Satellite data are grouped by region and by year. For most regions there are at least two types of measurements: sea surface temperature (SST) and surface Chl-a concentration (Chl). Other variables, such as Kd490 (diffuse attenuation of downwelling light at 490 nm), remote sensing reflectances at various wavelengths (e.g. Rrs443, Rrs490, Rrs555, etc) may be added. Under each year these data are composited over different time intervals: day, 5-day, 15-day, month.
If possible, similar data from multiple sensors are merged in order to increase coverage (reduce missing data due to clouds).
The files are identified by sensor, variable and compositing interval. The first letter of the file identifies the sensor or variable (for merged data) and means the following:
The full-resolution (1-km) datasets with numerical values are in HDF4 format. Reduced and annotated datasets are in both HDF4 and PNG. Attributes within HDF files provide information about the data. HDF files can be read with a number of programs.
The Scotia Sea region full image is 1600 pixels wide and 989 pixels high.
The latitudes and longitudes corresponding to each pixel can be obtained form HDF4 file http://spg.ucsd.edu/Satellite_Data/ScotiaSea/scotia_lamcon_1600x989_Latitude_Longitude.hdf
The products on this website are simple averages of all valid data from all available sensors: for SST that means MODIST and MODISA, for Chl-a that includes sensors like SeaWiFS, MODIST, MODISA, MERIS, VIIRS. Not all sensors are available for all the time.
More advanced merging of
Chl-a data is described in See Kahru, M., R.M. Kudela, M. Manzano-Sarabia and
B. G. Mitchell (2012), Trends in the surface chlorophyll of the California
Current: Merging data from multiple ocean color satellites, Deep-Sea Research.
II, 77-80, 89-98, http://dx.doi.org/10.1016/j.dsr2.2012.04.007
Both SST and Chl values in the HDF files use 1 byte per pixel with specific scaling. Linear scaling is used for SST and logarithmic scaling for Chl. The scaling equations using pixel value (PV) as unsigned byte (from 0 to 255) are:
SST (deg C) = 0.15 * PV - 3.0
Chl (mg m-3) = 10^(0.015 * PV - 2.0), i.e. 10 to the power of 0.015 * PV - 2.0.
Pixel values 0 and 255 (and the corresponding scaled values) are considered invalid and must be excluded in any statistics.
When reading with Matlab the unsigned byte variable is reported as signed byte (int8) and values over 128 become negative. A simple fix is to add 256 if the signed pixel value is negative.
Some products have a copy in KMZ file format. Clicking (double-clicking) on these will open Google Earth (if installed) and load the image into Google Earth for convenient navigation.
Note that all data are served through http and not ftp interface. For downloading many files in a batch mode you can use the wget utility that is either installed by default or available for free for most operating systems. Type wget --help to get all the options. There are way too many for me to grasp but you can try to use the following examples or try to understand wget on your own.
The following example downloads all *.png files from folder California_Current/1978/C1978_chl_month/:
wget -r -l1 -nd --no-parent -A.png http://spg.ucsd.edu/Satellite_Data/California_Current/1978/C1978_chl_month/
The following example
downloads all *.hdf files from folder California_Current/1978/C1978_chl_month/:
wget -r -l1 -nd --no-parent -A.hdf http://spg.ucsd.edu/Satellite_Data/California_Current/1978/C1978_chl_month/
For more information (not about wget!) please send email to mkahru-AT-ucsd.edu (replace -AT- with @).