Title

Port Phillip Bay seagrass mapping at nine aerial assessment regions in April 2011 

Alternate title

SEAGRASS_PPB_9REGIONS_APRIL11 

Resource Type

Dataset  

Purpose

The objective of the Baywide Seagrass Monitoring Program is to detect changes in seagrass health outside expected variability. Seagrass is being monitored at several spatial scales including aerial mapping undertaken annually in April/May. This data presents seagrass mapping from aerial photography at one of nine detailed aerial assessment regions established in the main seagrass areas in Port Phillip Bay. The seagrass mapping will be compared to the previous year's mapping and historic mapping to detect changes in percent cover outside expected variability. 

Credit

David Ball 

Supplemental Information

History: See seagrass mapping in these regions from 2008-10. Relationship to other Datasets: This data forms part of a range of seagrass health measurements being recorded across Port Phillip Bay at as part of the Baywide Seagrass Monitoring Program. Additional data collection at six of the regions (all except Altona-Point Cook, Curlewis Bank and Point Henry West) includes field-surveys using fixed quadrats at seagrass plots within the mapping area. Current Design Issues: The mapping design is described in the Baywide Seagrass Monitoring Program Detailed Design (CDP_ENV_MD_022 Rev5 - Port of Melbourne Corporation, 2010). Related Documents: None Underwater video ground-truthing 2011, April 2011 aerial photography 

Status

Completed  

Time period

2011-04-062011-05-30 

Spatial representation type

Vector  

Horizontal Accuracy

200m  

Code

4283 

 

Maintenance and update frequency

Annually  

Topic category

• Biota
• Farming
• Oceans

Use limitation

. 

Classification

Unclassified  

Statement

Dataset Source: Aerial photography flown by Aerometrex Pty Ltd 6-7 April 2011 and video ground-truthing by DPI Queenscliff from 19 April to 30 May 2011. Dataset Originality: Primary & Derived 

Description

Collection Method: Aerial photo interpretation, underwater video ground-truthing 

Description

Source description not available 

Description

Source description not available 

Description

Source description not available 

Description

Source description not available 

Description

Aerial photography of the Port Phillip Bay seagrass areas was flown by a consultant (Aerometrex Pty Ltd) on 6 April 2011 for the Kirk Point and Altona-Point Cook regions, and on 7 April 2011 for the other regions. As far as possible, the photography was flown under optimum conditions for seabed habitat. The orthorectified aerial photography was delivered to Fisheries Victoria Fisheries Research Branch (FRB) as unprocessed geotiff files for each frame and a colour balanced mosaic of all the photography in a compressed ECW format. The unprocessed geotiff files were used for the aerial mapping. Seagrass habitat mapping was undertaken with the remote sensing software ENVI. A subset of the area to be mapped was extracted from the photography and exported to a separate ENVI image file. For all regions except Blairgowrie and Point Henry West, the mapping approach applied a principal components transformation to the red, green, blue and infra red colour bands to create a four band principal component image. In Blairgowrie, a single band image of the first principal component was created from the red, green and blue colour bands only, while no principle components transformation was done in Point Henry West. An unsupervised classification was then applied to bands one and two of this image, except for the Altona-Point Cook, Kirk Point, Mud Islands and Point Richards-Bellarine Bank regions, where it was only applied to the first band, and Blairgowrie where it was applied to the whole image, to transform it into a thematic map. The output from this process was a single band image with 15 values representing the number of categories defined by the unsupervised classification. The classified image was overlayed on the aerial photography to allow a visual comparison. Groups of pixel values within the classified image were then assigned to a thematic classification based on the dominant seabed type (i.e. sparse seagrass, medium-dense seagrass or sediment) using the 'Combine Classes' function in ENVI. The classified image was then re-calculated so that a single pixel value corresponded to a single habitat type. The small pixel size of the aerial photography (0.3 m) meant that even the classified image retained isolated pixels or noise in the data. Following the unsupervised classification, a median filter was applied to smooth out any image noise in the data. The final phase of the seagrass mapping was to convert the classified raster image to a vector layer (shapefile) in ArcGIS. The classified raster data was made up of a grid of pixels with each pixel assigned a value representing the habitat feature at that location. Groups of adjacent pixels with the same value represented a single habitat feature such as a seagrass bed. A "smoothing" algorithm was applied during the raster to vector conversion process. The seagrass habitat shapefile from the raster to vector conversion was imported to a Geodatabase feature class in ArcGIS and polygon topology was created. To remove extraneous polygons generated by the raster to vector conversion, the Eliminate command was used to remove all polygons less than 2 m2, except in the Blairgowrie region, where polygons less than 1m2 were removed. The layer was then checked on screen and some manual edits were made to seagrass polygons which did not accurately match the aerial photography. A text attribute "Habitat" was then added to the layer and the habitat description was added for the relevant codes. The Geodatabase feature class was then exported back into a shapefile format to allow distribution. Seagrass mapping from the aerial photography was ground-truthed with underwater video. The ground-truthing sites were positioned to maximise coverage of field observations across the range of seagrass habitats present and to target any areas of uncertain classification. Video sites were also positioned amongst bare sediment to assist in assessing the mapping accuracy. The video was interpreted by a benthic ecologist at FRB to identify the dominant habitat. For each seagrass site the seagrass density was classified as Bare <1% cover, Very Sparse 1-10% cover, Sparse 11-20% cover, Medium 21-60% cover, Dense 61-100% cover. Seagrass densities <10% are typically below the level able to be interpreted from aerial photography. Seagrass epiphyte cover (%) was also estimated from the video. The video point data was overlayed on the seagrass habitat mapping to determine the mapping accuracy. Initially, nine separate layers were created, one for each region in Port Phillip Bay. Later, ArcToolbox's Merge tool was used to merge these nine shapefiles into a single shapefile, and a consistency check was applied to the final shapefile's attributes. 

Classification

Unclassified