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 > 9. Operational/Synoptic > 9.21 MODIS Images

9.21 Visualizing Satellite Images in IDV, Google Earth & SAGA:  MODIS Subsets

1.  Open the EOSDIS Rapid Response (RR) website, and browse the various product lines that are provided here.
2.  Select the MODIS SUBSETS.  From here you can to to programmatic collections ("Projects") or all of the program images arranged geographically ("Areas").

Select AREAS > GLOBAL

NOTE:  On your own time, check into all 6 of the PROJECTS to see more than we look at below.  There's a ton of great data here, and it's up to you to find it.

3.  Look over these offerings, and see if your area of interest is included.

NOTE:  This is only one of many hundreds of data sites that provide imagery, so don't be discouraged if you don't see your personal area outlined.  It's just your job to find suitable source sites.

4.  Find the Foreign Agricultural Service (FAS) images, and click on a tile that includes part of Liberia (X in this figure).
5.  Both MODIS Terra and MODIS Aqua images are available, at multiple resolutions (2km, 1km, 500m, 250m).
  • 721 - Means the image is composed from digital channels 7, 2 and 1
  • 367 - Image from channels 3, 6 and 7
  • NDVI - Normalized difference vegetation index (see above)

For an example, select the 250-m version of the image indicated with the X (a 721 image from MODIS Terra)

6.  Here is that image, a JPG file, showing moderate cloud interference.
7.  But look above the image, and you'll find many different data options. 

NOTE:  The VECTOR options look interesting, but if you add coasts or borders, they appear to be very crude and/or only roughly matched with the images.

8.  In the panels below, we'll explore the six items at the bottom of the list.
9.  Here is the result of the DISPLAY METADATA option.  One interesting item is that the "projection" is "Plate Carree," which means no projection at all...the data are pure latitude and longitude.
10.  Here is the result of the DISPLAY WORLDFILE option.  This is the standard format of all world files, no matter what graphic format.  Read about world files at the Auxiliary Formats article cited above.
11.  Here is the result of the DISPLAY PROJECTION FILE.  These files are usually one long line of text, but here it has been parsed into sections for ease of inspection.
GEOGCS
    ["GCS_WGS_1984",DATUM
        ["D_WGS_1984",SPHEROID
            ["WGS_1984",6378137,298.257223563]],
    PRIMEM["Greenwich",0],
    UNIT["Degree",0.017453292519943295]]
12.  Here are the files you get if you select the DOWNLOAD JPG IMAGE WITH ANCILLARY FILES option:
  • NAfrica_3_02.yyyyddd.terra.721.250m.jpg - The JPG image file
  • NAfrica_3_02.yyyyddd.terra.721.250m.jgw - Worldfile for the JPG image; same as above worldfile
  • NAfrica_3_02.yyyyddd.terra.721.250m.jpg.aux.xml - XML version of the projection file, for use with the JPG image
  • NAfrica_3_02.yyyyddd.terra.721.250m.prj - Same as above projection file
13.  The JPG file has a worldfile, so of course you can load it into any GIS program, where it will be georeferenced.

Here you can see how we can load it into Saga, using the MODULES > IMPORT/EXPORT-IMAGES > IMPORT IMAGE function.  Just click OK after you make these settings.

14.  Here's the georeferenced image in Saga.  Move your cursor over it to see that the X and Y coordinates look OK for this area.
15.  Here is the AUX.XML file.  It is an XML version of the projection file above. 

AUTHOR'S PERSONAL OBSERVATION: The official definition for the PAMDataset tag is so hairy nobody can possibly understand it.

<PAMDataset>
    <SRS> insert GEOGCS projection material from above panel
    </SRS>
</PAMDataset>

16.  Select DOWNLOAD KMZ FILE FOR GOOGLE EARTH.  Navigate to the folder DATA > OCEAN > MODIS_IMGS and save the KMZ file with the name provided by the server.
17.  KMZ files are actually just images (usually PNG) and KML auxiliary files, zipped together, with the extension ZIP changed to KMZ so that Google Earth recognizes them.

Here is the KML file inside the KMZ file.  You can see that it contains lots of location and visual presentation information for the PNG file.  The image itself is cited in the single location marked here in yello.

