Monitoring Heard Island remotely
Research on sub-Antarctic Heard Island, 4000 km south-west of Western Australia, has shown that local climatic conditions are continuing to change. These changes, in turn, are having an effect on the island's environment. Between 2000 and 2004, for example, Brown Glacier on the island's east coast, lost 8 million cubic metres of ice a year, compared to the 50 year average of 3 million cubic metres a year (Australian Antarctic Magazine 7: 9, 2004). In some coastal areas that were previously ice-covered, there are now large areas of bare ground and lagoons.
The Australian Antarctic Division manages the World Heritage listed Territory of Heard Island and McDonald Islands (HIMI) and the HIMI Marine Reserve. Part of this role includes monitoring and reporting on changes to the environment and conservation values, and pursuing necessary management actions. However, the cost and logistical challenges of getting to Heard Island preclude regular on-site monitoring. These practical constraints are acknowledged in the Heard Island and McDonald Islands Marine Reserve Management Plan, which promotes the development of practical, cost-effective and low-impact remote monitoring techniques.
Figure 4: This 3D view of Heard Island in Google Earth is comprised of WorldView-1 satellite imagery, in black and white, and IKONOS imagery of the eastern part of the island.
Photo: AADC/Google Earth
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The project utilises high resolution satellite images, such as from the WorldView-1 and QuickBird earth imaging satellites. These satellites detect the near-infrared light wavelengths reflected by vegetation, allowing different land cover types, such as bare rock and plant communities, to be identified. The satellites have a resolution of 50 cm and 60 cm, respectively, and can collect all the required information in one pass of the island.
The satellite images must be corrected for spatial distortions arising from topographical variations (such as mountains and gullies) in the earth's surface and the tilt of the satellite as it passes over these features. This 'orthorectification' is done with the help of a digital elevation model showing the terrain in 3D, on-ground photos, and previously collected global positioning system data (Australian Antarctic Magazine 7: 10-11, 2004).
Once corrected, important features such as the coastline, glacial extent, vegetation and lagoons are manually digitised in a geographic information system (GIS). The eventual aim is to automate this time-consuming process using pattern-recognition software. Instead of analysing each individual pixel, as is common in traditional feature extraction methods, homogeneous (similar looking) objects would be derived from the image by image segmentation (Figure 1). Object characteristics such as spectral (light) properties, shape, size, texture and context can then be used to classify the object into a meaningful class and produce up-to-date maps. This so-called 'object-based image analysis' approach aims to simulate the way humans visually analyse imagery.
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Figure 1: This small portion of a 60 cm resolution QuickBird image of Brown Lagoon, acquired on 30 January 2006, was used for testing automated feature extraction. The image on the left is a false colour composite highlighting vegetated areas in red. The image on the right is the result of object-based image analysis, which aims to simulate human vision. The first step is to identify image objects that correspond to real-world features (snow patches, lagoon, vegetation patches, etc.). The objects are then classified into land cover classes, resulting in a thematic map with information that can be integrated with an existing GIS database. This automated analysis allows for more efficient feature extraction than manual digitisation techniques.
The project also involves the development of automated techniques to identify changes in the coastline, glacial extent, and vegetation cover. Change detection is based on a comparison of two or more images. Preliminary results show that there has been an increase in vegetation between 1991 and 2006 (Figures 2 and 3). Comparing QuickBird images from 2003 and 2006 revealed that detailed changes in vegetation communities can be automatically identified and mapped.
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Figure 2: Comparing the 1991 SPOT image at 20 m resolution (left), showing the Stephenson Glacier (white) and plant communities (red), with a 2006 QuickBird image (right) of the same region (resampled to the 20 m resolution of the SPOT image), enables change detection over 15 years. It is immediately apparent that the Stephenson Glacier has altered snow cover and a reduced extent. Some cloud cover is visible in the 2006 image.
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Figure 3: This image (left) was produced by overlaying the change detections result from the 1991 SPOT and 2006 QuickBird image comparison over a digital elevation model of Heard Island (with the SPOT image in the background). The white arrow indicates an area of vegetation increase on the Stephenson moraine (significant changes appear in red). The photographs of Stephenson moraine taken in 1987 (centre) and 2004 (right), show the changes in this area as observed in the field – note the increase in cushion plant cover in the foreground of the 2004 photo.
By completion of the year-long project the intention is to have used orthorectified images from the Worldview-1 and QuickBird satellites to update topographic maps of the island, to map the coastline, human footprint, glacial extent and lagoons, and to map detectable changes in these features. Multimedia products such as interactive tours and animations in Google Earth are also being developed to publicly display the information and assist with the Antarctic Divisions efforts to present Heard Island to the community (Figure 4). At the end of the project, image change detection and classification results will be published on the HIMI website with downloadable Google Earth scenes and animations.
A major objective of the project is to further develop the capability of the AADC to assist with applying a similar approach to the remote monitoring of the Australian Antarctic Divisions other areas of management and research interest.
EWAN MCIVOR1, ARKO LUCIEER2, URSULA HARRIS3 and ANGELA BENDER3
1 Policy Branch, AAD
2 University of Tasmania
3 Australian Antarctic Data Centre, AAD
Big changes on a small island
The Australian Antarctic Division visited the island briefly in December 2008 to complete some aerial, ship-based and terrestrial photographic surveys to detect environmental change; inspect some of Heard Island's heritage sites; check and maintain scientific equipment, and make sure refuge huts were still sound. Expeditioners aboard the Aurora Australis also collected further bathymetric data to improve charting in the region.
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