138
C.I.S. Pater
The adoption of a tidal datum corresponds with the objective of producing a local
(embayment scale) spatial model. This datum allows information and data pertaining
to the maximum possible extent of the tidal area to be included, thereby allowing
consideration of the processes that are inherently responsible for the topography
contiguous between the highest and lowest levels of the tide. The charting resource of
the HO also represents the only cartographic record of the inshore submerged
channels. Within the Wash embayment, qualitative estimation of morphological
change using charts was first conducted by Hydraulics Research (HR Wallingford,
1953). Charts produced between 1828 and 1918 were redrawn with a common georeferencing system that allowed direct comparisons between editions. From this
analysis, estimates of change were calculated to ascertain locations of stability or
instability. GIS should represent a modern means of continuing this type of analysis.
Digital Representation of Subtidal and Intertidal Topography
During the course of the research both published charts and unpublished collector
charts have been digitised spanning the period 1924 to 1997. The data capture was
performed using Lites2 software, a component of the Laser-Scan Automated Map
Production System (LAMPS). A complete vector coverage was recorded including all
spot depths and isolines with attributable values. The data was subsequently used in
the production of Digital Terrain Models (DTMs). The purpose of producing DTMs
was two-fold, to provide a visualisation of an environment that would not otherwise be
readily accessible and for volumetric analysis between sequential chart editions. The
DTMs were produced within ArcInfo and preliminary terrain analysis of elevation and
declination was conducted through the use of Triangulated Irregular Networks (TINs).
The production, by Delaunay triangulation of a 2.5 dimensional or functional
representation of the terrain was obtained and viewed first by linear interpretation and
secondly via bivariate quintic interpretation (Fig. 3). Linear interpretation provides an
artificial angular view of the surface, whereas quintic interpretation has an emollient
effect producing a smooth surface, indicating a closer proximity with the actual
topography (ESRI, 1991). However, subsequent volumetric analysis of the surface can
produce different statistics depending on the surface interpolation adopted. Vertical
exaggeration is employed to lend emphasis to the undulation of the intertidal and
subtidal topography, an essential function in an area of low relief.
Boston Port Authority Surveys
The Boston Port hydrographic surveys are conducted on an ad hoc basis in two
locations in the western Wash, namely the ‘Approaches to Boston’ (Area 1, Fig. 3) and
the Freeman Channel (Area 2, Fig. 3). A good example of the visualisation provided
by the DTM can be seen in Fig. 3, Area 1 where a kidney shaped bar, located to the
north east of the Welland Cut, can be clearly identified. The routine survey work by
the Port of Boston, at this location, is concerned with charting the navigational status
of channels in relation to this bar. Surveys in June 1994 and December 1996, show the
C.I.S. Pater
The adoption of a tidal datum corresponds with the objective of producing a local
(embayment scale) spatial model. This datum allows information and data pertaining
to the maximum possible extent of the tidal area to be included, thereby allowing
consideration of the processes that are inherently responsible for the topography
contiguous between the highest and lowest levels of the tide. The charting resource of
the HO also represents the only cartographic record of the inshore submerged
channels. Within the Wash embayment, qualitative estimation of morphological
change using charts was first conducted by Hydraulics Research (HR Wallingford,
1953). Charts produced between 1828 and 1918 were redrawn with a common georeferencing system that allowed direct comparisons between editions. From this
analysis, estimates of change were calculated to ascertain locations of stability or
instability. GIS should represent a modern means of continuing this type of analysis.
Digital Representation of Subtidal and Intertidal Topography
During the course of the research both published charts and unpublished collector
charts have been digitised spanning the period 1924 to 1997. The data capture was
performed using Lites2 software, a component of the Laser-Scan Automated Map
Production System (LAMPS). A complete vector coverage was recorded including all
spot depths and isolines with attributable values. The data was subsequently used in
the production of Digital Terrain Models (DTMs). The purpose of producing DTMs
was two-fold, to provide a visualisation of an environment that would not otherwise be
readily accessible and for volumetric analysis between sequential chart editions. The
DTMs were produced within ArcInfo and preliminary terrain analysis of elevation and
declination was conducted through the use of Triangulated Irregular Networks (TINs).
The production, by Delaunay triangulation of a 2.5 dimensional or functional
representation of the terrain was obtained and viewed first by linear interpretation and
secondly via bivariate quintic interpretation (Fig. 3). Linear interpretation provides an
artificial angular view of the surface, whereas quintic interpretation has an emollient
effect producing a smooth surface, indicating a closer proximity with the actual
topography (ESRI, 1991). However, subsequent volumetric analysis of the surface can
produce different statistics depending on the surface interpolation adopted. Vertical
exaggeration is employed to lend emphasis to the undulation of the intertidal and
subtidal topography, an essential function in an area of low relief.
Boston Port Authority Surveys
The Boston Port hydrographic surveys are conducted on an ad hoc basis in two
locations in the western Wash, namely the ‘Approaches to Boston’ (Area 1, Fig. 3) and
the Freeman Channel (Area 2, Fig. 3). A good example of the visualisation provided
by the DTM can be seen in Fig. 3, Area 1 where a kidney shaped bar, located to the
north east of the Welland Cut, can be clearly identified. The routine survey work by
the Port of Boston, at this location, is concerned with charting the navigational status
of channels in relation to this bar. Surveys in June 1994 and December 1996, show the