<?xml version="1.0" encoding="UTF-8"?>

<kml xmlns="http://earth.google.com/kml/2.1">

<Document>

<name>NAfrica_3_02.2013314.terra.721.250m</name>

<LookAt id="khLookAtNAfrica_3_02.2013314.terra">

<longitude> -15.50</longitude>

<latitude> 5.69</latitude>

<altitude>0.00</altitude>

<range>888626.0</range>

<tilt> 0.0</tilt>

<heading> 0.0</heading>

</LookAt>

<Style id="caption">

<BalloonStyle>

<text>$[description]</text>

</BalloonStyle>

</Style>

<Snippet></Snippet>

<description><![CDATA[

]]>

</description>

<GroundOverlay>

<name>NAfrica_3_02.2013314.terra.721.250m</name>

<Icon>

<href>NAfrica_3_02.2013314.terra.721.250m.jpg</href>

</Icon>

<LatLonBox id="khLatLonBoxNAfrica_3_02.2013314.terra">

<north>12.209400</north>

<south>-0.830700</south>

<east>-10.382300</east>

<west>-20.623800</west>

<rotation> 0.00</rotation>

</LatLonBox>

</GroundOverlay>

<ScreenOverlay>

<name>MODIS Rapid Response, NASA Goddard Space Flight Center</name>

<overlayXY x="0" y="1" xunits="fraction" yunits="fraction"/>

<screenXY x="0" y="1" xunits="fraction" yunits="fraction"/>

<rotationXY x="0" y="0" xunits="fraction" yunits="fraction"/>

<size x="0" y="0" xunits="fraction" yunits="fraction"/>

</ScreenOverlay>

</Document>

</kml>

18.  Double-click on the KMZ file to open it in Google Earth (the easiest way to do that).  Here it is placed over the background "Blue Marble" default globe.
19. But can we use the image in IDV?  Just open IDV and we'll find out.
20.  In the dashboard, select DATA CHOOSERS > GENERAL > FILES

Then select DATA SOURCE TYPE = GOOGLE EARTH KMZ

Then navigate to the saved KMZ file and click on ADD SOURCE.

21.  After the data source appears in FIELDS, select DISPLAYS > IMAGERY > 3 COLOR (RGB) IMAGE and select CREATE DISPLAY.
22.  And here is the display in IDV.  All of the additional layers (coastline, relief, etc) seen in the Google Earth image above, can be added by you, if desired.
23.  To save your work, select FILE > SAVE AS and navigate to the folder PRODUCTS > IDV and use a filename like image_yyyymmdd_nliberia_modis_xxxx_250m_721.xidv
24.  Now the only option left (from Panel 7) is DOWNLOAD GEOTIFF FILE.  GeoTIFF files are TIF images with the georeferencing tags placed inside the file; there is no world file, although this is also an option for TIF images.

NOTE:  The spelling jumps between TIF and TIFF or GeoTIFF, without any difference in meaning.  The original spelling was TIFF for all versions, when the specification was first published.

25.  Download the GeoTIFF file to DATA > OCEAN > MODIS and open it in GEOTIFF EXAMINER.  This is a nice little utility program that tells you if a TIFF file already has georeferencing tags inside.  If so, then they appear on the left side, as you see here.

You have two choices:

  • Just use the existing internal tags, as most GIS programs will recognize and honor them
  • Click the right-pointing arrow and create a new TFW worldfile for the TIFF file

In the next panels we'll use the first method.

26.  Here in SAGA we've selected MODULES > IMPORT/EXPORT GDAL/OGR > IMPORT RASTER and made these choices.

NOTE:  This is a different module than above.

Then click OK.

27.  For this image, select LOAD ALL BANDS

NOTE:  Other TIFF images may not use this choice; do some experiments to get the best result for your own file.

Then click OK.

28.  You may need to make these choices (or similar settings...do some experiments) to get the image to look best.

Then click SETTINGS > APPLY at the bottom left.

29.  And here you can see the image in SAGA.  Because this has been loaded with a GeoTIFF raster, it is not as clean looking as the JPG above.

"The RGB Overlay color's method is just a fast and simple way to combine single bands to an RGB composite. For full control you can use the "Grid-Visualisation / RGB Composite" module."  [Note from Volker Wichmann at Saga]

30.  Now you can save the images (JPG or TIF) with the SAVE IMAGE function on the MAPS > THUMBNAILS tab of Saga.  In either case, navigate to PRODUCTS > SAGA  and use a filename like  image_yyyymmdd_nliberia_modis_xxxx_250m_721.xxx with the usual auxiliary file objects (world file, KML file, etc.).