Ratio Th/K - Ratio of Th over K derived from the filtered Th and K grids, units: dimensionless
The Radiometric Map of Australia dataset comprises grids of potassium (K), uranium (U) and thorium (Th) element concentrations, and derivatives of these grids. The third edition was derived by seamlessly merging 45 new survey grids with the Second Edition Radiometric Map of Australia (Minty et al., 2010).
Details of the specifications of individual airborne surveys can be found in the Fourteenth Edition of the Index of Airborne Geophysical Surveys (Percival, 2014), which is included with the grid as a PDF document. This Index is also available online at http://pid.geoscience.gov.au/dataset/ga/79134. Further up to date information about individual surveys can also be obtained online from the Airborne Surveys Database at http://www.ga.gov.au/oracle/argus/.
Matching of the grids in the database was achieved using a program called Gridmerge, which was originally developed within Geoscience Australia and has now been commercialised. This program was used to merge 45 new surveys to the Second Edition Radiometric Map of Australia. The second edition merged over 550 individual grids to create the compilation (Minty et al., 2009) and the Australia-wide Airborne Geophysical Survey (AWAGS) airborne radiometric data was used to control the base levels of those survey grids which overlapped the AWAGS data (Milligan et al., 2009). As the second edition was used as a base grid for the Gridmerge operation the new Third Edition is essentially levelled to AWAGS.
Cell sizes: The cell sizes of the original survey grids range from 50 m through 800 m, but most have a cell size of about 100 m. The 45 original survey grids were levelled and then re-sampled, using Newton 4th Order local operator onto the Second Edition Radiometric Map of Australia Grids with a cell size of about 100m (0.001 degrees).
Filtering: Potassium, uranium, thorium and dose rate grid are available in both filtered and unfiltered versions. The low-pass filtering was achieved by applying a 7-point, degree-3 Savitzky-Golay filter (Savitzky & Golay, 1964) to each of the original survey grids prior to grid merging.
Projections and Datums: The grids are stored as geodetic grids based on the GDA94 datum, but can be re-projected prior to downloading.
Grid downloads: The Radiometric Map of Australia grids can be downloaded using the Geophysical Archive Data Delivery System (GADDS) on the Australian Government's Geoscience Portal at http://www.geoscience.gov.au/gadds
File sizes: At full resolution, each Radiometric Map of Australia grid has 34761 rows and 40954 columns. Each grid has a file size of approximately 5.3 Gb in ERMapper format. Note that, because of the file sizes, GADDS will not allow users to do download the grids at full resolution. Users wishing to access the grids at full resolution should contact Geoscience Australia to make arrangements to have the data supplied on a portable hard drive.