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Introduction

The use of bathymetry is a very central element when working with the determination of the extension of the continental shelf according to UNCLOS Article 76. It is relevant both for the depth constraint (2500m depth +100M) and the determination of the foot of the continental slope.  In Geocap, bathymetry can have several different forms:

  • Singlebeam bathymetry may be imported as bathymetric profiles.
  • Multibeam bathymetry or other spread point data with depth measurements can also be imported.  In order to use the foot of slope analysis tool on point data, the dataset must be gridded first.
  • Grids may be imported directly into Geocap.  Geocap supports import of several different file formats; the most common and useful kind of bathymetric data is the ETOPO1 grid.  The ETOPO1 Grid is provided by NGDC (National Geophysical Data Center).  This grid will not be accepted by the Commission as a basis for your foot of slope points.  However once you have imported the grid you will be able to see the shape of the sea floor, and in particular the approximate position of the Foot of Slope or base of slope region.

Exercises

2500m isobath + 100M constraint

Computing the 2500m isobaths is necessary in order to determine the Depth constraint in UNCLOS Article 76, which is the 2500m isobaths plus 100 nautical miles.  The 2500m isobaths can be generated based on bathymetry grids or on bathymetric profiles (from single beam bathymetry). The only ones that can be used for an actual submission to the CLCS are 2500m isobath points generated from real bathymetry data.  For a grid, the resulting 2500m isobath will be a contour line.  A bathymetric profile will produce one or more points wherever the profile value is 2500m.

Exercise

Generate the 2500m isobath from a Seabed Surface

  1. Locate the etopo1 dataset which is located in 2. Seabed / Grids.  The dataset should have the schema Seabed Surface.  If it does not have this schema, right click it and select Set schema > Seabed Surface.
  2. Right click the dataset and select Display 2500m isobath. Black lines will appear where the 2500m isobaths are located.  You may notice that the isobaths cover a larger area than you are interested in so we will restrict the area where the isobath is calculated.
  3. Set the display window to 2D mode by clicking the 2D View button. or typing 2 on the keyboard.
  4. Zoom the display window so that it only covers the area you are interested in.
  5. Right click the seabed surface and use the command, Generate 2500m Isobath.  This will use the full resolution of the grid and calculate the 2500 isobath only in the area we are focused on.  If the display window is in 3D mode it will create the isobaths for the entire dataset.  If it is in 2D mode it will only create the isobaths inside the display window.
  6. Locate the result dataset which should be in 2. Seabed / 2500 Isobath. The Isobath dataset you have created may still have some line pieces which you want to remove before generating the 100M line, for instance the pieces that are seaward of the FOS points.
Exercise

Edit the generated 2500m isobath

  1. Clear the display window by clicking the button in the main toolbar.
  2. Right click the dataset and select the command Edit points and lines.
  3. Click the Display button near the bottom of the Edit points and lines dialogue box in order to see the isobaths.
  4. Select the Delete tab.
  5. Inside the Delete tab, select By closed line.
  6. Click the start button, then digitize a closed polygon around the part you want to delete by clicking with the left mouse button.
  7. Click Connect to Start and End in order to connect the last digitized point with the start point, and end the digitizing.
  8. Click Delete points INSIDE.
Exercise continued

The part inside the closed polygon is now deleted. You may repeat the three last points in order to delete some more. The changes you make are not performed on the dataset directly, but as a local copy.  In order to store the changes in the Edit points and lines menu, click the Execute button at the bottom.  A new copy of the data set is stored in the folder.

  1. Click the Cancel button in the Edit points and Lines menu.
  2. Clear the display window again.
  3. Display the edited isobaths and confirm that the unwanted parts have been removed.
Exercise

Generate 2500m isobath + 100M constraint line

  1. Display the 2500m isobath which you created in the previous exercise.
  2. Right-click the isobath and select the command Generate 100M Line.
  3. The new line should now be available under 1. Maritime Lines/100M Lines.
  4. Display the 100M line.

 

Generate Bathymetric Profiles

 

Exercise

Generate a Bathymetric Profile from a grid

  1. Locate the dataset you imported in the previous exercise, which should be located in 2. Seabed / Grids.  The data set should have the schema Seabed Surface.  If it does not have this schema, right click it and select Set Schema > Seabed Surface.
  2. Display the seabed by right clicking it and selecting Map Sea.
  3. Right click the dataset and select the command Display contours...
  4. A menu will appear. We will just use the default settings.  Click Execute, and Cancel.
  5. Right click the dataset and select the command Generate Bathymetric Profile.  A menu will appear.
  6. Click the Start button, then digitize the first point of the profile by clicking with the left mouse button where you want the profile to start on the seafloor in the display window.
  7. Digitize the last point of the bathymetric profile by clicking with the left mouse button where you want the profile to stop on the seafloor.
  8. After you have digitized both the start and stop point, you will be prompted with a dialog asking you to provide a name for the profile.  Keep the default name and click OK.
  9. Geocap will notify you where the resulting Bathymetric profile has been stored in the project.  Click OK.
  10. Locate the new profile in 2. Seabed / Bathymetric Profiles; this is the default position of bathymetric profiles.  In order to structure our project we want to move the profile into a sub folder and rename the profile.
  11. Right click the folder 2. Seabed / Bathymetric Profiles and select New > Folder.
  12. Change the folder name to Etopo1 and click OK.
  13. Right click the profile and select Cut.
  14. Right click the new folder and select Paste.  The profile is now moved into the new folder.
  15. Right click the data set and select Rename.  Give the profile a suitable name, for example the name of the area you are working in.
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