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 > 2. Marine GIS > 2.19 IDV & THREDDS

2.19 Visualizing Climatological Grid Analyses in IDV: THREDDS

  • Exercise Title:  Visualizing Climatological Grid Analyses in IDV: THREDDS

  • Abstract:  Although the IDV has been extensively use in other exercises to work with operational ocean data, it is also useful for the display of climatological grids, especially when they can be found in THREDDS data servers (TDS).  This makes their subsetting (by time, location and striding (i.e. decimation)) very efficient.  Many organizations and programs now publish their THREDDS data catalogs (usually with names like CATALOG.XML) which can also be used in IDV for browsing and access purposes. 

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

    • N/A

  • Required Software:

  • Other Resources: 

  • Author:  Murray Brown

  • Version:  9-11-2014

1.  Run IDV.
2.  On the dashboard, select DATA CHOOSERS > CATALOGS. 

Navigate in the catalog list to IDV CATALOG.  You might have to use the < or > arrows to find it.  This opens the main catalog of THREDDS catalogs.

At this very top level there are only 11 items, but if you explore them, you'll find hundreds of data collections.

3.  Within the THREDDS structure there are documentation files for many BUT NOT ALL items.  Some of the links are empty.  Some go to documents of limited use. 
4.  Just for an example, navigate down into the huge catalog structure to the COLOR section.
5.  You can browse deeper into this large section, until you find the PRIMARY PRODUCTION products from MODIS and Pathfinder.

To see what's here, burrow even deeper to find the DATASET SUMMARY.  Nearly all THREDDS datasets have these summaries for your information and possible selection.  Just double-click on the DATASET SUMMARY link to open it.

6.  If you decide you want to work with a specific dataset, then navigate back upwards to the item you want. 

You can tell when a dataset is available for use when the control ADD SOURCE (bottom of screen) is displayed with bold, dark letters, as you see here.

You might have to wait a minute or two for the item's listing to appear

7.  After the source is added. important PROPERTIES settings are necessary.
8.  First, check the available TIMES.  If you see a large list that you want to cut down, select USE SELECTED on the right margin, and then select only the dates you want.
9.  Now check the SPATIAL SUBSET.  If you want to limit the analyses to a particular area, then follow these steps:
  • With the cursor, draw a small box anywhere on the map
  • Now you can enter the area coordinates for your area of interest (Liberia in this case)
  • If the data are extremely dense, then you can use X SAMPLING and Y SAMPLING (which might be chained together as you see here) to decimate the data to a usable fraction.
  • You can also use LEVEL SAMPLING to cut the data down to specific depths (heights) if needed.

When finished with spatial considerations, click OK to continue.

10.  Now you can go to the DISPLAYS area (on the right) to select how the data will be displayed.  Try COLOR-SHADED PLAN VIEW for this exercise.  Of course, you're invited to experiment with all these methods, later on your own.

Then click CREATE DISPLAY.

11.  Here's the map you'll get. using the default color palette, and the existing data range for the subset
12.  To improve the display, which consists mainly of small values, you can click on DASHBORD > COLOR TABLE > DEFAULT and select RADAR > DBZ for a linear scaling of the colors.
13.  Note that the primary productivity (PP) values range from nearly zero to 7499 mg C m-2 day-1.  Usually with chlorophyll and PP you can get much better maps by reducing the highest displayed value to 1/2 or even 1/3 of the maximum value.

So click on COLOR TABLE > DEFAULT > CHANGE RANGE and cut the top value to about half of the maximum, as you see here.

14.  This is a much better map, showing very interesting features along the Equatorial Current.
15.  25.  Look carefully at the time control just over the map on the right, and you'll see that you can move through the frames, month-by-month.  In fact, you can easily animate the sequence, with a single button.
16.  On your own time, use VIEW > CAPTURE > MOVIE to explore various option for making movies in various formats (e.g. MOV and GIF).
17.  Now that covers THREDDS access to scalar grids.  Next we'll cover the use of vector component grids, the classic U and V files.
18.  The method below is completely described in Visualizing Satellite-Measured Winds in IDV: ASCAT so we only present the bare bones.  If you want to make these analyses, then go to that exercise for complete instructions.  The only difference would be that if you want to make an animation, then choose multiple data days rather than the single day chosen in the example.
19.  To find some wind vector components, navigate in the catalog to the location you see here.  
20.  Then you find these near-real-time files.
21.  Combining the component grids requires one of the special formulas available in IDV, found by clicking the FORMULAS control at the top of the list of data objects on the left.

22.

23.  Using a stride of 3 (i.e. using every third data point) here's the global map of the vectors.
24.  Don't worry if you can't see anything, just use the Shift-Cursor drag method to zoom in on any area of interest to see the winds.

You could also use PROPERTIES to set a specific region by exact latitude and longitude limits.

25.  This small overview of THREDDS and IDV should give you some ideas about additional data source for your work.  Although as stated above, there is a problem with good documentation within THREDDS catalogs, you can probably get some use from these URLs collected by the author over many months of use.  And you'll find that a feature of THREDDS is that any catalog can be a subdivision of another catalog, so at lease one item below is part of the first "master" catalog.

Don't use them as URLs, but just copy and paste them into the catalog name space in IDV, when needed.  You'll find that IDV "remembers" these sites for later use.