Working with Globcolour data

April 2, 2012
By

(This article was first published on me nugget, and kindly contributed to R-bloggers)




The Globcolour project (http://www.globcolour.info/) provides relatively easy access to ocean color remote sensing data. Data is provided at http://hermes.acri.fr/and the following parameters are available:
· Chlorophyll-a (CHL1 and CHL2)
· Fully normalised water leaving radiances at 412, 443, 490, 510, 531, 550-565, 620, 665-
670, 681 and 709 nm (Lxxx)
· Coloured dissolved and detrital organic materials absorption coefficient (CDM)
· Diffuse attenuation coefficient (Kd(490))
· Particulate back-scattering coefficient (bbp)
· Total Suspended Matter (TSM)
· Relative excess of radiance at 555 nm (EL555)
· Photosynthetic Available Radiation (PAR)
· Heated layer depth (ZHL)
· Secchi disk depth (ZSD)
· Primary production (PP)
· Aerosol optical thickness over water (T865)
· Cloud Fraction (CF)

Of particular interest to ecologists are the estimates of Chlorophyll a (chla) , which combines data from several satellites for better coverage - SeaWiFS (NASA), MODIS (NASA), MERIS (ESA).  Data is available at several temporal (daily, 8-days, and monthly averages) and spatial (4.63 km, 0.25°, and 1°) resolutions for the global domain. Several merged products are available: simple averaging (AV), weighted averaging (AVW), and Garver, Siegel, Maritorena Model (GSM) [for more information see the Product User Guide].

Due to the gappy nature of the data (e.g. due to land and clouds), many of the data products only provide values at grids where estimation was possible. For high resolution data, such as in 4.63 km resolution daily estimates, grids with values are often far fewer than the total number of ISIN grids (n=23,761,676) used by the product. This saves space in the files for download, but you may need to reconstruct the field (with NaN's included for grids without observations) for some analyses.

The following example shows how to retrieve Globcolour data and process it using R. Global data is available, but I have provided instructions for processing a smaller area of 4.63 km resolution chla data from the Galapagos archipelago. One can define lat/lon limits for the desired area on the http://hermes.acri.fr/ interface. An ftp address will be sent via Email as to the location of the data when finished.
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