Marine Data Literacy 2.0

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Home > 6. Motion Vectors > 6.4 Speed Grid

6.4 Using a Speed Grid for Visual Scaling of Vector Arrows

  • Exercise Title:  Using a Speed Grid for Visual Scaling of Vector Arrows

  • Abstract:  If an objective scale is required to facilitate visual estimation of the current or wind vector arrows, then you can color the arrows according to speed (as done in the previous exercise), or you can use a colored scalar speed grid as the background image on a map.  The speed grid is an easily made as a secondary product from the U and V grids.

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

  • Required Software:

  • Other Resources: 

  • Author:  Murray Brown

  • Version:  September 2012

1.  These are the wind vector maps for January (above) and July (below), made in the previous exercise ("Plotting Vector Arrows...").  Notice that although they clearly show the regional flow patterns qualitatively, there is no quantitative scale.  There is no arrow drawn below that is equal to, for example, 10 m/sec (a common scale arrow).
2.  Commercial software, and some more complex systems, can provide graphic legends that provide an objective numerical scale for the vector arrows.  The method below uses the scalar speed values to do the same thing with a color scale.  First we must create the speed grid from the U and V grids.
3.  Run Saga, and use MODULES > IMPORT/EXPORT-GRIDS > IMPORT ESRI GRID and load the ASC grids listed above.  Make sure to use these settings:
  • TARGET GRID TYPE = FLOATING POINT 8 BYTE
  • NODATA VALUE = USER DEFINED
  • USER DEFINED NODATA VALUE = -99999
4.  Here you can see the 2 grids loaded into Saga.
5.  Select MODULES > GRID-CALCULUS > CARTESIAN TO POLAR COORDINATES to load a module that will create the speed grid.
6.  Make these choices:
  • GRID SYSTEM = SYSTEM FOR THE WIND DATA
  • X COMPONENT = WIND U
  • Y COMPONENT = WIND V
  • DIRECTION = CREATE
  • LENGTH (SPEED) = CREATE
  • POLAR ANGLE UNITS = DEGREES
  • METHOD ANGLE = GEOGRAPHICAL
  • USER DEFINED ZERO DIRECTION = 0
  • USER DEFINED ORIENTATION = CLOCKWISE

Then click OK.

7.  These two new grids appear:  DIRECTION and LENGTH
8.  Now create the wind vectors again.  Select MODULES > SHAPES-GRID > GRADIENT VECTORS FROM DIRECTIONAL COMPONENTS.  Select the winds grid system.  Then carefully select the U component grid for Grid X and the V component grid for Grid Y. 

Now make these selections:

  • For GRADIENT VECTORS, select [CREATE].
  • For STEP, leave the value at 1 (to show every vector); larger incremental values (e.g. 2, 10) would cause subsampling.
  • For MINIMUM, select 10, arbitrarily
  • For MAXIMUM, select 100, arbitrarily
  • For AGGREGATION, either MEAN VALUE or NEAREST NEIGHBOR
  • For STYLE, select ARROW

Then click OK.

NOTE:  Do some experiments with the MIN/MAX values to see how they work.  Also with STYLE.

9.  You should see this new line shape for the vectors.
10.  Richt-click on the wind vectors and select ADD TO MAP to see them on a map.

 

NOTE:  The older command "Show Shapes" has been dropped.

11.  Right-click on the LENGTH (SPEED) grid and show it on the same map as the vectors.

The grid probably covers the vectors as you see here.

12.  To move the vectors to the top follow these steps:
  • Select the MAPS > TREE tabs in the lower right corner
  • Left-click on the vector object to select it
  • Right-click on the vector object
  • The select MOVE TO TOP
13.  After a little adjustment of the vector object's properties, you should be able to see something like this.
14.  After a little work with the PROPERTIES tab of the LENGTH (SPEED) grid, you should be able to display a LEGEND like this.
15.  Any and all of the above objects can be saved using appropriate filenames.  The suite of objects in the above figure can be saved as a Saga project.
16.  OPTION:  Make a similar figure for the July winds.