Bathymetric grid of Macquarie Island Region (2004)
Lineage: The grid is derived from data in Geoscience Australia databases and recent sources which will eventually be entered into those databases. Eight swath datasets acquired between 1994 and 2003 were used. Two of the surveys supply extensive areal coverage, whilst the others are basically multib...
| Summary: | Lineage: The grid is derived from data in Geoscience Australia databases and recent sources which will eventually be entered into those databases. Eight swath datasets acquired between 1994 and 2003 were used. Two of the surveys supply extensive areal coverage, whilst the others are basically multibeam transits of the area. Detailed hydrographic data (including a 1999 LADS dataset near the northern tip of Macquarie Island) exists around the island. A high quality coastline of Macquarie Island from the National Mapping Division of G.A. (1994) and a coastline of the New Zealand islands obtained from the NOAA website were used . High quality bathymetric data (post 1997) from Austral Fisheries Pty Ltd were made available via the AAD. Twenty-five surveys from the G.A. MARDAT database. These data range in vintage from 1965 to 1998 and were edited to remove suspect datapoints, but were left uncorrected with regards speed of sound issues. All available data in the area from the AAD voyages in the region. Extensive editing and a weighting scheme were employed to discriminate between closely spaced data of varying vintages. Other trackline data on NGDC and CSIRO websites that were not in MARDAT were also utilised. Satellite derived bathymetry from the SIO website was used to infill areas that had no higher quality observations within a nominal 10 kilometres. Positional accuracy: The grid incorporates data from surveys acquired since 1965. Modern surveys which used GPS have a positional accuracy of 5 - 30 m depending on several factors, while earlier surveys which used dead reckoning and Transit satellite fixes had positions accurate to 50-2000 m depending upon the water depth and strength of currents. These surveys overlap in an irregular distribution and the more extensive, higher quality swath datasets were used to mask data of lower quality and extent. Effectively there are two regions in the grid. One covered by multibeam data of high positional and bathymetric accuracy and the other heavily reliant on satellite data to provide infill between sparse ship track data of lower positional and bathymetric accuracy. The grid cell size is 0.00225 (nominal 250m). If displayed at 300dpi, nominal scale is 1:3,000,000 giving a plot size of (60cm x 53 cm) Attribute accuracy: The attribute accuracy varies depending upon the predominant data source in an area. Where modern, high quality swath bathymetric data that form an areal coverage exists, overlapping swaths and speed of sound corrections show that the data are quite accurate. Some transit swath data also have speed of sound corrections applied and are assumed to be highly accurate. Other transit swath surveys have utilized a nominal speed of sound of 1500 m/s and are not as accurate (problems were encountered trying to apply correction). Most other data assume a nominal speed of sound of 1500 m/s, although some The attribute accuracy varies depending upon the predominant data source in an area. Where modern, high quality swath bathymetric data that form an areal coverage exists, overlapping swaths and speed of sound corrections show that the data are quite accurate. Some transit swath data also have speed of sound corrections applied and are assumed to be highly accurate. Other transit swath surveys have utilized a nominal speed of sound of 1500 m/s and are not as accurate (problems were encountered trying to apply correction). Most other data assume a nominal speed of sound of 1500 m/s, although some have had corrections applied. For a significant portion of the region, predicted bathymetry from satellite data are the only source of data and the accuracy of these data are known to be good in a regional sense only. No attempts have been made to compare bathymetry from high quality datasets to that which would be obtained from lesser data sets, although all the data exist to perform such a task. Logical Consistency: Each of the input datasets was examined in detail and edited where necessary. Areas of poor navigation and obviously bad bathymetry were discarded. A hierarchical system was employed whereby the best and most extensive datasets were gridded first and applied as a mask to the next best dataset. A new masking grid would be formed from these datasets to pass non-overlapping data in the next best dataset. This procedure was employed until finally the satellite data were masked. All the various levels of masked data were then brought together by the gridding algorithm (Intrepid and Desmond Fitzgerald Associates) and an ERMapper format grid produced. Completeness: All of the known, available data (to 2004) were used in the production of the grid. The only data known to not be included are some surveys within the New Zealand EEZ, but these proved impossible to obtain. The G.A. databases which underpin this grid will be updated as new surveys are completed and older surveys have corrections applied to them. It is planned to be somewhat proactive and asking regular visitors to the region to vary their transits slightly to extend the coverage over time. A data density grid was produced as a means of assessing the completeness of coverage and it could possibly be used as a variably opaque overlay of the bathymetric grid to highlight good quality areas and darken lesser quality areas. This metadata record is a modified child record of an original parent record originating from custodians of data associated with Geoscience Australia (The identifier of the parent record is ANZCW0703006701, and can be found on the Australian Spatial Data Directory website - see the URL given below). A bathymetric grid of the Macquarie Island Region (Longitudes 151 E and 167 E, Latitudes 48 S and 62 S) was produced. In doing so, the individual datasets used were closely examined and any deficiencies noted for further follow up or were rectified immediately and the changes documented. These datasets include modern multibeam data, coastline data obtained from georeferenced SPOT imagery, hydrographic quality data, echosounder data from research and fishing vessels and satellite derived bathymetric data. A hierarchical system was employed whereby the best and most extensive datasets were gridded first and applied as a mask to the next best dataset. A new masking grid would be formed from these datasets to pass non-overlapping data in the next best dataset. This procedure was employed until finally the satellite data were masked. All the various levels of masked data were then brought together by the gridding algorithm (Intrepid and Desmond Fitzgerald Associates) and an ERMapper format grid produced. A grid cell size of 0.00225 (nominal 250m) was used with many iterations of minimum curvature gridding and several passes of smoothing. The final grid is available in geotiff, ArcInfo ascii and xyz text formats. A detailed report of the work completed is also available. |
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