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Quebec Bulletin
February - March 2013/Volume 16/Number 1

Hydrographers in Full Flight
Aerial Survey Technique

Aerial survey technique
Comparison of the coverage of two surveying technologies: LiDAR and multibeam.

Conducting hydrographic surveys in remote regions is an ongoing challenge for hydrographers. This is why last August the Canadian Hydrographic Service (CHS), Quebec Region, used the CCGS Matthew to survey various areas east of Anticosti Island and the Vieux Fort archipelago on the Lower North Shore. This ship and the two hydrographic vessels on board are equipped with the latest multibeam sounding systems.

Given the significant autonomy of the three vessels, it was possible to conduct extremely precise bathymetric surveys along the ranges and around the harbours in these areas. However, this technology has its limitations. Given that the width of the scan swaths is directly proportional to the depth, multibeam sounding systems become much less efficient in low depths, particularly when there are large areas to cover. This is where LiDAR comes in.

LiDAR (an acronym for Light Detection and Ranging) uses the same properties as a laser to model the earth’s surface but can also measure depths where the nature of the water allows it. The clearer the water and the fewer the suspended particles, the more deeply light can penetrate. The system uses a laser beam in the infrared band and another laser beam that is green. The green beam penetrates the water while the infrared beam is reflected off the water surface or land. The difference in transit time between the two beams determines the depth. These systems are generally mounted on small airplanes or helicopters and can cover a 250-metre-wide swath at an altitude of 500 metres and speeds of 150 knots or more. With depth soundings every 4 to 5 metres, the survey is less dense than multibeam sounding system surveys, but the compromise saves time and money.

In November, the CHS used LiDAR technology to cover an area of about 700 km² between Mistanoque Island and Blanc-Sablon (including the Vieux Fort archipelago) and the results were very positive. It took only four days in the air to survey almost all the areas between the shoreline and maximum depths of 15 to 30 metres, thus covering areas critical to coastal navigation. This survey will partly replace earlier surveys conducted between 1936 and 1939 using sextants and plumb bobs. Unfortunately, due to the nature of the water at the mouths of the Saint-Paul and Salmon rivers, it was not possible to measure the depths there.

Area covered by the November 2012 survey of the Lower North Shore.

The correlation between multibeam and LiDAR data is very good and the combination of the two will help CHS update nautical charts for this area more affordably and quickly than with conventional vessels. This approach fits perfectly with streamlining efforts over the past few years. The CHS will also benefit from reviewing the sectioning of nautical charts to improve its portfolio without compromising user safety or CHS product quality.

 

Roger Côté
Science
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