Geoscience Reference
In-Depth Information
N stands for nadir from the Arabic word for opposite ), as it does for all other
channels, and a second time with the sensor angled backwards at 27.6° (band
3B, where B stands for backward looking). h ese two bands are used to
generate ASTER digital elevation models (DEMs).
h e ASTER instrument produces two types of data: Level-1A (L1A) and
Level-1B (L1B) data (Abrams and Hook 2002). Whereas the L1A data are
reconstructed, unprocessed instrument data, the L1B data are radiometrically
and geometrically corrected. Any data that ASTER has already acquired are
available; they can be located by searching the Japan Space Systems GDS
ASTER/PALSAR Unii ed Search Site and can be ordered from
http://gds.ersdac.jspacesystems.or.jp/?lang=en
or from NASA Reverb
http://reverb.echo.nasa.gov/reverb/
As an example we process an image from an area in Kenya showing Lake
Naivasha (0°46'31.38"S 36°22'17.31"E). h e Level-1A data are stored in two
i les
AST_L1A_003_03082003080706_03242003202838.hdf
AST_L1A_003_03082003080706_03242003202838.hdf.met
h e i rst i le (116 MB) contains the actual raw data, whereas the second i le
(102 KB) contains the header, together with all sorts of information about
the data. We save both i les in our working directory. Since the i le name
is very long, we i rst save it in the filename variable and then use filename
instead of the long i le name. We then need to modify only this single line of
MATLAB code if we want to import and process other satellite images.
filename = 'AST_L1A_003_03082003080706_03242003202838.hdf';
h e Image Processing Toolbox contains various tools for importing and
processing i les stored in the hierarchical data format (HDF). h e graphical
user interface (GUI) based import tool for importing certain parts of the raw
data is
hdftool('filename')
h is command opens a GUI that allows us to browse the content of the
HDF-i le naivasha.hdf , obtains all information on the contents, and imports
certain frequency bands of the satellite image. Alternatively, the command
hdfread can be used as a quicker way of accessing image data. h e vnir_
Band3n , vnir_Band2 , and vnir_Band1 typically contain much information
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