Seagrass Classification

In a project initiated by the South Australian Environment Protection Authority, aerial photography captured by the Aerial Survey Unit of the Department for Environment, Heritage and Aboriginal Affairs (now the Department for Environment and Heritage), was used to produce digital orthophotography along the Adelaide metropolitan coastline between the seaside suburbs of Largs Bay and Aldinga Beach.

Seven different epochs between 1949 and 1996 were used to observe change along the coastline. Each epoch was classified into sand, seagrass/detrital matter and rock. Sand and clay showed as light areas on the image, and were relatively easy to interpret. Dark areas could be seagrass, detrital matter, rock, algal mats or deep water, or a combination of features.

Rock was identified using local knowledge and visual interpretation. Rock, land and deep water were digitally cut out of the image as seagrass loss or increase was assumed not to occur on rock or land, and could not be detected by the aerial photography in deeper water. The light areas were classified as sand and the remaining areas assumed to be seagrass, detrital matter and algal mats.

Once the interpretations for each epoch were put together then detrital matter could be separated from seagrass and algal mats. Any ‘short term gain’ of seagrass and then ‘loss’ from one epoch to another can be assumed to be detrital matter and therefore actually still sand.

Areas classified as sand were combined into a single file, with earlier epochs overlain on top of later epochs. This showed areas of sand for each epoch. Area measures of sand gain were extracted from the image automatically. By overlaying each “sand epoch” it is possible to observe if, when and where sand has increased or decreased over time. As sand increase can be assumed to be equivalent to seagrass loss, it is an effective way to measure these changes over time, and to highlight particular areas where these changes are more apparent.

Sample files are shown below of an area off Point Malcolm in 1949 and 1996. The resultant classification classes are represented by the following colours:

• Green: Seagrass present in 1996
• Yellow: Sand present in 1996
• Cream: Sand present in 1949
• Brown: Land

Click on the images for a more detailed view.
1949 Seagrass Orthophoto	1996 Seagrass Orthophoto	1949 - 1996 Seagrass Change Classification