D. Bathymetry

Owned by Harald Sund (Unlicensed)
Last updated: Oct 16, 2015 by Tore Sannes (Unlicensed)

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 data may be imported as bathymetric profiles.
  • Multibeam soundings 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. This can be done in Geocap.
  • Grids may be imported directly into Geocap. Geocap supports import of several different grid file formats. Most Shelf projects will consist of a low resolution grid (i.e Etopo or SRTM) covering the whole area of interest and then smaller high resolution grids from multibeam data.

The Etopo or SRTM grids 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 seafloor, 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. In the folder 2. Seabed / Grids right click the atlantis dataset and select Generate 2500m Isobath. 
  2. The command will generate a new dataset. Click OK.
  3. In the folder 2. Seabed / 2500 meter isobath check the box next to the dataset atlantis's 2500 meter isobath.

 

Notice how parts of the 2500m isobath are smaller contour lines around sea mounts etc. We want to remove these before we generate the 2500m + 100M constraint line.

Exercise

Edit the generated 2500m isobath

  1. Clear the display window by clicking the X button in the main toolbar.
  2. In the folder 2. Seabed / 2500 meter Isobath right click atlantis's 2500 meter isobath and select 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 tab.
  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.
  8. Click Delete points INSIDE. The part inside the closed polygon is now deleted.
  9. Change the drop down menu from Save as an extra copy to Overwrite input dataset.
  10. In order to store the changes in the Edit points and lines menu, click the Execute button at the bottom.
  11. Click Cancel to close the Edit points and Lines menu.
  12. Clear the display window by clicking the X button in the main toolbar.
  13. In the folder 2. Seabed / 2500 meter Isobath right click atlantis's 2500 meter isobath and select the Display and confirm that the unwanted parts have been removed.
  14. The Edit points and lines command stored the original 2500m isobath as a backup. Right click this dataset and select Delete.
  15. Click OK.

 

We can now generate the last constraint line.

Exercise

Generate 2500m isobath + 100M constraint line

  1. In the folder 2. Seabed / 2500 meter Isobath right click atlantis's 2500 meter isobath and select Generate 100M Line.
  2. Keep the default settings and click Execute.
  3. The new line should now be available under 1. Maritime Lines / 100M Lines. Click OK.
  4. Click OK to close the menu.
  5. In the folder 1. Maritime Lines / 100M Lines display atlantis's 2500 meter isobath + 100M.

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Bathymetric Profiles

To determine the foot of the continental slope we need bathymetric profiles. These profiles will be used as input to the foot of slope analysis tool in the next chapter. Bathymetric profiles can be imported from single beam or generated from bathymetric grids. In this tutorial we will use the atlantis seabed surface grid (similar to Etopo1) to generate bathymetric profiles. The project already contains several bathymetric profiles grouped into different areas. Our task is to complete the foot of slope analysis in region called East Sea. In this section we will generate the bathymetric profiles, give them a proper name and group them into a folder.


East Sea is the area in red

 

To ensure the most accurate calculation of the foot of slope the bathymetric profiles should run perpendicular to the seabed surface contours.

Exercise

Generate a bathymetric profile from a grid

  1. Display the atlantis seabed surface dataset in 2. Seabed / Grids and zoom in to the East Sea area.
  2. Right click the atlantis seabed surface dataset again and select the command Generate Bathymetric Profile.  A menu will appear.
  3. 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.
  4. 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.
  5. Click Stop
  6. You will be prompted with a dialog asking you to provide a name for the profile. Keep the default name and click OK.
  7. Geocap will notify you where the resulting Bathymetric profile has been stored in the project. Click OK.
  8. Locate the new profile in 2. Seabed / Bathymetric Profiles. This is the default position of bathymetric profiles.
  9. Display the profile.

 

 

It is possible to generate a bathymetric profile following a specific path or feature by digitizing more than two points.

Exercise

Generate more bathymetric profiles in the East Sea area

Repeat the steps in the previous exercise and generate a few more profiles in the same area. You should have 3-5 profiles in the area.

 

We should now group our bathymetric profiles into a folder and give them a proper name.

Exercise

Group your bathymetric profiles into a folder

  1. Right click the folder 2. Seabed / Bathymetric Profiles and select New > Folder.
  2. Select Generic and type in the name East Sea.
  3. Click OK.
  4. Right click the East Sea folder and select Set Schema > Bathymetric Profiles
  5. Locate the profiles you generated in 2. Seabed / Bathymetric Profiles.
  6. Select all the profiles, right click and select Cut
  7. Right click the new East Sea folder and select Paste. The profiles are now moved into the new folder.
Exercise

Rename your bathymetric profiles

  1. Right click one of the profiles in the East Sea folder and select Rename.
  2. Type in the name East Sea 1.
  3. Click OK
  4. Continue to rename the other profiles in the same manner. East Sea 2 etc.

It is advisable to name the profiles with numbers increasing from south to north.