Visualizing Satellite-Measured Optical Properties and Pigments in IDV:
Abstract: In this
exercise you will learn a method to obtain recent sea surface
pigment data (chlorophyll a) and many other optical properties from the MODIS-Aqua instrument. The
method, a global data download from the Ocean Color Web, obtains very
recent data (on the order of one day old) as an image that can be imported
into IDV, but the image is only qualitative. The visual color scale
is available for rough comparisons, but the raster contains only 256-color
palette values, not chlorophyll concentrations.
Preliminary Reading (in
OceanTeacher, unless otherwise indicated):
IMPORTANT NEW INFORMATION: Recently
the Saga GIS software has been shown to deal quite adequately with most
NetCDF (NC) data grids (and possibly also GRIB files). This should be
viewed as a useful addition to the rapid, direct NC analysis and display
capabilities of IDV (see below). If you are also interested in this
route for your data, then please check
Viewing and Managing
Well-Formed NetCDF Grids in Saga.
|1. Open the Ocean Color Web
home page. This site provides a wide range of remote sensing products,
ranging from climatologies to near-real time, at Levels 1, 2 and 3.
Only L3 products can be handled with the software described here.
LEVEL 3 BROWSER.
|2. This graphic and tabular
interface opens. Look along the top row to see the current settings,
then change them, as follows:
- Standard Products
- Aqua MODIS Chlorophyll Concentration
- 4 km
- 24 Thumbnails
|2. The latest image is in the
lower-right corner of the 24 thumbnails. Right-click on the exact
middle of the image, and select SAVE LINK AS.
NOTE: You do NOT
want to save the two binary data files which can be saved by clicking on the
lower left and right corners of the thumbnail.
|3. Navigate to
your folder DATA > OCEAN and save the
image as A2011007.L3m_DAY_CHL_chlor_a_4km.png. In your own case, the
date will be different, depending on instructions from your teacher or notes
in the course.
NOTE: The date format is YYYYddd, where ddd is the
number of the day in the year. Sometimes improperly called a Julian
Day, the real name for this value is the Year Day.
OPTION: You can also change the product type from DAILY to 3 DAYS, and
save the resulting composite image as A20110052011007.L3m_3D_CHL_chlor_a_4km.png.
In your own case, the date will be different, depending on instructions from
your teacher or notes in the course.
NOTE: The date format is
YYYYdddYYYYddd, to capture the beginning and end Year Days of the composite
|5. Open any good ASCII text
editor and COPY and PASTE this text into a blank document.
It is a
template for the XIMG file format used by IDV to geo-reference plate caree
<image name="Display Title for the Image"
|6. Edit the XIMG file to look
something like this, with obvious differences for the dates of the images
you really are using.
7. Navigate to the folder DATA > OCEAN, and
save the file with the name A2011007.L3m_DAY_CHL_chlor_a_4km_png.ximg,
or a similar name, depending on the data dates you are using.
|8. Run IDV.
|9. Select EDIT > PREFERENCES
> SYSTEM, and make sure these setting are already made by default:
- DISK CACHE SIZE: 60
- DATA CACHE MEMORY %: 50
- MAX IMAGE SIZE: 1000
This last item is counterintuitive, because the default -1 appears to
place no limit on size. This item is actually the size at which IDV
begins subsampling large images to display them, so a finite value is needed
to facilitate their display.
|10. Now you can select DATA >
CHOOSE DATA > DATA CHOOSERS. Set the DATA SOURCE TYPE to XML FILES,
and navigate to the DATA > OCEAN folder
Select the XIMG file for
the single-day image, and click ADD SOURCE.
|11. You should see this set
of options under the FIELD SELECTOR tab of the dashboard.
Select COLOR (RGB)
IMAGE and click CREATE DISPLAY.
|12. This is the global image.
Notice that it does not have a scale on the right relating the colors to
chlorophyll values. There is only a "gray scale" palette, due to the
fact that this raster is not a scientific data set, but merely a
georeferenced simple image.
In most cases, the
features of interest are obvious, and self explanatory.
can use the key combination SHIFT-Left Click to drag a zoom box around the
main data area for closer viewing. AND you can use the PROJECTIONS >
PREDEFINED controls to look at specific regions. Some areas don't work
very well, though.
|13. You can refer back to the
(logarithmic) color scale on the Color Web pages for a rough estimation
|14. This is the 3-day
composite image, mentioned in Panel 4 above, showing far more area of the
sea colored. Longer time means more coverage.
To save this product for other future work, select FILE > SAVE AS then
navigate to PRODUCTS > IDV and save it with the filename