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J. Populus et al.
The Depth Chart
THE INTERPOLATION METHOD
The whole area covered by oyster and mussel culture inside the Bassin is currently
surveyed by DDE (Harbour Maritime Service), in charge of coastal civil engineering.
Their main concern is a siltation trend that has been occurring for many years and that
is suspected to be the result of extensive oyster breeding on
i.e. metal
structures that stick out two feet off the ground. Depth surveys were conducted in 1970,
1985 and recently over the period 1994-96. Soundings were performed every ten
metres on transects 100 metres apart and corrected for tidal effects, to provide a
mapping on scale of 1/10 000. Tidal constraints limited transects ashore to an altitude
of approximately 4 metres above chart datum.
The data are delivered as (X,Y,Z) ascii files, X and Y being Lambert coordinates in
metres and Z in centimetres positive for depths above the chart datum. The variograms
generated by Isatis (de Chambure, 1991; Constantin, 1996) in two orthogonal
directions, i. e. NNW and ENE, both local (range of 500 metres) and egional (range of
1500 metres) are shown in Fig. 2.
The fitting using a Gaussian function is made good up to 400 metres. A slight
of a few centimetres is adopted, in order to avoid matrix inversion
problems. The NNW regional fitting (up to 900 metres) is made to be better than the
ENE one, as sounding profiles are run East-West. The neighbourhood is an oval one,
with 100 and 250 metres as respective horizontal and vertical semi-axis values. The
interpolation is performed on a 20 metre mesh size grid encompassing the whole basin.
A section of the the depth digital terrain model (DTM) is shown on Fig. 3, with depth
varying between -12 and +4 metres.
MERGING DEPTH TO LEASES
The aim of this operation is to provide the average depth of each lease. It is a classical
problem of raster/vector conversion (Durand, 1994), the depth DTM and the leases
being respectively under these forms. The choice of the method depends on the relative
size of the objects concerned. If the parcel size and the DTM pixel are of comparable
J. Populus et al.
The Depth Chart
THE INTERPOLATION METHOD
The whole area covered by oyster and mussel culture inside the Bassin is currently
surveyed by DDE (Harbour Maritime Service), in charge of coastal civil engineering.
Their main concern is a siltation trend that has been occurring for many years and that
is suspected to be the result of extensive oyster breeding on
i.e. metal
structures that stick out two feet off the ground. Depth surveys were conducted in 1970,
1985 and recently over the period 1994-96. Soundings were performed every ten
metres on transects 100 metres apart and corrected for tidal effects, to provide a
mapping on scale of 1/10 000. Tidal constraints limited transects ashore to an altitude
of approximately 4 metres above chart datum.
The data are delivered as (X,Y,Z) ascii files, X and Y being Lambert coordinates in
metres and Z in centimetres positive for depths above the chart datum. The variograms
generated by Isatis (de Chambure, 1991; Constantin, 1996) in two orthogonal
directions, i. e. NNW and ENE, both local (range of 500 metres) and egional (range of
1500 metres) are shown in Fig. 2.
The fitting using a Gaussian function is made good up to 400 metres. A slight
of a few centimetres is adopted, in order to avoid matrix inversion
problems. The NNW regional fitting (up to 900 metres) is made to be better than the
ENE one, as sounding profiles are run East-West. The neighbourhood is an oval one,
with 100 and 250 metres as respective horizontal and vertical semi-axis values. The
interpolation is performed on a 20 metre mesh size grid encompassing the whole basin.
A section of the the depth digital terrain model (DTM) is shown on Fig. 3, with depth
varying between -12 and +4 metres.
MERGING DEPTH TO LEASES
The aim of this operation is to provide the average depth of each lease. It is a classical
problem of raster/vector conversion (Durand, 1994), the depth DTM and the leases
being respectively under these forms. The choice of the method depends on the relative
size of the objects concerned. If the parcel size and the DTM pixel are of comparable