CoastGIS’99: geomatics and coastal environment
This means that all the elements are managed at time “t" and the original
data source can also be retained. Unfortunately, a certain number of problems appear during updating. It is not possible to lock definitively
some data or to ensure a certain hierarchy with respect to the integrity
of the defined elements. The introduction of new arcs, close to existing
ones, is likely to cause deformations and especially discontinuities
regarding those boundaries which have already been constructed. The
use ofvery short distances, during the topological construction phase,
slows down operations considerably when the base increases in size
and, above all, it does not provide absolute guarantee as regards the
eventual substitution of one arc by another in the case of arcs in proximity. Furthermore, these discontinuities are difficult to detect as they
are very small and there is a great number of non-significant intersections. The problem is raised systematically when areas which do
not share the same legal signification, or the same evolution, are superimposed on the same place.
A compromise has been achieved by keeping one single layer of arcs
and “routes”, from which the required entities are extracted in order
to build coherent sets of “regions”. These are managed independently
and, after the topological construction phase, modification control procedures are implemented. It is essential to optimise the number of
layers in order to reduce arc duplication (high watermark, for example)
with all the risks involved in differentiating round contours and in
updating.
Most of the data concerning a zone are defined by a restricted number
of summits, so that, when the duplication of complex arcs is reduced,
the volume of information obtained is relatively limited. This allows
vast geographical settings to be managed. Thus, it seems easier to
envisage coherent management by separating groups of entity types on
vast geographical settings rather than attempting to manage all the
information on a more limited territory.
Most regulatory arcs do not depend on the scales concerned. In general,
it is the direct or indirect participation of physiographical boundaries
in the composition of boundaries or zones which determines the scales.
However there is a problem of continuity concerning the representation of estuaries. Logically speaking, the coastline should extend landwards in the estuaries until the distance between the two banks corresponds to the level of precision given by the data. This principle has
been adopted by Shom but the IGN line is cut arbitrarily seawards. If
an exhaustive representation of legal boundaries or sections within
estuaries is required, the current system can only be used on the foreshore with respect to the boundary retained. Landward, another representation method, managed independently, must be implemented by
taking into account the arcs of the hydrographical network. The legal
boundary is thus represented by a node and the sector by arcs. The
switch from one mode to the other depends on the scale. The display
of exhaustive inventories of variable sized entities may also require
management in terms of node and/or polygon objects.
278
This means that all the elements are managed at time “t" and the original
data source can also be retained. Unfortunately, a certain number of problems appear during updating. It is not possible to lock definitively
some data or to ensure a certain hierarchy with respect to the integrity
of the defined elements. The introduction of new arcs, close to existing
ones, is likely to cause deformations and especially discontinuities
regarding those boundaries which have already been constructed. The
use ofvery short distances, during the topological construction phase,
slows down operations considerably when the base increases in size
and, above all, it does not provide absolute guarantee as regards the
eventual substitution of one arc by another in the case of arcs in proximity. Furthermore, these discontinuities are difficult to detect as they
are very small and there is a great number of non-significant intersections. The problem is raised systematically when areas which do
not share the same legal signification, or the same evolution, are superimposed on the same place.
A compromise has been achieved by keeping one single layer of arcs
and “routes”, from which the required entities are extracted in order
to build coherent sets of “regions”. These are managed independently
and, after the topological construction phase, modification control procedures are implemented. It is essential to optimise the number of
layers in order to reduce arc duplication (high watermark, for example)
with all the risks involved in differentiating round contours and in
updating.
Most of the data concerning a zone are defined by a restricted number
of summits, so that, when the duplication of complex arcs is reduced,
the volume of information obtained is relatively limited. This allows
vast geographical settings to be managed. Thus, it seems easier to
envisage coherent management by separating groups of entity types on
vast geographical settings rather than attempting to manage all the
information on a more limited territory.
Most regulatory arcs do not depend on the scales concerned. In general,
it is the direct or indirect participation of physiographical boundaries
in the composition of boundaries or zones which determines the scales.
However there is a problem of continuity concerning the representation of estuaries. Logically speaking, the coastline should extend landwards in the estuaries until the distance between the two banks corresponds to the level of precision given by the data. This principle has
been adopted by Shom but the IGN line is cut arbitrarily seawards. If
an exhaustive representation of legal boundaries or sections within
estuaries is required, the current system can only be used on the foreshore with respect to the boundary retained. Landward, another representation method, managed independently, must be implemented by
taking into account the arcs of the hydrographical network. The legal
boundary is thus represented by a node and the sector by arcs. The
switch from one mode to the other depends on the scale. The display
of exhaustive inventories of variable sized entities may also require
management in terms of node and/or polygon objects.
278