Collating the Past for Assessing the Future
143
Historical and Modern Inconsistencies
From an historical perspective, the system of using local datums for each chart was
abandoned in 1968, with the adoption of the LAT national datum and the metrification
of survey data. The system of chart projection has also changed from Gnomonic to
Transverse Mercator utilising Ordnance Survey Great Britain 1936 datum. On all
charts, the relative levels of the neap and spring tides are recorded. Within the Wash
the mean high water spring tide varies by 0.5 metres between Tabs Head (7.5 metres in
the ‘Approaches to Boston’) and West Stones (7.0 metres in the ‘Approaches to King’s
Lynn’). The level of Chart Datum to Ordnance Datum also varies around the
embayment (3.7 metres below Ordnance Datum at Tabs Head to 3.2 metres below
Ordnance Datum at Wisbech Cut), which does complicate the use of Chart Datum. In
the absence of intertidal levelling data for the rest of the Wash, the Mean High Water
143
Historical and Modern Inconsistencies
From an historical perspective, the system of using local datums for each chart was
abandoned in 1968, with the adoption of the LAT national datum and the metrification
of survey data. The system of chart projection has also changed from Gnomonic to
Transverse Mercator utilising Ordnance Survey Great Britain 1936 datum. On all
charts, the relative levels of the neap and spring tides are recorded. Within the Wash
the mean high water spring tide varies by 0.5 metres between Tabs Head (7.5 metres in
the ‘Approaches to Boston’) and West Stones (7.0 metres in the ‘Approaches to King’s
Lynn’). The level of Chart Datum to Ordnance Datum also varies around the
embayment (3.7 metres below Ordnance Datum at Tabs Head to 3.2 metres below
Ordnance Datum at Wisbech Cut), which does complicate the use of Chart Datum. In
the absence of intertidal levelling data for the rest of the Wash, the Mean High Water