1. Altimetry and the Law of the Sea definition of the Continental Shelf
Dave Monahan
Canadian Hydrographic Service and
Ocean Mapping Group, University of New Brunswick
Global Bathymetry for Oceanography, Geophysics, and Climatology
Altimetry and UNCLOS

2. The United Nations Convention on the Law of the Sea (UNCLOS)
The biggest single boundary-making event in human history.
Within a short period of time, two-thirds of the earth’s surface will be divided into zones fringing the coasts and over which individual Coastal States exercise varying degrees of sovereignty, with the residual “Area” being administered by the UN.
Reasonably straightforward measurements of 12, 24 and 200 M establish the Territorial Sea, the Contiguous Zone and the Exclusive Economic Zone
The Outer Limit of a Coastal State’s Continental Shelf, or its Exclusive Economic Zone where there is no Shelf, forms the boundary of the “Area”.
A Coastal State must establishing the existence of, and delineate the limits to, a juridical Continental Shelf, as part of a claiming process
Altimetry and UNCLOS

3. The idea behind the definition in Article 76
Broad-margin states wanted the definition to permit an extensive shelf, in opposition to those with no physiographic shelves who would benefit from the Area being as large as possible.
Eventually settled on a principal that would see “the natural prolongation” of the continent available to the Coastal State, with the true oceanic ocean floors being included in the Area.
According to prevalent knowledge, this granted hydrocarbons to the Coastal State and minerals like “manganese nodules” to the International Seabed Authority.
Altimetry and UNCLOS

4. The model that influenced the wording of Article 76
Foot of the Slope was a sea floor feature that was seized on to mark the division between continental and oceanic crust.
Article 76 definition: ‘In the absence of evidence to the contrary, the foot of the continental slope shall be determined as the point of maximum change in the gradient at its base.’
Clearly influenced by the models of what are now called passive margins that were available to the legal drafters in the 1970s, paragraph 4(b) defines the Foot of the Slope as a feature between the Slope and the Rise In the absence of evidence to the contrary, the foot of the continental slope shall be determined as the point of maximum change in the gradient at its base. No depths are given, but in physiographic terms, the Foot of the Slope tends to lie in depths of 2-5 k m (check reference).
Altimetry and UNCLOS

5. The Foot of the Slope and Beyond
Since the Continental Rise is composed of sediment, and hydrocarbons occur only in sediment, it was argued that the shelf should include portions of the Rise.
Options: either a measurement of 60 M seawards or the point where the thickness of sedimentary rocks is at least 1 per cent of the shortest distance to the Foot of the Slope.
Altimetry and UNCLOS

6. Constraints to unbounded expansion
The disadvantaged states forced the inclusion of a finite Outer Constraint
Continental Shelf can not extend beyond the most seaward of a line 350 M from Baselines, or a line 100 M seaward of the 2500 m isobath.
Outer Constraint restricted to 350 M over “submarine ridges”
Altimetry and UNCLOS

7. Where are the Juridical Continental Shelves likely to be?
Altimetry and UNCLOS

8. The first claim, Russia, December 2001
Wakey, Wakey!
Altimetry and UNCLOS

9. The Boundary Making Process
1. Boundary is defined through words of the Convention (Treaty)
2. Boundary is delineated on maps and diagrams
3. Boundary is demarcated - on land by constructing markers, at sea by depositing coordinates with the Secretary-General of the UN.
Continental Shelf boundaries must be submitted to the Commission on the Limits of the Continental Shelf (CLCS) for review as part of Step 2
The CLCS has issued Guidelines on the quantity and type of data they consider acceptable for delineation, (but in places may overstepped their mandate and entered the realm of definition)
Coastal States have to steer a course between the Convention and the Guidelines
Altimetry and UNCLOS

10. Elements of the delineation process that altimetry might contribute to
Determining the 2500 m isobath for use in the 2500 m plus 100 nm outer constraint constraint
Locating the “Foot of the Slope” if it exists as a physiographic feature, or, where it does not, locating the continent/ocean boundary
Determining the geological nature of submarine “ridges”
Altimetry and UNCLOS

11. World (excluding Polar Regions) 2500 m contour
World (excluding Polar Regions) 2500 m contour. Extracted from GEBCO Digital Atlas
Worldwide, 2500 m isobaths fringe continents and islands, and lie on the flanks of mid-ocean ridges. (Figure x) Not all will have relevance to Article 76. Those that do not surround a land mass will only be a factor where they surround an isolated elevation that may be natural component of a Continental Shelf. Isobaths on the flanks of ridges may be rendered inapplicable by the exception provided in paragraph 6. … on submarine ridges, the outer limit of the continental shelf shall not exceed 350 nautical miles from the baselines from which the breadth of the territorial sea is measured. This paragraph does not apply to submarine elevations that are natural components of the continental margin, such as its plateaux, rises, caps, banks and spurs. This paragraph is controversial and remains to be tested. Furthermore, some 2500 m isobaths are so close to land that a line 100 nm seawards from tem will lie inside 350 nm or even inside 200 nm. Those 2500 m isobaths that will be used for Article 76 purposes primarily lie on the Continental Slope of passive margins.
Altimetry and UNCLOS

