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 data may be imported as bathymetric profiles.
- Multibeam bathymetry 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; 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 . 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.
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However once you have imported the grid you will be able to see the shape of the
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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.
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Download the ETOPO1 grid from the Internet- Open a web browser, and go to the following address: http://www.ngdc.noaa.gov/mgg/global/global.html
- Click the link: Extract Custom Grids - in the left column on the page, below the globe. A new web page with a interactive map should appear.
- Zoom into the area where you want to extract the grid.
- Click the icon with and i in the upper left corner of the map window.
- Drag a rectangle over the area where you want to extract the grid. You should see a red rectangle on the map.
- Keep Layer as ETOPO1 (ice)
- Select Output Format XYZ
- Click "click here to download"
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The following exercise can be used on most binary grids which Geocap supports. You normally do not have to specify the grid file format. Geocap will automatically recognize the file format.
Generate the 2500m isobath from a Seabed Surface- In the folder 2. Seabed / Grids right click the atlantis dataset and select Generate 2500m Isobath.
- The command will generate a new dataset. Click OK.
- In the folder 2. Seabed / 2500 meter isobath check the box next to the dataset atlantis's 2500 meter isobath.
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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.
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Import the grid into a folder in your projectLocate the folder called Grids in your Geocap project under 2. SeabedRight click the folder and select Import > Generic…The format should be set to Automatic.Click the browse  Image Removed button and locate the file you downloaded in the previous exercise.If Specify import area is checked, uncheck this option. This will import the entire grid.Click on the Reader Options tab and under polydata and check the box next to Invert (The Grid we will import has negative values for sea depths, and positive values for heights above sea level. Usually bathymetry data has positive values for depths. We therefore have to multiply the depth and height values by -1 with this option to "flip" the data).Click Execute.You will now be prompted with a question of the coordinate system and datum. Select: World Geodetic system 1984, Geodetic and click OK.Geocap will report if the grid has been imported correctly; click OK and close the import dialog by clicking Cancel.Observe that the new data set has appeared in the Grids folderEdit the generated 2500m isobath- Clear the display window by clicking the X button in the main toolbar.
- In the folder 2. Seabed / 2500 meter Isobath right click atlantis's 2500 meter isobath and select Edit points and lines...
- Click the Display button near the bottom of the Edit points and lines dialogue box in order to see the isobaths.
- Select the Delete tab.
- Inside the Delete tab, select By closed line tab.
- Click the Start button, then digitize a closed polygon around the part you want to delete by clicking with the left mouse button.
- Click Connect to Start and End in order to connect the last digitized point with the start point.
- Click Delete points INSIDE. The part inside the closed polygon is now deleted.
- Change the drop down menu from Save as an extra copy to Overwrite input dataset.
- In order to store the changes in the Edit points and lines menu, click the Execute button at the bottom.
- Click Cancel to close the Edit points and Lines menu.
- Clear the display window by clicking the X button in the main toolbar.
- 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.
- The Edit points and lines command stored the original 2500m isobath as a backup. Right click this dataset and select Delete.
- Click OK.
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We can now generate the last constraint line.
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Convert the grid to Mercator coordinate systemThe data is imported in Geodetic, with latitudes as Y coordinates and longitudes as X coordinates. In order to view the grid with the data we have already imported and converted to a Mercator coordinate system, we need to convert this grid to match the same coordinate system as the other data in the project (Mercator). - Click the dataset and go to the Shared commands section in the Toolbox.
- Double click the Convert to Mercator... command in the Operations folder. (or another relevant datum / coordinate system)
- Change the result combo box to Replace input.
- Click Execute.
- Agree to replace the existing dataset by clicking Yes.
- Geocap will report if the grid has been converted correctly, click OK and close the conversion dialog by clicking Cancel.
- Check that the schema of the dataset is seabed surface. If not then right click the dataset, choose Set Schema and choose seabed surface.
The data is now converted and ready to be displayed: - Right click the dataset and select Zoom to Data
- Right click the dataset and select Map Sea
- Right click the dataset and select Map Land
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When you have made changes to the project it is always a very good idea to store them. Click the save button Image Removed in the project tool bar in order to save your projectGenerate 2500m isobath + 100M constraint line- In the folder 2. Seabed / 2500 meter Isobath right click atlantis's 2500 meter isobath and select Generate 100M Line.
- Keep the default settings and click Execute.
- The new line should now be available under 1. Maritime Lines / 100M Lines. Click OK.
- Click OK to close the menu.
- 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.
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East Sea is the area in red
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To ensure the most accurate calculation of the foot of slope the bathymetric profiles should run perpendicular to the seabed surface contours. |
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Generate a Bathymetric Profile bathymetric profile from a grid- Locate Display the dataset you imported in the previous exercise, which should be located atlantis seabed surface dataset 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.Display the seabed by right clicking it and selecting Map Seaand zoom in to the East Sea area.
- Right click the dataset and select the command Display contours...
- A menu will appear. We will just use the default settings. Click Execute, and Cancel.
- Right click the dataset atlantis seabed surface dataset again and select the command Generate Bathymetric Profile. A menu will appear.
- 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.
- 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.After you have digitized both the start and stop point, you
- Click Stop
- You will be prompted with a dialog asking you to provide a name for the profile. Keep Keep the default name and click OK.
- Geocap will notify you where the resulting Bathymetric profile has been stored in the project. Click Click OK.
- Locate the new profile in 2. Seabed / Bathymetric Profiles; this . 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.
- Display the profile.
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It is possible to generate a bathymetric profile following a specific path or feature by digitizing more than two points. |
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Generate more bathymetric profiles in the East Sea areaRepeat 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.
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Group your bathymetric profiles into a folder- Right click the folder 2. Seabed / Bathymetric Profiles and select New > Folder.Change
- Select Generic and type in the folder name to Etopo1 and click name East Sea.
- Click OK.
- Right click the profile East Sea folder and select Set Schema > Bathymetric Profiles
- Locate the profiles you generated in 2. Seabed / Bathymetric Profiles.
- Select all the profiles, right click and select Cut.
- Right click the new East Sea folder and select Paste. The profile is The profiles are now moved into the new folder.
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Rename your bathymetric profiles- Right click the data set one of the profiles in the East Sea folder and select Rename. Give the profile a suitable name, for example the name of the area you are working in
- Type in the name East Sea 1.
- Click OK
- Continue to rename the other profiles in the same manner. East Sea 2 etc.
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It is advisable to name the profiles with numbers increasing from south to north. |
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