Bathymetric Grid of Heard Island - Kerguelen Plateau Region (2005)

Brief description

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 ANZCW0703009248, and can be found on the Australian Spatial Data Directory website - see the URL given below).

Taken from the report:
A bathymetric grid of the Heard Island-Kerguelen Plateau Region (Longitudes 68 degrees E - 80 degrees E, Latitudes 48 degrees S - 56 degrees S) is produced. In doing so, the individual datasets used have been closely examined and any deficiencies noted for further follow up or have been rectified immediately and the changes documented. These datasets include modern multibeam data, coastline data obtained from the World Vector Shoreline, echosounder data from research, fishing and Customs 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 - Desmond Fitzgerald Associates) and an ERMapper format grid produced.

A grid cell size of 0.005 degrees (nominal 500m) was used with many iterations of minimum curvature gridding and several passes of smoothing.

The final grid is available in ERMapper, ArcInfo and ASCII xyz formats.

Lineage

Progress Code: completed

Statement: Lineage:

The grid is derived from data in Geoscience Australia databases and recent sources which will eventually be entered into those databases. One swath dataset acquired in 2004 was used but covers only a small area in the south-east of the region. An acceptable quality coastline of the islands in the region was obtained via the NGDC web-site. Thirty-three surveys from the G.A. MARDAT/OZMARII database, ranging in vintage from 1956 to 1999, and data from five ANARE years were incorporated into the gridding process. Finally satellite derived bathymetry from the SIO website was used to infill areas that had no higher quality observations within a nominal 10 kilometre radius.

Positional accuracy:

The grid incorporates data from surveys acquired since 1956. 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. The principal dataset was sourced from recent fishing ventures in the region. Whilst G.A. has no documentary evidence of the data quality (or metadata), the internal consistency of the data leads us to believe that it is a good dataset. As it is also the largest dataset, it was chosen to be the principal dataset for masking purposes

The grid cell size is 0.005 degrees (nominal 500m).

If displayed at 2 pixels = 1mm, nominal scale is 1:1,000,000 giving a plot size of (1.2 m x 0.8 m).

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 allow one to determine the bathymetry quite accurately. Unfortunately this situation exists in only a small portion of this region.

The fishing fleet data have documented that a speed of sound of 1498 m/sec has been used. Presumably appropriate draft corrections have also been applied.

The data from the AAD probably used a value of 1500 m/sec in determining the water depth although we have no documentary evidence that this is the case.

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.

The data from MARDAT/OZMARII used a range of values for the speed of sound in water and some have even been Carter corrected. Details can be found in the individual database entries.

Logical Consistency:

Each of the input datasets were 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 - Desmond Fitzgerald Associates) and an ERMapper format grid produced.

Completeness:

All of the known, available data (to 2005) were used in the production of the grid. 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.