E. Foot of Slope Analysis & Hedberg points

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

Introduction

UNCLOS Article 76 states that "the foot of the continental slope shall be delineated as the point of maximum change in the gradient at its base". FOS analysis is crucial because both formula lines (Gardiner and Hedberg) are calculated using FOS positions. In Geocap there are several tools for analyzing the gradient of the slope, based on different data sets. The Gradient Band Analysis is often used for identifying the approximate location of the base of slope region, based on bathymetric grids. To calculate the accurate location of a foot of slope (FOS) point the Bathymetric Profile Analysis panel is used. 

In this section we will analyze one foot of slope point for each of the bathymetric profiles we created in the previous section. Then we will proceed to generate 60M lines (Hedberg) from these FOS points.

Exercises

Base of Slope

Paragraph 3 of article 76 states that "The continental margin comprises the submerged prolongation of the land mass of the coastal State, and consists of the sea-bed and subsoil of the shelf, the slope and the rise. It does not include the deep ocean floor with its oceanic ridges or the subsoil thereof". This means that the base of slope region is the transition region from the rise to the deep ocean floor. We can try to analyze and visualize this transition zone using the Gradient Band Analysis. 

The Gradient Band Analysis command will divide a grid into different areas depending on the steepness of the grid. The areas will be display as color bands. It is possible to adjust the boundaries between the different areas. The boundaries are set in degrees.

Exercise

Visualizing gradient bands

  1. In the folder 2. Seabed / Grids right click atlantis and select Advanced Display > Gradient Band Analysis.
  2. Set the Smoothing steps to 5.
  3. Keep the rest of the settings as default and click Execute.
  4. Investigate the color bands and the steepness. Are the boundaries reasonable? 
  5. Try to change the angles and click Execute. 
  6. Can you find values that could represent the shelf, slope, rise and the deep ocean floor in the East Sea region?


Gradient Band Analysis

Note that the Gradient Band Analysis only takes morphological data into account. Geological and geophysical data can also be used to identify the base of slope region.
For more information refer to the Scientific and Technical Guidelines of the Commission on the Limits of the Continental Shelf found here: http://www.un.org/Depts/los/clcs_new/commission_guidelines.htm 

Foot of Slope Analysis

The Analysis Panel is one of the most important tools in Geocap because it allows you to determine the Foot of the Continental Slope in accordance with UNCLOS Article 76.  


The elements of the FOS analysis panel

Important elements in the FOS analysis panel:

  • The Select area button lets you select the area you are interested in analyzing. The FOS point will end up inside this area.
  • The Calculate change in gradient is where you set up the mathematical methods and variables that go into the FOS point calculation.
  • The Select Foot of Slope lists the possible FOS points in the area. The point with the larges change in gradient will be listed at the top and selected by default.
  • The Plot item list the different elements in the plot. Elements can be turned on and off or edited. It is also possible to add additional elements.
  • The Save Analysis As... button lets you save an analysis and the corresponding FOS point.

A typical workflow for analyzing a foot of slope point would be:

  1. Select an area of interest - This should represent your base of slope region.
  2. Select a method for calculating the gradient - Observe the difference for the three methods.
  3. Select a calculation interval - These values should represent the general trend in the area. 
  4. Save the Analysis and FOS point.

 

 

The analysis and FOS point will be stored under the bathymetric profile


Profile, Analysis and FOS point


Exercise

Analyze a bathymetric profile and create a FOS point

  1. In the folder 2. Seabed / Bathymetric Profiles / East Sea right click the East Sea 1 profile the and select Analyze Profile. The profile analysis tool will appear at the bottom of your screen. The same profile is also visible in the 3D window above the analysis panel where you will notice the red vertical line marking the maximum change of gradient.
  2. Click the Select area button.
  3. Press the left mouse button in the profile window, drag the mouse to the side, and release the button. You have now selected the area where the software will find the FOS point.
  4. Try increasing the Calculation interval on both sides (blue and green) by typing in a higher value (i.e 25 000).
  5. When you think you have identified the FOS point. Click the Save Analysis as.. button. 
  6. Keep the default name for the Analysis and click OK.
  7. Type in the FOS name ES 1 and click OK.
  8. The FOS point is now saved. Click OK.
  9. Click the triangle (or plus sign) next to the East Sea 1 profile in the project and you will see that the FOS analysis is stored as a child of the profile in the tree.
  10. Click the triangle (or plus sign) next to the FOS analysis dataset, and you will see the ES 1 foot of slope dataset. This is the data set holding the foot of slope point we just made.
  11. Click the Exit button in the Profile Analysis Panel to close it.

