During the acoustic field survey a GPS point is logged every 2 sec, resulting in habitats being attributed from the echogram on average every 2.7 m at a vessel speed of 5 knots. A minimum of six similar attributed points, relating to an average distance of around 15 m, are required for a polygon to be generated. Attributed points are connected at a scale of 1:2,000 to form polygons through a knowledge based interpolation process where the points are joined with similar neighboring transect points to make one habitat polygon.

For shallow Tasmanian coastal waters where rocky reef and unconsolidated habitats are the dominant habitat types this methodology appears to best represent the complex pattern of habitats. In addition, much of the area of the coastal seabed exists within depths that are suitable for defining habitat boundaries through both aerial photography and real-time attribution (generally less than 10 m depth). Combining this from polygon boundaries identified from acoustics in deeper water provides a consistent data model across all depths, but obviously greater uncertainties will exist with the acoustic data due to the need for interpolation.

It is difficult to verify the knowledge-based interpolation technique with unconsolidated habitats, as the boundaries are not visual or object based and the boundary is attributed by defining a position in the echogram. However, this is generally only an issue in sheltered waters in Tasmania where silt through to sand substrates exist over large distance due to the lack of steep depth gradients, in contrast to the majority of coastal waters that is exposed resulting in few boundaries between unconsolidated substrate (which is predominantly sand). While each habitat polygon can only occur as one category in the hierarchical classification table, within one map of coastal seabed habitats there may be varying levels of classification, spatial patterns (i.e. minimum mapping units) and uncertainties introduced due to differences in the resolution of airborne and vessel based instrumentation and survey design.

Depth measurements taken from the sounder are used to construct a contour layer. Data are cleaned for erroneous points and are corrected to Mean Sea Level after the depths are adjusted for tidal variation. Arc Info is used to interpolate a TIN (Triangular Irregular Network) surface bathymetric model. Contour lines are generally created for 5 m depth intervals out to the maximum depth of the acoustic transects (usually 40 m).