Unclos Tutorial K. The 2500m Isobath

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). For a Grid, the result will be a contour line. A bathymetric profile will produce one or more points, where the profile crosses 2500m. In the next exercise we will produce the 2500m isobaths based on Etopo1 data.

• Locate the data set you imported in Exercise 2, 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”
• Right click the dataset and select “Display 2500 meter isobath”
  A black line will appear where the 2500m isobaths is located. You may see that the isobaths covers a larger area than you are interested in. In that case we will restrict the area where the isobath is calculated.
• Set the display window in 2D mode by clicking the 2D button.
• Zoom the display window so that it only covers the area you are interested in.
• Right click the dataset and select the command “Generate Isobath”
  This will use the full resolution of the grid and calculate the 2500 isobath. 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.
• Locate the result dataset which should be in “2. Seabed / 2500 Isobath”
  The Isobath you have created may still have some line pieces which you want to remove before generating the 100M line.
• Clear the display window by clicking this button in the main toolbar
• Right click the data set and select the command “Edit points and lines”
  The “Edit points and lines” menu should appear.
• Click the “Display” button near the bottom in order to see the isobaths.
• Select the “Delete” tab
• Inside the “Delete” tab, select the “Inside closed line”
• Click the “start” button, then digitize a closed polygon around the part you want to delete by clicking with the left mouse button.
• Click the button “Connect to Start and End” in order to connect the last digitized point with the start point, and end the digitizing.
• Click Delete

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 do are not performed on the dataset directly, but a local copy. In order to store the changes you do 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.

• Click the Close button in the “Edit points and Lines” menu.
• Clear the display window again by clicking this button in the main toolbar
• Display the edited line and confirm that the unwanted parts of the line have been removed.
• When you have made changes to the project it is always a good idea to store them. Click the save button in the project tool bar in order to save your project.

• Display the 2500 isobath which you created in the previous exercise.
• Right-click the isobath and select the command object “Generate 100M Line”
• The new line should now be available under “1. Maritime Lines/100M Lines”
• Display the 100M line
• When you have made changes to the project it is always a good idea to store them. Click the save button in the project tool bar in order to save your project.