact a bottom hit or submerged vegetation, instead of doing the classification clean up in a third party application without access to the waveform information.
The Chiroptera II is a well-built, solid solution for shallow water collection. Our team was able to achieve excellent penetration results, but as expected, the system is heavily dependent on good water clarity. Areas such as the St John River would immediately block return data when entering from a tributary. This was not unexpected and murky water will remain an obstacle to this system. However, in areas where the water clarity is acceptable, the system performs admirably. The estuaries on the New Brunswick Northumberland shore yielded excellent results with almost all of the sea bottom mapped.
The Sable Island data showed surprising clarity with penetration to 15m. The topographic scanner (sold stand alone as the Dragoneye) was tested on Sable Island in isolation from the bathymetric scanner. This scanner performed well at 1400m and provided ground sampling at better than one point per metre. The multiple look angle of the nutating scan pattern ensures minimal LiDAR shadows in the data, which could possibly eliminate the need for multiple passes. Overall the Chiroptera is an extremely capable, flexible and welldesigned Topo-Bathymetric system.
Originally posted at http://canadiangis.com/initial-flights-with-airborne-hydrography-ab-chiroptera-ii-system.php
Wayne Richardson is the Manager of Positioning and Quality Control and a Project manager at Leading Edge Geomatics (LEG) in New Brunswick Canada.
A 1.202Mb PDF of this article as it appeared in the magazine complete with images is available by clicking HERE