Application of Airborne Laser Terrain Mapper technology to coastal areas of Nova Scotia
Introduction
The Airborne Laser Terrain Mapper (ALTM) is an airborne sensor that uses the latest laser technology to collect thousands of spot elevations per second as the aircraft flies over a land surface. Two Global positioning System (GPS) receivers are used to locate the aircraft with accuracy better than 1 metre. One receiver is installed in the aircraft, while the other is situated at a known ground location. The ground receiver identifies and corrects errors in the aircraft's position. A high accuracy laser rangefinder scans beneath the aircraft to produce a wide swath over which the distance from the aircraft to the ground is measured. The laser angles are also measured and corrections are applied to eliminate motions of the aircraft. Once gathered, the angles and distance determine the position of points on the Earth's surface. The ALTM provides data similar to that of a conventional ground surveying technology but at a much faster speed and with both day and night operations.
High-resolution terrain elevation data are of great value in coastal areas. For example, in coastal lowlands it can be used to indicate the extent of areas susceptible to flooding, or to provide information on dyke elevations and arable land. Repeat surveys can be used to quantify coastal retreat rates and and areas of deposition.
Data was collected in late December 1998 by the Canadian Hydrographic Service, the Geological Survey of Canada (Atlantic), and the Transportation Safety Board of Canada using an Airborne Laser Terrain Mapper (ALTM) operated by Optech Inc., based in Toronto, Ontario. The ALTM system was mounted in a twin engine Piper Navaho. On average, flight lines were spaced 200 to 300 m apart, and the survey altitude was typically between 500 to 1000 m. Calibration was performed by flying several overlapping lines over an industrial site in Dartmouth. Numerous objects of various sizes were used to groundtruth the highest resolution of the system to objects smaller than 1 metre in size. Data were processed by Optech, and elevation information was provided on several CD-ROMs. Coordinates were expressed in UTM northings and eastings, and heights were expressed in centimeters relative to the WGS 84 ellipsoid.
Shaded relief of Story Head Beach
A small part of a shaded relief image from data collected at Story Head Beach is shown. Story Head Beach is a rapidly migrating, mixed sand and gravel barrier that has been the object of continuous monitoring by GSCA over the past 15 years. Much detail can be seen in overwash fans and beach ridges on the flying spit that extends north from Story Head. Note that the ALTM system is designed for land use, and wide data gaps occur over the sea.
Other areas which have been mapped using ALTM technology are Chezzetcook Inlet and the Truro area with a case study of flood water levels during the Saxby Gale of 1869.
Chezzetcook Inlet
Chezzetcook Inlet, located about 60 km northeast of Halifax, Nova Scotia, contains a variety of coastal features including: salt marsh, tidal channels, barrier and fringing beaches, multiple beach ridges, spits and shore cliffs. Rapid erosion of the outer coastal features and sediment deposition within the inner inlet, make Chezzetcook Inlet an ideal location to test the ALTM for measuring specific coastal features and also to establish a baseline data set for measuring future changes within the entire estuary.
The shores surrounding Chezzetcook Inlet, Nova Scotia, are increasingly being developed as a residential area for people working in Halifax, only 30 km to the west. Rapidly migrating barrier beaches and eroding shore cliffs at the mouth of the inlet are also sites for detailed investigations of shoreline change by the Geological Survey of Canada. Increased pollution and increased physical changes within the estuary have become major concerns of the local communities. The following images illustrate the usefulness of using ALTM data to visualize the impacts of rising sea level and for monitoring changes in the physical shoreline.
