Creating Bathymetric Contours from Grids: GEBCO 08 Shelf Estimations

Marine Data Literacy

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7.2 Creating Bathymetric Contours from Grids: GEBCO 08 Shelf Estimations

Exercise Title: Creating Bathymetric Contours from Grids: GEBCO 08 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 08 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):
- Bathymetry and Topography
Required Software:
- Saga
Other Resources:
- bathymetry_cons_liberia_gebco2003_1000s - Depth contour vectors (shapefile) for Liberia, created in Adding GEBCO Depth Contours to a Project Map in Saga
- coastline_liberia_wvs_gebco2003 - Coastline for Liberia (shapefile), originally published as the World Vector Shoreline, created in Adding GEBCO Depth Contours to a Project Map in Saga
- relief_liberia_gebco08_saga_1min.sgrd - GEBCO 08 bathymetry grid (Saga format) for Liberia, created in GEBCO 08 Global Relief Grid: Local Processing, or in the exercise GEBCO 08 Global Relief Grid: Online Processing
Author: Murray Brown
Version: November 2012

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 above resource files.

2. Show the relief grid with ADD TO MAP.

3. SHOW the GEBCO depth contour lines (here with a legend) and the GEBCO/WVS coastline (here in white) on the same map.

Seen up close, the GEBCO contours are quite detailed. Nearly identical indentations oriented toward the north or northeast indicate canyon-like features.

We want to create and add contours between 1000 m (the shallowest contour) and the coastline.

4. Here the map has been zoomed to show better the northern area.

5. Select MODULES > SHAPES-GRID > CONTOUR LINES FROM GRID.

5. Set the GRID SYSTEM to the ETOPO1 grid system, and the GRID to ETOPO1.

Make sure CONTOUR LINES is set to CREATE.

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

[NOTE: We are avoiding creation of the 0-m, or coastline shape. This is usually a very poor simulation from gridded data, even of the highest quality.]

Then click OK.

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

8. SHOW the new shape on the map.

9. 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.

10. To properly display the color coding (i.e. the palette) for the new contours, select COLORS > TYPE and pick GRADUATED COLOR. Then for the ATTRIBUTE, don't use the ID, but select the name of shape object.

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

12. 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.

13. Now to save the new shape, right-click on it and select SAVE AS.

14. Navigate to PRODUCTS > SAGA > VECTORS and use the name bathy_estim_cons_500m_100m_liberia_geobco08_gridviewer_saga

15. In case you need to make some "special" contours, the following 2 frames should be helpful.

16. Here's how to do exactly 2 contours, in this case 300 m and 100 m depth.

17. Here's how to do only one contour, in this case 250 m depth. A very small (not zero!) EQUIDISTANCE is required or the module fails.

7.2 Creating Bathymetric Contours from Grids: GEBCO 08 Shelf Estimations