Title

Port Phillip Bay seagrass mapping at nine aerial assessment regions in April 2008 

Alternate title

SEAGRASS_PPB_9REGIONS_APRIL08 

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. This dataset replaces the datasets SEAGRASS_ALTONA_2008, SEAGRASS_BLAIRGOWRIE_2008, SEAGRASS_CURLEWIS_BANK_2008, SEAGRASS_KIRK_POINT_2008, SEAGRASS_MUD_ISLANDS_2008, SEAGRASS_PT_HENRY_WEST_2008, SEAGRASS_PT_RICHARDS_2008, SEAGRASS_ST_LEONARDS_2008 and SEAGRASS_SWAN_BAY_2008. 

Credit

David Ball 

Supplemental Information

Relationship to other Datasets: This data forms part of a range of seagrass health measurements being recorded across Port Phillip Bay 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 2008, April 2008 aerial photography 

Status

Completed  

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 from 20-24 April 2008 and video ground-truthing by DPI Queenscliff from May-August 2008. 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

Aerial photography of the Port Phillip Bay seagrass areas was flown by a consultant (AEROmetrex Pty Ltd) on 20 April 2008 for the Altona-Point Cook and Kirk Point regions, 23 April 2008 for the Curlewis Bank, Point Henry West and Point Richards-Bellarine Bank regions and 24 April 2008 for the Blairgowrie, Mud Islands, St. Leonards and Swan Bay 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 ER Mapper. 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 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). 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 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 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 1 m2 from the Blairgowrie region and less than 2 m2 from all of the other regions. The 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 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, Sparse 1-20% cover, Medium 21-60% cover, Dense 61-100% cover. The sparse seagrass was further split into a very sparse (1-10%) category as 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, one for each region in Port Phillip Bay were created. Later, the nine shapefiles were merged into a single shapefile using ArcToolbox's Merge tool and the attributes of the final shapefile were checked for consistency. 

Classification

Unclassified  

Attribute Quality

Comments

Description: The habitat layer attributes describe the dominant habitat (substratum and vegetation) and vegetation density. The accuracy of the mapping was determined by comparison to point observations from underwater video. Determination: Overall mapping classification accuracy was determined by generating an error matrix from the video ground-truthing overlayed on the mapping. The overall mapping accuracies for the regions are: Altona-Point Cook: 100% based on 15 video ground-truthing points. Blairgowrie: 92% based on 25 video ground-truthing points. Curlewis Bank: 100% based on 9 video ground-truthing points. Kirk Point: 100% based on 3 video ground-truthing points. Mud Islands: 100% based on 13 video ground-truthing points. Point Henry West: 100% based on 12 video ground-truthing points. Point Richards-Bellarine Bank: 58% based on 26 video ground-truthing points. St Leonards: 50% based on 6 video ground-truthing points. Swan Bay: 77% based on 13 video ground-truthing points. 

Positional Accuracy

Comments

Description: Positional accuracy of the mapping is dependent on the accuracy of the source aerial photography, and the accuracy of the habitat boundaries is related to the mapping accuracy. Determination: The estimate of positional accuracy for the aerial photography was determined by comparison to known ground-control points. Overall mapping accuracy was determined by comparison with video-ground-truthing, and 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

All of the regions mapped are covered by this layer Completeness Verification: The dataset has been verified for all attributes. 

Excess Data

Comments

Attributes describe the dominant habitat type but are not exhaustive. The habitat classification groups all seagrass and macroalgae into single categories as it is not possible to derive species type from aerial photography interpretation.