It is possible to display Base of slope (BOS) polygon or other lines which crosses the bathymetric profile in the profile window. Click the browse icon on the analysis panel, browse in the line or polygon and it will be displayed as a vertical line.

Exercise

Analyze and create FOS points for the remaining bathymetric profiles in the East Sea area

  1. In the folder 2. Seabed / Bathymetric Profiles / East Sea right click the next East Sea profile the and select Analyze Profile.
  2. Use the analysis panel to find the FOS point and click Save Analysis as..
  3. Keep the analysis name and type in the FOS name ES <number corresponding to bathymetric profile> and click OK.
  4. Click the Exit button in the Profile Analysis Panel to close it.
  5. Follow the steps above until you have an analysis and FOS point for all bathymetric profiles in the East Sea area.

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In many cases you want to go back and look at the analysis you did for a specific FOS point and maybe change the analysis and update the FOS point. In the next exercise we will change the FOS point for an existing analysis.

Exercise

Change an analysis and FOS point

  1. In the folder 2. Seabed / Bathymetric Profiles / East Sea, expand the East Sea 1 profile so you can see the FOS analysis dataset.
  2. Right click the FOS analysis dataset and click Analyze. This will open the profile with the settings you used for this analysis.
  3. Change some of the settings in the analysis in order to move/change the FOS point. (i.e selected area, calculation interval, method).
  4. Click Save Analysis changes.
  5. This will overwrite the previous settings. Click OK.
  6. Click Exit to close the analysis panel.
Exercise

Display FOS points

  1. In the folder 2. Seabed / Bathymetric Profiles / East Sea expand the East Sea 1 profile and analysis so you can see the ES 1 dataset.
  2. Right click ES 1 and select Display FOS points.
  3. Notice that the FOS point is displayed as a sphere in the graphics window.
  4. Repeat the steps above to display all your FOS points in the East Sea area.
Exercise

Investigate the FOS point table view

  1. In the folder 2. Seabed / Bathymetric Profiles / East Sea expand the East Sea 1 profile and analysis so you can see the ES 1 dataset.
  2. Right click ES 1 and select Table View. 
  3. Notice that the various field data contains the information from the analysis panel.
  4. Click Close.

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Generating Hedberg points (FOS + 60M)

Once we have our FOS points we can generate the 60M Hedberg points which is one of the formula lines in article 76. It is possible to generate a 60M circle around each FOS point but it is often useful to work with a group of FOS points instead of working with them individually. The next exercises will show you how to collect many FOS points into one foot of slope dataset and then generate 60M Hedberg points around those FOS points.

 

Exercise

Joining FOS points into one FOS group dataset

  1. In the folder 2. Seabed / Bathymetric Profiles / East Sea expand the East Sea 1 profile and analysis so you can see the ES 1 dataset.
  2. Right click ES 1 and select Merge Foot of Slope Points. This will open a menu with a list containing your first FOS point. We are now going to populate this list with the rest of the FOS points.
  3. Keep the menu open and select the ES 2 dataset in your project. Click the icon in the merge menu. This will add the FOS point to your list.
  4. Repeat the previous step to add all your FOS points in the East Sea area to the list.
  5. In the folder 2. Seabed / Foot of slope select the folder Region Groups and click the icon in the merge menu to select the Result folder.
  6. Click Execute.
  7. Type in the name East Sea and click OK. This will store your FOS group in the Region Groups folder.
  8. Click Cancel to close the menu.
  9. In the folder 2. Seabed / Foot of slope / Region groups right click East Sea and click Display FOS points.

 

We are now going to create the Hedberg lines from these FOS points.

Exercise

Generating 60M line to the FOS points

  1. In the folder 2. Seabed / Foot of slope / Region groups right click East Sea and click Generate 60M Line.
  2. Keep the default settings and click Execute.
  3. The new line will be stored in 1. Maritime Lines / 60M Lines. Click OK.
  4. Click OK to close the menu.
  5. In the folder 1. Maritime Lines / 60M Lines right click East Sea + 60M and click White (FOS+60).

 

The Table View contains important metadata about the Hedberg points.

