Marine Data Literacy 2.0

Providing instruction for managing, converting, analyzing and displaying oceanographic station data, marine meteorological data, GIS-compatible marine and coastal data or model simulations, and mapped remote sensing imagery

 

 

 

 

Home > 4. Ocean Data View > 4.10 Ocean Fronts

4.10 Creating Isotherm-Depth Ocean Front Climatologies in ODV

1.  Run Ocean Data View, and load the special collection for the northwest Atlantic, analyses_annu_nwatl_woa09_odvpages.odv
2.  Select VIEW > LAYOUT TEMPLATES > 1 SURFACE WINDOW to see this graphic (or something very close to it).
3.  Select VIEW > ISOSURFACE VARIABLES
4.  This screen shows the current isosurface variables.  Unless you have already added some, there will be only a few available.
5.  We want to add an isosurface that displays the depth of the 15-degree temperature isotherm.

Change the top line to read:

  • DEPTH[M] @ TEMPERATURE [C] = 15

Then click ADD to effect the addition.

6.  Make sure you see the new isosurface in the list before continuing.

Click OK.

7.  Right-click on the graphic and select Z-VARIABLE.
8.  Select the new isosurface (DEPTH AT TEMPERATURE = 15) to be the Z variable.  You should see this graphic.

Notice that the isotherm of interest is several hundred meters deep in the entire area.

9.  Right-click on the graphic and select PROPERTIES
10.  Select the CONTOURS tab.  Enter these values:
  • START = 200
  • INCREMENT - leave blank
  • END = 200

Then click the << control to effect the addition.

11.  You should see the 200-m depth contour under ALREADY DEFINED.  Then click OK.
12.  This depth contour should appear on the graphic.
13.  To save the new line as a vector object, right-click on the graphic and select EXTRAS > EXPORT AS GRAPHICS OBJECT > CONTOUR LINES
14.  In the same location as the northwest Atlantic special collection, save the lines with the filename 15deg_at_200m_contour_annu_nwatl_woa09_odvwebsite_odv.gob - The GOB will be automatically added by ODV.
15.  Open the GOB contour line file in any ASCII editor.
  • Block and delete the text rows you see here

NOTE:  The data is arranged in columns of longitude (X) and latitude (Y).  And the longitude values are in the 0-360 system, that must be changed below.

16.  Go to the middle of the file to find that the POLYLINE is repeated. 
  • Block and delete the entire second appearance of the line
17.  Save the resulting file at the same location with the name 15deg_at_200m_contour_annu_nwatl_woa09_odvwebsite_odv.txt  (Note the extension is TXT, not GOB!)
18.  Run Saga and select MODULES > IMPORT TEXT TABLE WITH NUMBERS ONLY
19.  In the module, make these selections:
  • File contains headline - Uncheck
  • Separator - Other
  • Separator (other) - Type a single space
  • File - point to the new TXT file you just made above

Then click OK.

20.  Now you have the table in Saga, but you need to convert it to a shape.  Select MODULES > SHAPES POINTS > CONVERT A TABLE TO POINTS
21.  In the module, make these selections:
  • <<POINTS - Leave as CREATE
  • >>Table - Point to the table you just imported
  • X - Set to 1, the first column in the table
  • Y - Set to 2, the second column in the table

Then click OK.

22.  You can view the new point shape, if you wish.
23.  Now you must "transform" the point shape from a 0-360 system to a -180-to-+180 system.  Select MODULES > SHAPES TOOLS > TRANSFORM SHAPES
24.  In the modules, make these selections:
  • <<OUTPUT - Leave as CREATE
  • >>SHAPES - Point at the new point shape
  • dX - change to -360

Then click OK.

25.  You can use the usual method to save the new "transformed" shape in the folder PRODUCTS > SAGA > VECTORS with the filename 15deg_at_200m_contour_annu_nwatl_woa09_odvwebsite_odv_180_saga
26.  Here is the new shapefile displayed with the global coastline file.

NOTE1:  The file is best left as a point shape, because conversion to a line in Saga results in an attempt to close it like a polygon.

NOTE2:  Use the MAP tab in the lower left corner of Saga to get to the controls for the placement of the layers, then move the global map to the top.  The contour line produced by ODV actually extends to the left (i.e. below the North American continent), which needs to be covered up for a better display.

27.  This method just reveals the basic functions that can be used to draw, save and use the specific isotherm depth files you may need to display an ocean front.  The original data source, the World Ocean Atlas 2009 has seasons and months for this same area, which should be much better for climatological products.  Also, you must perform numerous experiments with different isotherms and depths -- alongside separately obtained maps of the currents of interest -- to find out what combination works best for you.  As you can clearly see above the Fuglister pair (15 degrees at 200 m) only works down to the latitude of Cape Hatteras, where he remarked that you would "run into the marshes" if you kept following these same values.  An entirely different pair of values should be used south of the Cape. etc. etc.  Make sure to use the CIMAS Ocean Surface Currents Atlas (reference above) to find technical descriptions of the major currents, including temperature-depth information in many cases.