Constructing a Database of Coastal Change Using GIS
407
records: the UK Meteorological Office has developed a wave hindcasting system which
has been validated against a number of physical wave measurement stations (Reeve et
al., 1996). Tidal stage in the form of an ‘average’ tidal curve is available for a number
of standard ports around the coast of Britain; however, these are dominated by
estuarial rather than open coast sites where the tidal behaviour is often quite different.
Tidal flow rates are available for a number of points around the coastline, usually in
shipping lanes and not in the nearshore zone.
Integration of Coastal Spatio-Temporal Data Sources
While capturing data on coastal landforms and processes is problematic given the
difficulties of ground control for indirect methods and the difficulties of access and
instrumentation for direct methods, the integration of the collected data is also
challenging. Specific concerns include the handling of coordinates and datums, the
regionalisation of data collected at points and the structuring of spatio-temporal
datasets.
COORDINATE SYSTEMS AND DATUMS
Coastlines are difficult environments in which to integrate different data sources. In
general terrestrial and marine coordinate systems are different and require
transformations between projections and datum spheroids. There are different mapped
coastlines that differ by date of survey (problematic in a highly dynamic environment
like the coast) and also by the definition of sea level used. In elevation terms the
terrestrial map datum is usually related to mean high tide while the marine chart
datum is expressed as positive value below lowest astronomical tide (LAT). The
difference between the terrestrial datum and the marine datum varies according to
tidal range - which is spatially variable and often only measured at certain points e.g.
ports. This makes it difficult to create integrated terrestrial and marine elevation
models.
The ideal solution to this integration problem is to add the actual tidal range
at each point to the measured depths: this would then allow the terrestrial and marine
elevation models to be connected along the mean high tide line. Since tidal range data
is only available at a limited number of points on the coastline a method of
interpolation is required. One method that has been developed experimentally in
Norfolk is to linearly interpolate tidal range between known points along the coastline.
Where there are barrier islands, only the coastline with a long fetch to the open sea is
used and inlets are ignored. Using this method, where interpolated steps of 0.25m
change in tidal range are reached a line is drawn orthogonal to the coast until it exits
the area, reaches another coastline or until it intersects another such line.
This semi-automated procedure defines zones within which the tidal range
can be considered to be within 0.25m of each other. By adding this tidal range to all
measured depths below LAT within each zone, and changing the sign on the aggregate
value to negative, a depth below the mean high tide terrestrial datum is derived.
407
records: the UK Meteorological Office has developed a wave hindcasting system which
has been validated against a number of physical wave measurement stations (Reeve et
al., 1996). Tidal stage in the form of an ‘average’ tidal curve is available for a number
of standard ports around the coast of Britain; however, these are dominated by
estuarial rather than open coast sites where the tidal behaviour is often quite different.
Tidal flow rates are available for a number of points around the coastline, usually in
shipping lanes and not in the nearshore zone.
Integration of Coastal Spatio-Temporal Data Sources
While capturing data on coastal landforms and processes is problematic given the
difficulties of ground control for indirect methods and the difficulties of access and
instrumentation for direct methods, the integration of the collected data is also
challenging. Specific concerns include the handling of coordinates and datums, the
regionalisation of data collected at points and the structuring of spatio-temporal
datasets.
COORDINATE SYSTEMS AND DATUMS
Coastlines are difficult environments in which to integrate different data sources. In
general terrestrial and marine coordinate systems are different and require
transformations between projections and datum spheroids. There are different mapped
coastlines that differ by date of survey (problematic in a highly dynamic environment
like the coast) and also by the definition of sea level used. In elevation terms the
terrestrial map datum is usually related to mean high tide while the marine chart
datum is expressed as positive value below lowest astronomical tide (LAT). The
difference between the terrestrial datum and the marine datum varies according to
tidal range - which is spatially variable and often only measured at certain points e.g.
ports. This makes it difficult to create integrated terrestrial and marine elevation
models.
The ideal solution to this integration problem is to add the actual tidal range
at each point to the measured depths: this would then allow the terrestrial and marine
elevation models to be connected along the mean high tide line. Since tidal range data
is only available at a limited number of points on the coastline a method of
interpolation is required. One method that has been developed experimentally in
Norfolk is to linearly interpolate tidal range between known points along the coastline.
Where there are barrier islands, only the coastline with a long fetch to the open sea is
used and inlets are ignored. Using this method, where interpolated steps of 0.25m
change in tidal range are reached a line is drawn orthogonal to the coast until it exits
the area, reaches another coastline or until it intersects another such line.
This semi-automated procedure defines zones within which the tidal range
can be considered to be within 0.25m of each other. By adding this tidal range to all
measured depths below LAT within each zone, and changing the sign on the aggregate
value to negative, a depth below the mean high tide terrestrial datum is derived.