Visualizing Satellite Images in IDV & SAGA: MODIS Subsets
Abstract: In this
exercise you will be introduced to one method and source to obtain very
recent (within hours!) satellite visual-channel imagery. The website
below includes many views of the earth, but is not universal, so users
must do their own research about possible expansion of the products here
(actually offered in the documentation) or about other websites containing
Preliminary Reading (in
OceanTeacher, unless otherwise indicated):
|1. Open the EOSDIS Rapid
Response (RR) website, and browse the various product lines that are
|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
|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.
|12. Here are the
files you get if you select the DOWNLOAD JPG IMAGE WITH ANCILLARY FILES
- 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
|13. The JPG file has a
worldfile, so of course you can load it into any GIS program, where it will
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.
<SRS> insert GEOGCS projection material from above panel
|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.
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.
Response, NASA Goddard Space Flight Center</name>
y="1" xunits="fraction" yunits="fraction"/>
y="1" xunits="fraction" yunits="fraction"/>
y="0" xunits="fraction" yunits="fraction"/>
<size x="0" y="0"
|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
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
|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
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.
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.).