Chezzetcook Inlet at mean tide
This colour shaded relief image shows detailed ground elevations as well as all other positive relief features such as trees, buildings, and roads along Chezzetcook Inlet. The elevations, in decimeters, are colour coded (see colour bar) with red being the highest elevation, blue the lowest. ALTM technology does not penetrate water; areas of white are where the laser beams have been dispersed by the water surface resulting in no recorded data. Features most visible on the image are forest cover, drumlins (hills), barrier beaches, spits, intertidal flats and channels. Artificial features such as the road network are also easily identified but buildings commonly resemble trees. Lobes at the landward side of barrier beaches provide an indication of wave washover processes. One of the most useful capabilities of this technology is to be able to assign colours to elevations. In this image all areas below mean sea level have been coloured blue. To visualise the impact of higher sea levels caused by storm surges or natural sea level rise, the elevations only need to be redefined at specific levels such as higher high tide level.
Chezzetcook Inlet at high tide
In this image, sea level (denoted by blue) has been raised to higher high tide level. Marshes, mud flats and several of the outer low barrier beaches and spits are now submerged. By changing the water levels it is easier to visualise the decreased protection provided by the outer shores to the inner estuary, during high water storm events. The numerous spits growing outward from the islands are also better defined on this image. The distribution of spits is an indicator of where sediment is being deposited and its direction of transport.
Story Head Beach
This is an enlargement of the eastern entrance to Chezzetcook Inlet showing Story Head Beach and Fishermans Beach. Story Head Beach is a rapidly migrating mixed sand and gravel barrier that has been the object of continuous monitoring by GSCA over the past 15 years. Between 1996 and 1998, Story Head Beach has migrated 60m landward in response to a quick succession of storms, especially in 1998 The barrier has been unable to rebuild itself and much of it now remains submerged at high tide level. The left side of Story Head barrier was pushed by waves against Fishermans Beach and merged with it by February 2000.
Fishermans Beach profile
Shown here is a comparison of a ground survey and ALTM survey across a series of beach ridges on Fishermans Beach (Line A-B in the image).
To test the accuracy of the ALTM surveys, ground surveys were completed across both Fishermans Beach and Story Head Beach. The ground survey was completed using a Geotracer 2000 dual-phase real-time kinematic GPS (global positioning system) with horizontal and vertical resolution of ±0.1 m. Next, elevations from the same survey points were selected from the ALTM survey. Elevations from both surveys were referenced to the same vertical (CGVD28) and horizontal (WGS84) datums.
The profile from ALTM data closely mirrored the beach ridge morphology obtained from the ground survey but the absolute elevations were generally higher by a mean of 0.258 m which has been subtracted from the ALTM data for this presentation. The cause of the difference in elevations obtained from the two survey methods is unknown. For now, the difference is attributed to the poorer GPS positioning accuracy during the airborne survey. Further research will be required to resolve the problem.
The ease with which beach profiles above water level can be obtained is exciting. Even more exciting is the possibility of obtaining profile data anywhere on the ALTM image. Volumetric change could be calculated when successive surveys are available for comparison. It is intended that Chezzetcook Inlet will be reflown in several years and comparison of the two ALTM data sets will provide a quantitative measure of the redistribution of sediment resulting from erosion and deposition within the estuary.
The Saxby Gale of 1869 - A case study of flood water levels in Truro, Nova Scotia, Canada
The Bay of Fundy, on Canada's eastern seaboard, is the site of the world's highest recorded tides. On 5 October 1869, a storm with hurricane force winds travelled up the Bay of Fundy causing a 2 metre tidal surge resulting in extensive flooding. This event is known as the Saxby Gale.
The lowlands at the head of the Bay of Fundy are very susceptible to flooding by storm surges. High resolution digital elevation models (DEMs) acquired by remote sensing are a valuable tool when used to accurately estimate potential flooding over such extended areas. To evaluate the use of ALTM to this purpose, much of the floodplain and dykelands of Truro, Nova Scotia was surveyed which provided an extremely accurate elevation model. The flood water levels associated with the Saxby Gale storm surge have been superimposed on the terrain model to reconstruct the extent of flooding that occurred with the event.
ALTM image - Truro region - area (in blue) which is supposed to have flooded during the Saxby Gale
The town of Truro is built on the floodplains of the Salmon River. Extensive dykes contain the river within its channel and protect the rich farmlands of the marshes from inundation by seawater during the high tides that occur in the area. These dykes have served the town well for the past 2-3 centuries.