12. How well are continental slopes mapped worldwide?
Relatively fewer investigations concentrated on continental slopes
Much of the data over them was not collected by systematic surveys but by transit tracks.
Water properties over Slopes often complicated and poorly known, leading to higher uncertainty in acoustic measurements
Altimetry and UNCLOS

13. Sources of data Instruments Data bases and maps
Single beam echo sounder, variable sounding line spacing
Sidescan, most notably GLORIA.
Multibeam coverage is only increasing at a slow rate.
Data bases and maps
Contour maps, grids and raw sounding data bases exist, based on the sporadic acoustic data.
Limitation to acoustic methods
Vessel speed and availability. GoMAP project estimates 200 ship years to survey the world ocean (excluding the Arctic) from the 500 m contour out
Altimetry and UNCLOS

14. 2500 m isobath specified by the Convention
Definition “the 2,500 metre isobath, which is a line connecting the depth of 2,500 metres.”
Implies an absolute 2500m isobath without specifying surface that depth is measured from
Refers to “the 2500 m isobath” as if there were only one fringing a Continental Shelf, while allowing for “submarine elevations that are natural components of the continental margin” which might mean that there is more than one 2500 m isobath off a margin
Altimetry and UNCLOS

15. Requirements specified by Guidelines
Uncertainty assessment based on International Hydrographic Organization standard S44
“ no need to submit ... a full 2,500 m isobath…” but “the full bathymetric database will be regarded as an essential component of the supporting scientific and technical data.”
Want to use the most landward 2500 m isobath where multiple isobaths exist
Want to judge whether submarine elevations are part of the continental margin on a “case-by-case basis”.
Altimetry and UNCLOS

16. Which 2500 m isobath to use
a) where 2500m contours surround “submarine elevations”
b) where the 2500m contour doubles back on itself
c) whether the contour is on a “ridge”
Altimetry and UNCLOS

17. Vertical uncertainty at 2500 m from IHO S44
Combines fixed (constant) and variable ( with depth) errors as Root Sum of Squares.
Provides multipliers and constants for depth measurement and isobath
classic Continental Slope and Rise have very low gradients
eg the US Continental Slope steepest area regional slope of 2.5 degrees and a local slope of 7.6 degrees.
Accurately locating a contour on such low gradients is extremely demanding on measurement systems.
Altimetry and UNCLOS

18. Horizontal uncertainty at 2500 m from IHO S44
classic Continental Slope and Rise have very low gradients
eg the US Continental Slope steepest area regional slope of 2.5 degrees and a local slope of 7.6 degrees.
Accurately locating a contour on such low gradients is extremely demanding on measurement systems.
Altimetry and UNCLOS

19. Comparison of four different 2500 m isobaths
Off New Jersey, plotted the 2500m contour from
ETOPO-5 data
GEBCO-97 Digital Atlas CD
NGDC’s Coastal Relief Model CD’s
Predicted Topography Smith and Sandwell (1997 )
Altimetry and UNCLOS

20. mbes Altimetry and UNCLOS
Visual inspection shows that the three ocean-scale data sets interweave each other and form a corridor or confidence zone approximately 10 km wide.. Naturally, these contours contain only long wavelengths. The much shorter wavelengths captured by the multibeam contour weave amongst the other three, and appear to be centred on the zone created by the older three.
Altimetry and UNCLOS

21. Horizontal differences between 2500m contours from ETOPO5, Predicted, GEBCO and multibeam. Multibeam is assigned the value of zero.
Magnitude of horizontal differences between 2500m contours produced from ETOPO5, NOAA Predicted (Satellite) Bathymetry, GEBCO and a multibeam survey from NGDC Coastal Relief Model. The multibeam is taken as the datum and assigned the value of zero. Since the multibeam is newest, should be better positioned, internally consistent, suffer little from beamwidth problems, and have no gaps in its coverage of the sea floor it was considered to be the datum and the horizontal distance between it and the other data sets were measured along a section of the continental slope. Assuming the multibeam-derived contour to be true, the horizontal distances from it to the each of the other contours were measured at intervals of 1 km along a 70-km stretch. The magnitude of these differences, as shown in Figure 1, is never more than 10 km, and is usually less than 5 km.
Altimetry and UNCLOS

22. Histogram of horizontal differences between 2500m contours from ETOPO5, Predicted, GEBCO and multibeam survey. Multibeam is considered true and the displacement of the other three measured seawards (+) or landward (-).
Figure 2 Histogram of horizontal differences between 2500m contours produced from ETOPO5, NOAA Predicted (Satellite) Bathymetry, GEBCO and a multibeam survey from NGDC Coastal Relief Model. The multibeam data is considered as true and the displacement of the other three measured seawards (+) or landward (-).
From a histogram of these differences, (Figure 2) it appears that the predicted bathymetry has a systematic horizontal bias of 2-3 km. GEBCO and ETOPO5 do not appear to have a bias, with GEBCO being more closely located to the multibeam contour. Maximum differences between the predicted contour and the MBES contour are less than those between the ETOPO5 contour and the MBES.
Altimetry and UNCLOS

