Collating the Past for Assessing the Future
139
main approach channel currently runs down the north west side of this bar (Fig. 4 and
5). However, sedimentation is so rapid in this area that on almost a monthly basis, the
approach can switch from one side to the other (Davies, Port of Boston, pers com).
The sounding location points are produced through the use of a Differential
Global Positioning System. The Differential Global Positioning System (DGPS) datum
uses the World Geodetic System 1984, which differs from the Ordnance Survey of
Great Britain 1936 datum used in plotting HO charts. Therefore, positions of
soundings obtained using GPS require a conversion in order to align with the chart.
The error expected in the conversion process is in the order of two metres in the
horizontal plane. However, given the degree of uncertainty in the original plotting of a
transient feature such as an intertidal bar, this estimate would seem acceptable. The
precise location and plotting of an amorphous soft sediment bar means that the
increasing demand for absolute accuracy has to be balanced against the function of
accretion or deposition that can be expected per tide, per season and per year.
139
main approach channel currently runs down the north west side of this bar (Fig. 4 and
5). However, sedimentation is so rapid in this area that on almost a monthly basis, the
approach can switch from one side to the other (Davies, Port of Boston, pers com).
The sounding location points are produced through the use of a Differential
Global Positioning System. The Differential Global Positioning System (DGPS) datum
uses the World Geodetic System 1984, which differs from the Ordnance Survey of
Great Britain 1936 datum used in plotting HO charts. Therefore, positions of
soundings obtained using GPS require a conversion in order to align with the chart.
The error expected in the conversion process is in the order of two metres in the
horizontal plane. However, given the degree of uncertainty in the original plotting of a
transient feature such as an intertidal bar, this estimate would seem acceptable. The
precise location and plotting of an amorphous soft sediment bar means that the
increasing demand for absolute accuracy has to be balanced against the function of
accretion or deposition that can be expected per tide, per season and per year.