The file magmap_V5_2010 is a composite TMI grid of the Australian region with a grid cell spacing of ~3 seconds of arc (approximately 80 m). This grid only includes airborne-derived TMI data for onshore and near-offshore continental areas. Data used to compile this grid form the basis for the concurrent release of a new fifth edition of the Magnetic Anomaly Map of Australia. This edition is underpinned by the concept of a database of matched TMI grids, from which any map at any resolution can be extracted. Details of the specifications of individual airborne surveys can be found in the Eleventh Edition of the Index of Airborne Geophysical Surveys (Percival, 2010), which is included with the grid as a PDF document called magmap_V5_2010.pdf. This Index is also available online at http://www.ga.gov.au/minerals/research/methodology/geophysics/acquisition.jsp#reports. 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/. The grid results from a completely new compilation of TMI grid data, an updated method of matching the individual survey grids, and use of independent data to constrain long wavelengths. 795 individual grids have been matched and merged into the composite grids supplied here. The resolution of each grid is optimal for the specifications of the source survey line data (Briggs, 1974). Since the fourth edition was released in 2004 data from many new surveys have been added to the database, acquired mainly by the State and Territory Geological Surveys. It is estimated that 27 000 000 line-kilometres of survey data were acquired to produce the grid data, 8 000 000 line-kilometres more than for the previous edition. The index map (Figure 1) shows the distribution of original survey line spacings from which the grids are derived, Figure 2 shows the distribution of all survey line spacings, and Figure 3 shows the survey ownerships. 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. It uses statistics of the overlap regions between adjacent surveys to globally minimise the differences in the base levels of all surveys (Minty et al., 2003). There is also provision to remove higher-order surfaces from grids. To constrain long wavelengths, an independent data set, the Australia-wide Airborne Geophysical Survey (AWAGS) airborne magnetic data, was used to control the base levels of those survey grids which overlapped the AWAGS data (Milligan et al., 2009). The high-frequency differences between the grids were smoothed using a convolution operator. Very long wavelengths greater than 1000 km have been constrained by using the MF6 data derived from the CHAMP satellite (Maus et al., 2007). Grid downloads: The Magnetic Map of Australia grid can be downloaded using the Geophysical Archive Data Delivery System (GADDS) on the Australian Government's Geoscience Portal at http://www.geoscience.gov.au/bin/mapserv36?map=/public/http/www/geoportal/gadds/gadds.map File size: At full resolution, the Magnetic Map of Australia grid has 41876 rows and 50592 columns and has a file size of approximately 8.3 Gb in ERMapper format. Note that, because of GADDS file size limits, it is not possible for clients to download very large areas of the grid at full resolution. Clients wishing to do so should contact Geoscience Australia to make special arrangements to have the complete grid dataset provided on a user-supplied portable hard drive.
