Collating the Past for Assessing the Future
141
embankment, across the intertidal area for an average distance of 1270 metres.
Additional information on the survey transect includes a description of the ground at
each levelling point. For the intertidal area in proximity to the ‘Approaches to Boston’
three sets of transect data are available (summer 1993, winter 1994 and summer 1994).
Ordnance Survey Great Britain 1936 datum is used for the co-ordinate locations and is
therefore compatible with Ordnance Survey maps and HO charts. There are however,
several issues concerning Environment Agency levelling that will be examination
further. For example the optimum distance between transects, the reliability of
benchmarks, seasonal variability between surveys and quality assurance for contract
survey work. The debate will also be continued between the relative merits of
Ordnance Datum and Chart Datum in the maritime environment.
Remote Sensing and Aerial Imagery
The use of remote sensing presents a convenient means to obtain intertidal
information, which through the use of ground data can be used to produce a DTM that
can be updated rapidly. Synthetic Aperture Radar, which operates independent of
meteorological conditions and the sun, provides a viable means of producing
topographic details from a satellite platform. Intertidal DTMs in the Wash have
recently been produced using this technology (Mason et al., 1995 and Mason, 1997).
Aerial photography sorties are also flown over the intertidal zone each year by the
Environment Agency as part of a monitoring programme. The survey height accuracy
in the intertidal environment needs to be balanced against the resolution that is feasible
in the subtidal environment. For nearly the entire intertidal zone, high resolution
calibration is possible with the Environment Agency bi-annual levelling data, but this
is not available as a matter of course for the subtidal zone. The Port of Boston provides
high resolution soundings for particular locations on a seasonal basis, but a panembayment subtidal survey is scheduled by the Environment Agency on a five year
cycle.
Analysis must allow for variable vertical accuracy, variable rates of
morphological change and variable survey dates.
Critique of Methodology
There are several stages at which error may be introduced to GIS analysis. These can
be highlighted as error into the system, error in the system and errors in interpretation
of the GIS output. In the first instant, digitising by hand requires quality control and
patience. The interpretation of the chart by the digitiser is also subject to the original
interpretation of the data by the hydrographer. In the second error bracket,
consideration is needed of the processing technique and interpolation between data
points. Finally, training and experience is required to enable accurate interpretation of
the models produced from the original data.
For the long term perspective, between 1924 and 1985 datums, projections,
scales and area of coverage all vary for the different editions of published charts.
Although it may be possible to conduct qualitative estimation over such a temporal
141
embankment, across the intertidal area for an average distance of 1270 metres.
Additional information on the survey transect includes a description of the ground at
each levelling point. For the intertidal area in proximity to the ‘Approaches to Boston’
three sets of transect data are available (summer 1993, winter 1994 and summer 1994).
Ordnance Survey Great Britain 1936 datum is used for the co-ordinate locations and is
therefore compatible with Ordnance Survey maps and HO charts. There are however,
several issues concerning Environment Agency levelling that will be examination
further. For example the optimum distance between transects, the reliability of
benchmarks, seasonal variability between surveys and quality assurance for contract
survey work. The debate will also be continued between the relative merits of
Ordnance Datum and Chart Datum in the maritime environment.
Remote Sensing and Aerial Imagery
The use of remote sensing presents a convenient means to obtain intertidal
information, which through the use of ground data can be used to produce a DTM that
can be updated rapidly. Synthetic Aperture Radar, which operates independent of
meteorological conditions and the sun, provides a viable means of producing
topographic details from a satellite platform. Intertidal DTMs in the Wash have
recently been produced using this technology (Mason et al., 1995 and Mason, 1997).
Aerial photography sorties are also flown over the intertidal zone each year by the
Environment Agency as part of a monitoring programme. The survey height accuracy
in the intertidal environment needs to be balanced against the resolution that is feasible
in the subtidal environment. For nearly the entire intertidal zone, high resolution
calibration is possible with the Environment Agency bi-annual levelling data, but this
is not available as a matter of course for the subtidal zone. The Port of Boston provides
high resolution soundings for particular locations on a seasonal basis, but a panembayment subtidal survey is scheduled by the Environment Agency on a five year
cycle.
Analysis must allow for variable vertical accuracy, variable rates of
morphological change and variable survey dates.
Critique of Methodology
There are several stages at which error may be introduced to GIS analysis. These can
be highlighted as error into the system, error in the system and errors in interpretation
of the GIS output. In the first instant, digitising by hand requires quality control and
patience. The interpretation of the chart by the digitiser is also subject to the original
interpretation of the data by the hydrographer. In the second error bracket,
consideration is needed of the processing technique and interpolation between data
points. Finally, training and experience is required to enable accurate interpretation of
the models produced from the original data.
For the long term perspective, between 1924 and 1985 datums, projections,
scales and area of coverage all vary for the different editions of published charts.
Although it may be possible to conduct qualitative estimation over such a temporal