Title

Corangamite Coast Marine Habitat December 2009 

Alternate title

CORANGAMITE_MARINE_HAB2009 

Purpose

This dataset aimed to improve knowledge about the location, spatial distribution, condition, and extent of shallow marine habitats (less than 15 m) and associated biodiversity in the Central Victorian marine bioregion. The project filled a knowledge gap and increased the capacity of natural resource managers to make informed decisions concerning asset identification, risk assessment, and establishment of management action targets for nearshore marine habitats in the region. 

Credit

David Ball 

Supplemental Information

History: This layer combines two datasets for east and west Corrangamite into a single layer. Relationship to other Datasets: This dataset includes basic habitat attributes relating to habitat substratum with related video ground-truthing data providing further information relating to biota. Current Design Issues: This dataset was created to improve knowledge about the location and spatial distribution of shallow marine habitats in the Corangamite Bass Strait coastal region. The mapping covers the marine habitats to a depth of up to 15m, dependent on water clarity in the aerial imagery. The dataset is split into two shapefiles covering the MGA Zone 54 and Zone 55 areas in the Corangamite region. Future Design Issues: None. Related Documents: None Underwater video ground-truthing shallow marine habitats Corangamite. 

Status

Completed  

Horizontal Accuracy

200m  

Code

4283 

Maintenance and update frequency

As needed  

Topic category

• Oceans

Use limitation

. 

Classification

Unclassified  

Statement

Dataset Source: Aerial photography flown for DSE at various dates between 27 August 2002 and 17 December 2007. Video ground-truthing by DPI Queenscliff between 27 December 2007 and 9 April 2008. Dataset Originality: Primary & Derived 

Description

Collection Method: Aerial photography interpretation and underwater video ground-truthing 

Description

Source description not available 

Description

Aerial photography of the Corangamite coastal region flown for DSE was delivered to Fisheries Victoria Fisheries Research Branch (FRB) as geotiff files for each frame and a colour balanced mosaic of all the photography in a compressed ECW format. The geotiff files were used for the aerial mapping. Marine habitat mapping was undertaken with the remote sensing software ER Mapper and ENVI. A mapping region was extracted from the photography and exported to a separate geotiff file. The mapping approach applied a principal components transformation to the red, green and blue colour bands to create a single-band image of the first principal component. An unsupervised classification was then applied to this 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 image 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. Reef, Patchy Reef, Rock Platform or Sediment). 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 low pass filter was applied to smooth out any image noise in the data. The final phase of the 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 rock reef. A "smoothing" algorithm was applied during the raster to vector conversion process. The marine habitat shapefile from the raster to vector conversion was converted to an ArcInfo layer using the Regionpoly command in ArcInfo, and polygon topology was created by running the Clean command. To remove extraneous polygons generated by the raster to vector conversion, the Eliminate command was used to remove all polygons less than 5 m2. A text attribute "Habitat" was then added to the layer and the habitat description was added for the relevant codes. The ArcInfo layer was then exported back into a shapefile format. Some areas were not able to be mapped using the automated mapping process outlined above due to poor image quality. These areas were mapped by manually digitising habitat features with the ArcGIS software. Habitat mapping from the aerial imagery was ground-truthed with underwater video. The ground-truthing sites were positioned to maximise coverage of field observations across the range of shallow marine 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 marine scientist at FRB to identify the dominant habitat and biota categories described in Blake et al. (2009). Dominant habitat categories included Low Profile Reef (less than 1 m), High Profile Reef (greater than 1 m), and Bare and Vegetated Sediment. Dominant biota categories described the macroalgae and seagrass species and presence of sessile invertebrates. The video point data was overlayed on the habitat mapping to determine the mapping accuracy. 

Classification

Unclassified  

Attribute Quality

Comments

The habitat layer attributes describe the dominant habitat (substratum). Overall mapping classification accuracy was determined by generating an error matrix from the video ground-truthing overlayed on the mapping. The overall mapping accuracy was 81.8% based on 143 video ground-truthing points. A lower producer's accuracy in classifying patchy reef (15.4% based on 26 video ground-truthing points) was primarily due to the different classification scale between the mapping from aerial imagery and underwater video interpretation. Areas classified as patchy reef in the video may have been mapped as multiple small separate areas of reef separated by sand. The mapping accuracy for all reef combined was 96% (91 of 95 video sites). 

Positional Accuracy

Comments

Positional accuracy of the mapping is dependent on the accuracy of the source aerial photography (see the source imagery's metadata for more information), and the accuracy of the habitat boundaries is related to the mapping accuracy, which was determined by comparison with video-ground-truthing. No quantitative measure of positional accuracy was undertaken. 

Conceptual Consistency

Comments

Polygon topology was created in ArcInfo and all polygons were checked to ensure they had labels and an attribute value. 

Missing Data

Comments

This layer is complete for the western Corangamite coastal region. Completeness Verification: The dataset has been verified for all attributes. 

Excess Data

Comments

Attributes describe the dominant habitat type.