Exercise

Investigate the FOS + 60M table view

  1. In the folder 1. Maritime Lines / 60M Lines right click East Sea + 60M and click Table View.
  2. Notice how the FOS Point Name changes as you scroll down the list of points.
  3. Click Close

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Real Bathymetric Profiles

Real bathymetric profiles are measured, usually by sonar. They are not generated by extracting values from a seabed grid or from any other data derived mainly from satellite altimetry, such as ETOPO1. An example of real bathymetry is "Corrected Depth Bathymetry" from the U.S. National Oceanic and Atmospheric Administration's (NOAA) Geophysical Data System (GEODAS). Bathymetric profiles often come in ASCII formats with X Y Z or Lat Lon Z values. These type of files can be imported using the ASCII importer in Geocap. In the next exercise we are going to import a bathymetric profile using this import menu.

 

Exercise

Import a bathymetric profile

  1. In the folder 2. Seabed / Bathymetric Profiles right click Singlebeam and click Import > ASCII Column.
  2. Click the browse button and browse for the dataset called Cruise-12  (Data/Atlantis/Bathymetry/Singlebeam/Cruise-12.txt). Click Open
  3. Make sure Adjust header is set to 0.
  4. Make sure Coordinate format is set to XY.
  5. Make sure Z is checked.
  6. Make sure Scalars is unchecked.
  7. Make sure that File column X=1, Y=2, Z=3
  8. Make sure that Cell type is set to Line
  9. Make sure that Schema is set to Bathymetric Profile
  10. Click Execute.
  11. You will be prompted with a dialog asking you for Coordinate System and Datum. Select World Geodetic System 1984 and Mercator (all values 0).
  12. Click OK.
  13. The file is now imported. Click OK.
  14. Click OK to close the import menu.

 

 

The GEODAS bathymetric profiles can sometimes be very long, covering areas that you are not interested in. In addition to this they can contain noise etc. It is possible to adjust the length of the profiles in Geocap, so that you will only analyse the part you need. It is also possible to filter/smoothen the data. We are now going to adjust the length of the profile we just imported and then use filters to smoothen it.

 

Exercise

Save an adjusted bathymetric profile as a new bathymetric profile

  1. In the folder 2. Seabed / Bathymetric Profiles / Single Beam right click Cruise-12 and click Analyze Profile.
  2. In the 3D graphics window find the white spheres at the end of the profile and adjust them by dragging the spheres to the desired position.
  3. In the bottom left corner of the analysis panel click on Save profile As...
  4. Type in the name Cruise-12-East-Sea and click OK.
  5. The new profile will be saved in the same folder as the original. Click OK. 
  6. Click Exit to close the analysis panel.
  7. Right-click the new bathymetric profile in your project and click Analyze Profile.  Notice the new bathymetric profile is now a sub-set of the original profile.

 

 

Tip

In some instances the white spheres may be very small when the analysis panel is first opened. It may be necessary to zoom in closer to find one of the ends of the profile. Once the white sphere is clicked on once, the white spheres on both ends will become larger and easier to see when dragging them to the desired position.

 

The bathymetry in the Cruise-12 singlebeam dataset is very "choppy" for some reason, and we want to smoothen the dataset,

As a general rule we do not recommend filtering and smoothing of bathymetric profiles unless it is absolutely necessary. If you do use filtering in an FOS analysis the Commission might request full disclosure of the original data, the mathematical details of the smoothing algorithm and the output data. For more information refer to section 5.3 in the Scientific and Technical Guidelines of the Commission on the Limits of the Continental Shelf found here: http://www.un.org/Depts/los/clcs_new/commission_guidelines.htm 

Exercise

Smooth the bathymetry

  1. In the analysis panel check the box in front of 1.Filter. Notice that a thin yellow line is shown on top of your seabed in the analysis window.
  2. Change the filter to e.g. Moving Average.
  3. Try changing the Calculation point radius number and observed the difference in the filtered line.
  4. Try to find a FOS point you think you can justify and save the analysis by clicking Save Analysis As
  5. Give the FOS point a proper name and click OK.
  6. Click OK again.
  7. Click Exit to close the analysis window.

It is possible to store the filtered (e.g. smoothed) bathymetry as a separate dataset. Click the Tables of values button to the right in the analysis panel. The filtered result is then available in a tab, and may be extracted as a project dataset.