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7.2 Creating Bathymetric Contours from Grids: GEBCO 14 Shelf Estimations

  • Exercise Title:  Creating Bathymetric Contours from Grids:  GEBCO 14 Shelf Estimations

  • Abstract:  In most areas of the world there are no GEBCO depth contours shallower than 100 m.  In this exercise you'll learn how to estimate shallow contours with Saga, using the contour lines module and the GEBCO 2014 gridded bathymetry.  It's important to note here and on any relief products you make that these are roughly estimated locations that should not be relied on for navigation or any other practical purpose.  This exercise demonstrates the general method to create contour lines from all grids.

  • Preliminary Reading (in OceanTeacher, unless otherwise indicated):

  • Required Software:

  • Other Resources: 

  • Author:  Murray Brown & Lilian Krug

  • Version:  30-Jan-2019 (with SAGA 7.1.0)

THIS LESSON IS PROVIDED FOR RESEARCH PURPOSES ONLY.  NO ONE SHOULD ATTEMPT TO RELY ON THESE ANALYSIS PRODUCTS FOR NAVIGATION OR FOR ANY OPERATIONAL PURPOSES AT SEA.

1.  Run Saga.  Close any open datasets, then load the GEBCO 14 relief for Liberia.
2.  Show the relief grid with ADD TO MAP.
3.  We want to create and add contours between 1000 m (the shallowest contour) and the coastline.
4.  Select MODULES > SHAPES > SHAPES-GRID TOOLS > CONTOUR LINES FROM GRID.
5.   Set the GRID SYSTEM and the GRID to GEBCO14.

Make sure CONTOUR LINES  is set to CREATE.

Just for demonstration purposes, set the MINIMUM VALUE to -500, and the MAXIMUM VALUE to -100.  Then set EQUIDISTANCE (=contour interval) to 100.

 

Then click OK.

 

6. This new shape object appears.  Notice that it is named for the grid it is based on.
7.  SHOW the new shape on the map.

 

8.  If you ZOOM it up, and use the PAN tool, you can move around and examine the relation between the GEBCO contours and the new ones.  In many areas there is acceptable agreement with the general trends.  In some other areas there are obvious disagreements.
9.  To properly display the color coding (i.e. the palette) for the new contours, select COLORS > TYPE and pick DISCRETE COLOR.  For the ATTRIBUTE, use Z.

10.   Now, after adjusting the number of colors and selecting the RAINBOW preset palette, you can get this reasonable legend.

 

 

11. Now to save the new shape, right-click on it and select SAVE AS.
12.  Navigate to PRODUCTS > SAGA > VECTORS and use the name bathy_estim_cons_500m_100m_liberia_geobco08_gridviewer_saga
13.  Here's how to do only one contour, in this case, the coastline (0m depth).

When the EQUIDISTANCE is set to 0, the MAXIMUM CONTOUR VALUE option disappears.

 

14.  Here's a ZOOM showing the Coastline (black line), relief grid (mostly orange) and the World Borders (gray).

 

[NOTE:  This is for exercise-purposes only, since the coastline derived from a grid is usually a very poor simulation from gridded data, even of the highest quality.]

15.  In general, the attempt has been a modest success, but any use of these contours in other work should be accompanied by very strong warnings about the very approximate nature of these results.  This example has posed one of the premier hand-drawn charts in the world, GEBCO, against a fairly rough grid, so the differences were to be expected.  It's important to note here and on any relief products you make that these are roughly estimated locations that should not be relied on for navigation or any other practical purpose.