Regulatory aspects
and they do not provide sufficient accuracy. Users hesitate to use them
and errors may increase with the succession of operations.
In the sea environment, most “elementary bricks”, especially reference
elements, do not exist in a stable and standard form, and they are currently
under study. The Shom coastline is only available in a form which meets
navigational safety standards. There is no compliance with the IGN high
watermark, as regards the tidal term and in estuaries the digitized bank
lines do not reach the salt water boundary. Similar problems exist in terms
of the hydrographic zero. Moreover, problems linked to operating rights
(mainly with IGN) are proving to be considerable stumbling blocks as
regards the dissemination of regulatory data for the general public.
There is a great diversity of demarcation methods in use, even for the
same zone (all the following examples are extracts from various bylaws relative to the sanitary classification of shellfish production zones).
The support base could be geographical co-ordinates, arbitrary lines such
as meridians or parallel lines, constant azimuth lines, alignments as well
as preexisting jurisdictional boundaries or zones. These zones can also
choose as references natural or artificial geodesic elements, some of
which are displayed on marine charts or land maps. Whereas others are
only known locally or are imprecise. It could be a question of distances
in relation to these objects. Some of these construction modes cause transcription orprecision problems when detailed research is required (IHO,
1990):
- the transfer of geographical co-ordinates demands a certain knowledge
of the geodesic system and the reference ellipsoid. However this information is not usually displayed in national documents;
- the distances shown on the charts must take account of scale variations
which exist in many projections. Large distances must be calculated in
geodesic terms;
- in the case of demarcations drawn up according to the principle of an
equidistance line between two coasts (opposite or adjacent) automatic
calculation processes are not necessarily the most suitable;
- lines perpendicular to the general direction of the coast can be
constructed if the direction has actually been determined. Protocols have
been introduced using the azimuth of lines linking regularly spaced out
points on a stretch of coast which is yet to be defined. This mean direction
is a loxodrome which can easily be deduced in a Mercator projection,
but errors will probably appear using other types of projections or geodesic calculations. Definitions such as “the prolongation of land frontiers”
are even more inaccurate and difficult to implement;
- the notion of “thalweg”, a maximum depth line along a channel deduced
from geographical data, is used when it is advisable not to place a
navigational channel under one sole control of one state. In unstable
areas, stationary lines can be used by defining fixed extremities on land
and on the sea. In order to determine the foot of the continental shelf,
detailed protocols have been drawn up to define the point of maximum change of gradient;
275
and they do not provide sufficient accuracy. Users hesitate to use them
and errors may increase with the succession of operations.
In the sea environment, most “elementary bricks”, especially reference
elements, do not exist in a stable and standard form, and they are currently
under study. The Shom coastline is only available in a form which meets
navigational safety standards. There is no compliance with the IGN high
watermark, as regards the tidal term and in estuaries the digitized bank
lines do not reach the salt water boundary. Similar problems exist in terms
of the hydrographic zero. Moreover, problems linked to operating rights
(mainly with IGN) are proving to be considerable stumbling blocks as
regards the dissemination of regulatory data for the general public.
There is a great diversity of demarcation methods in use, even for the
same zone (all the following examples are extracts from various bylaws relative to the sanitary classification of shellfish production zones).
The support base could be geographical co-ordinates, arbitrary lines such
as meridians or parallel lines, constant azimuth lines, alignments as well
as preexisting jurisdictional boundaries or zones. These zones can also
choose as references natural or artificial geodesic elements, some of
which are displayed on marine charts or land maps. Whereas others are
only known locally or are imprecise. It could be a question of distances
in relation to these objects. Some of these construction modes cause transcription orprecision problems when detailed research is required (IHO,
1990):
- the transfer of geographical co-ordinates demands a certain knowledge
of the geodesic system and the reference ellipsoid. However this information is not usually displayed in national documents;
- the distances shown on the charts must take account of scale variations
which exist in many projections. Large distances must be calculated in
geodesic terms;
- in the case of demarcations drawn up according to the principle of an
equidistance line between two coasts (opposite or adjacent) automatic
calculation processes are not necessarily the most suitable;
- lines perpendicular to the general direction of the coast can be
constructed if the direction has actually been determined. Protocols have
been introduced using the azimuth of lines linking regularly spaced out
points on a stretch of coast which is yet to be defined. This mean direction
is a loxodrome which can easily be deduced in a Mercator projection,
but errors will probably appear using other types of projections or geodesic calculations. Definitions such as “the prolongation of land frontiers”
are even more inaccurate and difficult to implement;
- the notion of “thalweg”, a maximum depth line along a channel deduced
from geographical data, is used when it is advisable not to place a
navigational channel under one sole control of one state. In unstable
areas, stationary lines can be used by defining fixed extremities on land
and on the sea. In order to determine the foot of the continental shelf,
detailed protocols have been drawn up to define the point of maximum change of gradient;
275