Marine Data Literacy

Providing instruction for handling (managing, converting, analyzing and displaying) oceanographic station data, marine meteorological data, GIS-compatible marine and coastal data, and mapped remote sensing imagery.

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9.1 IDV Setup
9.2 SST Fields
9.3 Modeled Waves
9.4 Pigments/Color
9.5 GlobWave Points
9.6 Sea Surface Salinity
9.7 Sea Ice
9.8 Sea Surface Height
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9.15 Real-Time GTSPP

Home > 9. Operational/Synoptic > 9.5 GlobWave Points

9.5 Visualizing Satellite-Measured Wave Analyses in IDV: GlobWave Point Data

  • Exercise Title:  Visualizing Satellite-Measured Wave Analyses in IDV: GlobWave Point Data

  • Abstract:  In this exercise you'll learn how to use satellite "overpass" data that comes in the form of points rather than neat grids.  These data are all from wave-sensing instruments that look downward and only "see" the surface wave heights (SWH) directly below.  The exercise is also good for showing how IDV handles such point data.

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

  • Required Software:

  • Other Resources: 

    • GlobWave Home Page

    • GlobWave Product User Guide

    • GlobWave FTP server:  ftp://eftp.ifremer.fr/ - after entering the site, change folders to waveuser/globwave/data/l2p

      • "Username and password details for the ftp site can be obtained by e-mailing the CERSAT Help Desk at fpaf@ifremer.fr with the subject: “GlobWave Data Access”.  Four data streams are available from 3 operational satellites in NRT [near-real-time]. Both Altimeter and SAR [synthetic aperture radar] data are available from Envisat, with Altimeter data also being available from Jason-1 and Jason-2. All data [are] disseminated in the common L2P format within 1-4 hours of the observation from space." [From the GlobWave website]

  • Author:  Murray Brown

  • Version:  April 2011
1.  In case the above description lost you, here's the "tree" of the available products:

  • Altimeter

    • Envisat

    • Jason 1

    • Jason 2

  • Synthetic Aperture Radar (SAR)

    • Envisat
2.  Use any good FTP client, plus the ID and passwords provided to you, and open the folder l2p to see its contents.

We'll use the altimeter data, so click on that folder.
3.  Here you see the contents are of two types:
  • GDR - generally means geophysical data record, and is often used for final, quality-controlled data
  • NRT - near-real-time, or quick delivery data

Click on the NRT folder.
4.  Here you see the available satellites.  We'll take them in alphabetical order, so click on ENVISAT.
5.  Select the most current year (in the author's case, 2011).
6.  Select the most current year-day (i.e. the 122nd data of the year is May 2).
7.  Examine the available files, and you'll find that many overpasses were recorded for this day.
8.  Right-click on the most recent data file (bottom of the list), and save it (using the existing filename) in the folder DATA > OCEAN
9.  In the same way, find and download the most recent JASON 1 and 2 files in the same folder

NOTE:  The GlobWave files represent satellite overpasses (ascending or descending) or sometimes only parts of overpasses.  Real operational studies should take this into account by downloading multiple files and combining them.  For purposes of brevity here, we are using only the most recent files.
10.  There has been a problem with the format of the SAR files, so ask your instructor if you should even try to use any files from that source.
11.  Open IDV.
12.  Click twice on DATA to get the dashboard to open at the DATA CHOOSERS tab.

Select DATA SOURCE TYPE = NETCDF/GEMPAK POINT DATA FILES.

Then navigate to the ENVISAT folder and select the file you just downloaded.

Then click ADD SOURCE..
13.  Select FIELDS > POINT DATA and DISPLAYS > POINT DATA PLOT.
14.  In the lower right corner, you can see a REGION selection option.  Usually we'd be interested in this, but we don't yet know where this particular overpass was located, and we run the real risk of having no data in view if we specify a region now.  So, for these types of overpass point data, let's skip the REGION function for now.
15.  In the lower right corner, select LAYOUT MODEL.  This is a recipe for how IDV displays all the different types of data it can handle.

You can see that NONE is the current choice.  Click on the double down arrows to open the menu of available layout.
16.  Here is the list of layouts.  You can pick one and use it directly, or modify it and save it with a new name.

Select LOCATIONS - CIRCLES

Then click on CREATE DISPLAY at the bottom of the dashboard.
17.   Answer this question NO.
18.  On the LAYOUT tab, set the DECLUTTER DENSITY control to mid-range.
19.  On the TIMES tab, set the SHOW control to MULTIPLE.
20.  Please also note these items:
  • PLOT - Lets you plot graphs of the data, such as time series
  • FILTERS - Lets you choose a subset of the data, according to various boolean criteria.  We'll come back to this function below.
21.  Click on the IDV map window to see your new data map.  It looks promising, but the color palette needs fixing.