However, there have been some notable exceptions when the dykes could not hold back the onset of the tide. One of these events occurred during the Saxby Gale, of 5 October 1869, when a 2 metre storm surge coincided with an extreme high tide event. Extensive flooding and storm damage occurred. A brief history of the events leading to the gale were presented at the 23rd Conference on Hurricanes and Tropical Meteorology.
ALTM data of Truro
This colour shaded relief image was created from ALTM data collected in December 1998. The image uses colour to show the relative heights of features in the area. Features shown in red have the lowest elevation and those in green depict the highest elevations. The above image (click to enlarge) shows the general layout of the town with the commercial district and residential areas situated on the floodplain of the Salmon River.
ALTM data of Truro showing a perigean Spring tide
Every 18 years, the orbits of the sun, earth and moon align to produce an extreme high tide known as a perigean spring tide. In this colour shaded relief image of Truro, NS, the blue colour shows the area that would flood during a perigean spring tide if there were no dykes present. Note that there are no breaches in the dykes along the Salmon River, indicating that they are high enough to hold back these tides. Without the existing dykes, much of the agricultural lands near the river and an area south of the commercial district would flood.
ALTM data of Truro showing a 2 metre storm surge - the Saxby Gale
During the Saxby Gale of 5 October 1869 a 2 metre storm surge coincided with the perigean spring tide. In this image, the blue colour shows the approximate area that would flood if these conditions re-occur. Much of the present commercial area and most of the residential area built on the flood plains would be submerged. All dykes would be overtopped. Historical documents on the Saxby Gale, report that flood water remained trapped behind the dykes for several days after the storm.
The sequence of images below show the effects of rising tide levels on the main commercial area of Truro.
Air photo data of Truro
This aerial photograph, taken on 5 June 1994, shows the commercial area of Truro near Highway 102, as well as the brownish waters of the Salmon River, dyked farmlands, shopping malls and commercial areas.
ALTM data of Truro showing a normal high tide
ALTM data, from the same area as shown in the air photo, were used to create this shaded relief image. The data were processed to provide a horizontial resolution of 2.5 metres with a vertical resolution of several centimeters. This allows detection of objects the size of small cars. The features shown in the overview image above are quite evident in this image. Note that the water level in the Salmon River is shown in blue.
ALTM data of Truro showing a perigean Spring tide
Truro has a dyke system designed to prevent flooding from daily high tides of the Bay of Fundy. The blue colour in the ALTM image shows the area that would be flooded by a perigean spring tide if the dykes were not present.
ALTM data of Truro showing 1 metre storm surge
The blue colour in this image shows then area that would flood by a 1 metre storm surge coincident with a perigean spring tide. Note that parts of the dykes have been inundated. As the tide drops, the dykes would retain the flood waters.
ALTM data of Truro showing 2 metre storm surge - the Saxby Gale
Records from the Saxby Gale indicate that the water levels were equivalent to a 2 metre storm surge superimposed on a perigean spring tide. The blue colour shows the area that would flood during such an event. Most of the dykes would be inundated, large areas of Truro would flood, and the dykes would retain the water, compounding the problem.
Air photo and ALTM data of Truro showing the Saxby Gale
This image, a montage created from ALTM data and an air photo of Truro, presents a birds-eye view of conditions that would occur during an event like the Saxby Gale. ALTM data is shown for areas that would be flooded and overlain on an air photo. Data were manipulated within the GIS system to produce this image.
Affiliations and contacts
The partners in this project were:
- Canadian Hydrographic Service
- Geological Survey of Canada (Atlantic)
- Transportation Safety Board of Canada (TSB)
More examples on the use of ALTM:
For further information, contact:
- Russell Parrott (GSC Atlantic)
- John Shaw (GSC Atlantic)
- Robert Taylor (GSC Atlantic)
- Charles O'Reilly (Canadian Hydrographic Service) (1-902-426-5344)