Title

East Gippsland Marine Habitats November 2009 

Alternate title

EAST_GIPPS_MARINE_HAB_2009 

Purpose

The purpose of this dataset is to improve knowledge about the location, spatial distribution, condition, and extent of shallow marine habitats (<15 m) and associated biodiversity in the Twofold Shelf 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 management action targets for nearshore marine habitats in the region. 

Credit

David Ball 

Supplemental Information

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 East Gippsland region. The mapping covers the marine habitats to a depth of up to 15 m, dependent on the water clarity in the aerial imagery. Related Documents: None Underwater video ground-truthing shallow marine habitats East Gippsland. 

Status

Completed  

Horizontal Accuracy

200m  

Code

4283 

Maintenance and update frequency

As needed  

Topic category

• Oceans

Use limitation

Unrestricted 

Classification

Unclassified  

Statement

Dataset Source: Some data was obtained from aerial photography flown for DSE on 20 November 2007. For the areas not covered by this aerial photography, QuickBird satellite imagery was acquired from the DigitalGlobe archives through Sinclair Knight Merz. This imagery was captured on the following dates: 14 Dec 2003 (Long Reef region from The Skerries to Sandpatch Point), 21 Aug 2005 (Gabo Island region), 8 Sep 2005 (Seal Cove and Mallacoota regions), 7 Dec 2005 (Long Reef region from Benedore River to Little Rame Head), 26 Sep 2006 (Ewing Marsh region), 6 Oct 2007 (Marlo region) and 6 Nov 2007 (Pearl Point region). Dataset Originality: Primary & Derived 

Description

Collection Method: Interpretation of aerial photography and satellite images, video ground-truthing 

Description

Aerial photography of the East Gippsland region was provided to Fisheries Victoria Fisheries Research Branch (FRB) by DSE 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. The Quickbird satellite imagery was delivered as ortho ready images and orthorectified by FRB with ENVI using the provided RPC files, a DEM created from VicMap topographic data and ground control points selected from the aerial photography where it overlapped the satellite imagery. The orthorectified satellite images were saved in geotiff format. Marine habitat mapping was undertaken with the remote sensing softwares ER Mapper and ENVI. A mapping region was extracted from the image 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 imagery (0.3-0.6m) 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. The final shapefile was then dissolved using arc Toolbox based on the Habitat, Location and Source fields. 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 (<1 m), High Profile Reef (>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 80.3% based on 127 video ground-truthing points. A lower producer's accuracy in classifying patchy reef (8% based on 25 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 94% (80 of 85 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

The entire mapping region is covered by this layer Completeness Verification: The dataset has been verified for all attributes. 

Excess Data

Comments

Attributes describe the dominant marine habitat type.