NOTE:  You might have to use PROJECTIONS > PREDEFINED > WORLD  to get this view of the data.
22.  Just for for your information, at this point you could reduce the map to a REGION.  On the dashboard, select EDIT > PROPERTIES
23.  And one of the available tabs is SPATIAL SUBSET, where you could enter coordinates if you want to.  We will not use this now.  You can close this window by clicking CANCEL.

NOTE:  To make the spatial subset function work, you must click on the map (anywhere) to initiate the algorithm.
24.  Select EDIT > LAYOUT MODEL.
25.  This is the layout editor, a very flexible function in IDV for defining the ways the maps and graphs are drawn.  The symbol showing here is the LOCATIONS - CIRCLES layout IDV has been using as the default layout for our data.
26.  Right-click on the example symbol, and select PROPERTIES.
27.  Select the COLOR BY tab, then navigate to MAP VALUE OF > CURRENT FIELDS > POINT DATA > GROUP #2 > SWH (= surface wave height).

You can go through all the other tabs and available goodies on your own later.
28.  As soon as you pick SWH for the variable, then IDV attempts to make other choices to help you.  You must change the data range to something like 0-5 and change the units to METERS.

Later, you can explore other ranges on your own.

Then click on APPLY.
29.  And here is the wave height figure.  Notice that there are values over land!  Sometimes satellite datasets include measurements where you do not expect them.  We'll eliminate these spurious values below.
30.  Although you can clearly see in Panel 27 above that the datafile includes many other fields beside the SWH (sea wave height), none of these was read into memory when the figure in Panel 29 was created, except the SWH itself.  To access and use the SWH_QUALITY field, we must take a minute to read that into memory also, as you'll see in the next few steps.
31.  You are currently viewing a POINT DATA PLOT, selected in the DISPLAYS window.
32.  Change the selection to POINT DATA LIST, and then click CREATE DISPLAY.
33.  This list of the data appears, showing that only DATE/TIME, LAT, LON and ALTITUDE are really in memory now (probably also SEA WAVE HEIGHT but that is assumed).

Click on SELECT FIELDS.
34.  Select SWH and SWH_QUALITY and move them to the right side with the ADD> tool.

Then click OK.
35.  Now you see these fields in the table.  Click on the FIELD SELECTOR tab at the top of the page.
36.   This is what you should be seeing.  There is only one dataset under POINT DATA on the right, the one we've been working with.

Do not click on CREATE DISPLAY, or it will add an unnecessary data object to the map.

Click on the label of the object in the right-hand menu, in this case NETCDF POINT DATA - POINT DATA PLOT.
37.  This properties window for the point data plot opens.  It includes the FILTERS control that we can use now.

Click on FILTERS.
38.  This window opens, showing up to 5 filters that you can create and apply to the data.
  • The first column contains drop-down menus of the available field; examine them
  • The second column contains drop-down menus of various Boolean operators; examine them
  • In the third column you can enter data values or strings to complete the Boolean filters
39.  At this point you should check the GlobWave Product User Guide (link above) to see what values the SWH_QUALITY can have, and which values you want to discriminate against (i.e. not plot).  In it, you'll find that values of 2 indicate non-valid data.
40.  Set up the Boolean filter this way.  It means that the SWH_QUALITY MUST NOT EQUAL 2.

Then click FILTERS ENABLED.
41.  Immediately, all values on land disappear.  You can use this method to "clean up" all GlobWave datasets.
42.  Now that we've solved the points-on-land problem, it's time for some usual housekeeping.
43.  This new layout is working pretty well, so let's save it for future use.  In the layout editor (TOOLS > LAYOUT MODEL EDITOR), select FILE > SAVE AS.
44.  Save the layout with an appropriate name.  Click OK to complete the job.
45.  With overpass data like these, it's best to combine more than one file to see the global pattern.    Here are all 3 of the datasets from the altimeters (see Panel 1 above), before filtering.   Just be sure that you select the new layout GLOBEWAVE SURF. WAVE HT. PTS. to display each one.

NOTE:  You can use the key combination SHIFT-Left Click to drag a zoom box around the main data area for closer viewing.
46.  This exercise not only introduced the great GlobWave products, but also showed you how to use NetCDF point data files.  If you wish, you can use FILE > SAVE AS to save this collection of products as an IDVX "bundle".

 

Earlier versions of his material were in the public domain, but this is no longer true.  Due to unattributed "borrowing" of text, I am now asserting my copyright and deny permission to copy any text or figures found here without my express approval in writing ------ Murray Brown m.brown.nsb <at> gmail.com