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 Risk management
 Prolongation of the Land Mass
 Ridges and submarine elevations
 Accretion and suturing

Straight bridging lines

Continental margins are rarely straight. They are interrupted by ridges, plateaus, canyons, embayments and other features. The outer limit of the continental shelf that derives from the formulae set out in paragraphs 4, 5 and 6 of article 76 can be correspondingly complicated.
 
To simplify the potentially very complex definition of the outer limit of the continental shelf, paragraph 7 of article 76 provides for the use of straight bridging lines between fixed points. These are;

 straight lines not exceeding 60 nautical miles in length, connecting fixed points, defined by coordinates of latitude and longitude

The application of this rule helps determine the level of detail required for surveys of the margin. Foot of continental slope positions may be up to 180 nautical miles apart, but the area of continental shelf will generally be larger when they are closer together.


A schematic diagram showing smoothing of the outer limit of the continental shelf by straight bridging lines between fixed points (yellow circles). Areas of the continental shelf that are within 60 nautical miles of a foot of the continental slope position are shown in green. Areas in pink are parts of the continental shelf that have not been surveyed as being within 60 nautical miles of a foot of the continental slope position, but are delimited by straight bridging lines. In these examples the area of continental shelf increases as the foot of continental slope positions get closer together.


Constructing the outer limit of the Continental Shelf

The Commission Guidelines (1999) (2.3.8) state that

These straight lines can connect fixed points located on one of, or any combination formed by, the four outer limits produced by each of the two formulae and the two constraints contained in article 76.

Paragraph 5 of article 76 states that all the fixed points used to delimit the continental shelf must lie within constraint lines

 The fixed points comprising the line of the outer limits of the continental shelf on the seabed, drawn in accordance with paragraph 4 (a)(i) and (ii), either shall not exceed 350 nautical miles from the baselines from which the breadth of the territorial sea is measured or shall not exceed 100 nautical miles from the 2,500 metre isobath, which is a line connecting the depth of 2,500 metres.

Therefore straight bridging lines can include fixed points that lie within or on the constraint lines, but not points that lie outside constraint lines.

The Commission Guidelines (1999) (2.3) describe a four stage process that first constructs an outer envelope “formulae line”, then constructs an outer envelope “constraint line”, and then combines these to form the “inner envelope of formula and constraint lines”. The outer limit of the continental shelf is formed by the last stage of constructing straight bridging lines not more than 60 nautical miles in length.


Stage 1. Formula line

A schematic diagram depicting the construction of the outer envelope formulae line (black) that comprises areas within 60 nautical miles of a foot of the continental slope position (green region) and areas defined by fixed points not more than 60 nautical miles apart where sediment thickness is greater than 1% of the distance to the nearest foot of the continental slope (purple region).
 

Stage 2. Constraint line

A schematic diagram depicting the construction of the outer envelope constraint line (black) that encompasses the area in yellow and comprises regions within 350 nautical miles of base lines (blue dashed line) and areas within 100 nautical miles of the 2500 m isobath (green dashed line).

Stage 3. Formula and constraint line

A schematic diagram depicting the construction of the inner envelope of formula and constraint lines (black).
 
Stage 4. Straight bridging lines

A schematic diagram depicting straight bridging lines between fixed points (yellow circles) that define the outer limit of the continental shelf. Areas in pink are parts of the continental shelf that have not been proven to be within 60 nautical miles of a foot of the continental slope position or are within regions defined by sediment thickness, but are delimited by straight bridging lines between fixed points.

 
Bridging outside constraint lines

In the above figures fixed points used to delimit the continental shelf all lie on or within constraint lines. There is, however, potential for straight bridging lines to delimit the continental shelf beyond the constraints contained in article 76, even though all fixed points lie within, or on the constraint lines. This occurs where straight bridging lines, between two fixed points less than 60 nautical miles apart, cross over constraint lines. In this case the continental shelf extends beyond the constraint line but is defined by fixed points that meet the formulae described in article 76.

 

A schematic diagram depicting straight bridging lines between fixed points (yellow circles) that define the outer limit of the continental shelf. The pink regions are areas of continental shelf that have not been proven to be within 60 nautical miles of a foot of continental slope positions, but are within the constraint line. The region coloured in red is more than 350 nautical miles from the baselines and more than 100 nautical miles from the 2,500 m isobath, but is part of the continental shelf because it is defined by fixed points that lie within, or on the constraint line.

 
Area enclosed by straight bridging lines

In most cases, the use of straight bridging lines will smooth the outer limit of the continental shelf. Provided the fixed points lie within constraint lines, there is no restriction on which points may be used, or limit to the size of the enclosed area.

Careful analysis of the pattern of fixed points that are able to be used to delimit the continental shelf is required to maximise the enclosed area. In some cases, straight line segments that are tangential to arcs used to define the limit of the continental shelf enclose less area than straight line segments that connect different sets of fixed points. Judicious selection of fixed points allows placement of straight bridging lines that enclose the greatest possible area.


Schematic diagrams depicting straight bridging lines between fixed points (yellow circles) that define the outer limit of the continental shelf. In the diagram on the left, bridging lines are tangential to arcs formed 60 nautical miles from foot of the continental slope positions. The area enclosed by straight bridging lines is less than in the diagram on the right where the fixed points at the each end of the straight bridging line are chosen to maximise the area of continental shelf.

The area enclosed by straight bridging lines may include relatively large areas of continental shelf, particularly where there are “embayments” in the margin. The method and qualifying criteria for constructing straight bridging lines is the same no matter how large or small the enclosed area.
 

A schematic diagram showing the outer limit of the continental shelf delimited by straight bridging lines between fixed points (yellow circles). Area “A” (orange colour) includes a large embayment included in the continental shelf by a straight bridging line. The “B” areas show how the arcs defining the outer limit of the continental shelf are smoothed by straight bridging lines. The only difference between areas “A” and “B” is one of scale. All these areas conform to the terms of article 76 (7) although they may include seafloor not established as being within the limits of the continental shelf as defined in article 76 (4).


In the case of fixed points based on sediment thickness, the Guidelines (2.3.9) state that

These straight lines should not be used to connect fixed points located on opposite and separate continental margins

The meaning of “separate continental margins” is unclear. Article 76 (1) states that continental margins derive from the prolongation of the land territory. The concept of “separate continental margins” therefore implies prolongation from separate land masses. Fixed points that are located on prolongations of the same land mass are part of the same continental margin.

For fixed points other than those based on sediment thickness, there is no restriction on their use to construct straight bridging lines.