23. Histogram of horizontal differences between 2500m contours from ETOPO5, Predicted, GEBCO and multibeam survey. Multibeam is considered true and the displacement of the other three measured seawards (+) or landward (-).
S44 Uncertainty
Figure 2 Histogram of horizontal differences between 2500m contours produced from ETOPO5, NOAA Predicted (Satellite) Bathymetry, GEBCO and a multibeam survey from NGDC Coastal Relief Model. The multibeam data is considered as true and the displacement of the other three measured seawards (+) or landward (-).
From a histogram of these differences, (Figure 2) it appears that the predicted bathymetry has a systematic horizontal bias of 2-3 km. GEBCO and ETOPO5 do not appear to have a bias, with GEBCO being more closely located to the multibeam contour. Maximum differences between the predicted contour and the MBES contour are less than those between the ETOPO5 contour and the MBES.
Altimetry and UNCLOS

24. Findings for 2500 m isobath
Over the sample area, the 2500 m isobath based on altimetric data
a) is as accurate as those from other world-scale maps and
b) meets the IHO spec S44.
Altimetry and UNCLOS

25. Maximization of area claimed
Outer Limit can be delineated by straight lines not exceeding 60 nautical miles in length, connecting fixed points.
To maximise area must select the fixed points carefully
Any line defined by points 60M apart is not very sinuous, and can be portrayed at a small scale, although Coastal States seeking to maximize their area will use spacing of less than 60 where it is to their benefit (generally where the limit is convex wrt the coast). Where it is based on the 2500 m isobath, the Outer Limit will be created by swinging arcs 100 M long from points on the isobath. Clearly, not all of the isobath will determine the Outer Limit: it would be possible to construct it from points 60M apart, and even further apart in areas where the margin is convex wrt the land.
Altimetry and UNCLOS

26. Optimal selection of points
Long wavelength contours may miss localised seaward protrusions
Strategy conduct large scale, short wavelength MBES surveys along the altimetric based 2500 m isobath
Select most seawards points on which to base 100 nm arcs
Altimetry and UNCLOS

27. Roles for predicted bathymetry in Continental Shelf delineation
“desk –top study” to determine the likely area to be included within a juridical Continental Shelf
Quality control device to assess the accuracy of existing sounding data
In developing a plan for building the case data to substantiate it.
Infer morphologic trends between existing sounding lines
Plan the orientation of any new sounding lines.
Altimetry and UNCLOS

28. Such use of altimetric data is consistent with the view expressed by the Commission on the Limits of the Continental Shelf
“ Other sources of evidence, such as satellite altimetry-derived bathymetric data or imaging side-scan sonar information, will not be regarded as admissible for the purpose of delineating the 2,500 m isobath. This information, however, might be useful as additional qualitative information in support of other parts of a submission but will not be considered during the determination of this or any other isobaths. However, this data will be considered admissible as supporting information in a submission.”
Altimetry and UNCLOS

29. Roles for predicted bathymetry in Continental Shelf delineation
“desk –top study” to determine the likely area to be included within a juridical Continental Shelf
Quality control device to assess the accuracy of existing sounding data
In developing a plan for building the case data to substantiate it.
Infer morphologic trends between existing sounding lines
Plan the orientation of any new sounding lines.
That’s what I thought before yesterday’s presentations!
Altimetry and UNCLOS

30. Presence/absence of sediment Slope/Rise junction
Locating the “Foot of the Slope” if it exists as a physiographic feature, or, where it does not, locating the continent/ocean boundary
Presence/absence of sediment
Slope/Rise junction
Altimetry and UNCLOS

31. Determining the geological nature of submarine “ridges”
Outer Constraint at 350 nm along “ridges”
Article 76 “...on submarine ridges, the outer limit of the continental shelf shall not exceed 350 nautical miles... This paragraph does not apply to submarine elevations that are natural components of the continental margin...”
CLCS Guidelines “As it is difficult to define the details concerning various conditions, the Commission feels it appropriate that the issue of ridges be examined on a case-by-case basis.”
Altimetry and UNCLOS

32. Example from the Arctic
Maximum area would terminate at 350 nautical miles if Alpha Ridge is ruled to be oceanic ie not a natural prolongation of land
Altimetry and UNCLOS

33. Example from the Arctic 2
Yellow line is median
Maximum area would terminate at median if Alpha Ridge is ruled to be continental ie a natural prolongation of land
Altimetry and UNCLOS

34. Take Home Message
Delineating a juridical Continental Shelf under Article 76 of UNCLOS is complicated and contains many judgmental elements
Altimetry can contribute to:
2500 m isobath
Foot of the Slope or evidence to the contrary
nature of ridges
Altimetry and UNCLOS