source: production/onslow_2006/report/data.tex @ 3390

The calculated run-up height and resulting inundation ashore is determined by
the input topographic and bathymetric elevation, the
initial and boundary conditions, as well as the cell area of the computational
mesh. 
Ideally, the data should adequately capture all complex features
of the underlying bathymetry and topography. Any limitations
in the resolution and accuracy of the data will introduce 
errors to the inundation maps, in addition to the range of approximations
made within the model.

In this study, we used the Australian Height Datum (AHD)
as the vertical datum. Mean Sea Level (MSL) is approximately equal to
0m AHD with the Highest Astronomical Tide (HAT) 
and Lowest Astronomical Tide (LAT) defined as 1.5m AHD
and -1.5m AHD respectively for Onslow \cite{antt:06}. 
These values are tidal
predictions based on continous tidal observations from Standard Ports
over a period of
at least one year, with the Australian Hydrographic Service
recommending this be extended to three years to capture
changes to the mean sea level. Onslow is listed as
a Standard Port. As an aside, current work at GA is
extracting information from LANDSAT imagery to reconstruct the 
tidal variations for various WA locations. Future modelling of
these areas will incorporate this information.

Data for this study have been sourced from a number of agencies. With
respect to the onshore data, the Defence Imagery and Geospatial
Organisation (DIGO) supplied the Digital Terrain Elevation
Data Level 2 (DTED) which has been authorised for Australian Tsunami
Warning System use only. The resolution of this data is 1 second
(about 30 metres), and has been produced from 1:50 000 contours, elevations and
drainage. In addition, the Department of Land Information (DLI) has provided a
20m Digital Elevation Model (DEM) and orthophotography 
covering the NW Shelf. The DTED Level 2 data is ``bare earth'' while the 
DLI data is distorted by vegetation and buildings.  

Figure \ref{fig:contours_compare}(a) shows the contour lines for
HAT, MSL and LAT for Onslow using the DTED data where it is evident 
that the extent of the tidal inundation is exaggerated. In particular,
parts of Onslow town appears to be inundated at HAT before a tsunami has
even been generated. This is due to
short comings with the digital elevation model (DEM) created from 
the DTED data. 
Figure \ref{fig:contours_compare}(b) shows
the contour lines for HAT, MSL and LAT for Onslow using the WA DLI data.
It is obvious that there are significant differences in each DEM with
total station survey information and the knowledge
of the HAT contour line pointing to increased confidence in the WA DLI
data over the DTED data for use in the inundation modelling.
The impact difference based on these two onshore data sets 
will be discussed in Section \ref{sec:issues}.


\begin{figure}[p]
\center{(a)}
  \centerline{ \includegraphics[width=150mm, height=100mm]
{../report_figures/onslow_dted_contour.jpg}}

 % \caption{Onslow region showing the -1.5m AHD (LAT), 0m AHD (MSL)
 %and -1.5m AHD (LAT) contour lines using the DTED Level 2 data.}
 % \label{fig:contours_dted}
%\end{figure}

%\begin{figure}[hbt]
\center{(b)}
  \centerline{ \includegraphics[width=150mm, height=100mm]
{../report_figures/onslow_dli_contour.jpg}}

  \caption{Onslow region showing the -1.5m AHD (LAT), 0m AHD (MSL) 
and 1.5m AHD (HAT) contour lines using the (a) DTED Level 2 data and
the (b) WA DLI data.}
 % \label{fig:contours_dli}
 \label{fig:contours_compare}
\end{figure}

With respect to the offshore data, the Department of Planning and
Infrastructure (DPI) have provided state digital fairsheet data around
Onslow. This data cover only a very small geographic area. (Note,
similar data have been provided by DPI for Pt Hedland and Broome.) 
The Australian Hydrographic Office (AHO) has supplied extensive
fairsheet data which has also been utilised. In contrast to the onshore data, the offshore data is a series of survey points which is typically not supplied on a fixed grid. In addition, offshore data typically does not have the coverage of the onshore data, and often the offshore data will have gaps where surveys have not been conducted.
The coastline has been generated by
using the aerial photography, two detailed surveys provided
by WA DPI and a number of total station surveys of Onslow. 
The WA DLI data surrounding the coast are error prone and
have been clipped at the derived coastline. 
Appendix \ref{sec:metadata} provides more details and the supporting metadata 
for this study, including images of the data extent.
Table \ref{table:data} summarises the available data. 

\begin{table}
\caption{Available data for the North West shelf tsunami inundation studies.}
\label{table:data}
\begin{center}
\begin{tabular}{|l|l|}\hline
Data & Detail \\ \hline 
DIGO DTED Level 2  & Onshore, 1 second $\approx$ 30m \\ \hline 
DLI & Onshore, 20m DEM and orthophotography \\ \hline
DPI & Offshore, fairsheet data around Onslow \\ \hline
AHO & Offshore, fairsheet data for North West Shelf region \\ \hline
\end{tabular}
\end{center}
\end{table}


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