Christiaan M Roelfsema

Georeferenced photographs of benthic photoquadrats acquired along 63 transects distributed over 12 reefs in the Far North region of the Great Barrier Reef, December, 2017

Underwater georeferenced photo-transect surveys were conducted at different locations: in the shallow waters of the Reef Flat (0.5-2.5m) by a snorkeller, or, along a 5m depth contour of the Reef Slope by a diver. For these surveys, the snorkellers or divers traversed a pre-determined transect 250-1000 m in length, while taking photos of the benthos at a set height using a standard digital camera and towing a surface float GPS which was logging its track every five seconds. The camera lens provided a 1.0 m x 1.0 m footprint, at 0.5 m height above the benthos. Horizontal distance between photoquadrats was estimated by fin kicks of the surveyor, and corresponded to a surface distance of approximately 2.0 - 4.0 m. The coordinates of each photoquadrat were approximated based on the timestamp of the photoquadrat and the GPS timestamp. Coordinates of each photoquadrat were interpolated by finding the GPS coordinates that were logged at a set time before and after the photoquadrat was captured. 12 Datasets in Pangaea Photoquadrats were collected with the purpose of determination of the benthic composition of each photoquadrat and to subsequently use this georeferenced field data for calibration and validation of benthic habitat maps. Transect location, direction and depth were chosen for this purpose and so as to characterise the variation in benthic cover types present on coral reefs. Photoquadrat interval along the transects was chosen to reflect the resolution of high spatial resolution satellite image data types.

Mapped distribution of intertidal habitats in Australia between 1999 and 2014, link to data in ArcGIS format (29 MB)

Mapping of distribution of intertidal habitats in Australia, and identification of percentage of marine and terrestrial protected areas.

Spectral reflectance library of algal, seagrass and substrate types in Moreton Bay, Australia.

Ten samples of each substrata were collected randomly and placed on a black background where each sample covered a homogenous 5 cm x 5 cm area. Radiance-reflectance measurements were collected using an Analytical Spectral Devices spectrometer (ASD VNIR), which recorded in the visible to infrared (400-1050 nm) wavelengths in 1024 bands (at 2 nm intervals, with a 7 nm Full-Width-Half Maximum power resolution), using a 68 field of view. Upwelling radiance measurements (Lu) were obtained in sunlight by placing the spectrometer optics 15 cm vertically above the target. A SpectralonTM panel, approximating a 100% reflective Lambertian surface, was measured prior to each sample as a surrogate for down-welling irradiance (Ed). A radiance-reflectance signature was then calculated for each sample by dividing the target radiance by the Spectralon panel radiance (Lu/Ed). A mean spectral reflectance signature was produced by averaging 10 measurements for each target. Spectral signatures include: seagrass and algae species, sand, mud and cyanobacteria (Lyngbya majuscula).

Habitat map of seagrass cover derived from a supervised moderate-spatial-resolution multi-spectral satellite image, integrated with manual delineation and coincident field data, Moreton Bay, 2004, with link to shapefile

The substrate DN signatures were extracted from the Landsat 5 TM image for field survey locations of known substrate cover, enabling a characteristic qspectral reflectance signatureq to be defined for each target. The Landsat TM image, containing only those pixels in water l 3.0m deep, was then subject to minimum distance to means algorithm to group pixels with similar DN signatures (assumed to correspond to the different substrata). This process enabled each pixel to be assigned a label of either seagrass cover (0, 1-25 %, 25-50 %, 50-75 % and 75-100 %). The resulting raster data was then converted into a vector polygon file. Species information was added based on the field data and expert knowledge. Both polygon files were joined by overlaying features of remote sensing files with the EHMP field data to produce an output theme that contains the attributes and full extent of both themes. If polygons of remote sensing were within polygons of field data the assumption was made that the remote sensing polygon was showing more detail and the underlying field polygon was deleted.