The Ocean Colour Database (OCDB)

This database consists of processed Sea-viewing Wide Field-of-view Sensor (SeaWiFS) measurements of chlorophyll. SeaWiFS is the name of the sensor on board the SeaStar polar-orbiting satellite (also called Orbview-2). The SeaWiFS data are obtained from GeoEye (formerly Orbimage, the company which owns the SeaStar) via NASA.

There are two SeaWiFS chlorophyll products maintained in the database:

  • SeaWiFS Local Area Coverage (LAC) (1 km x 1 km, Local Area Coverage) 1.5 x 1.5 km semi-monthly composites for the North Atlantic from the second half of September 1997 to December 23, 2004.
  • SeaWiFS Global Area Coverage (GAC) (4 km x 4 km, Global Area Coverage) 4 km x 4 km weekly composites for the North Atlantic from the second half of September 1997 to the present.

For a description of SeaWiFS, see http://oceancolor.gsfc.nasa.gov/SeaWiFS/.

SeaWiFS Local Area Coverage (LAC) Processing Overview

These data represent semi-monthly composites of SeaWiFS chlorophyll (chl_oc4) estimates for the North Atlantic (39N - 62.5N, 42W to 71W), from the second half of September 1997 to December 23, 2004. The raw SeaWiFS LAC data were captured at a nominal resolution of 1 km (at nadir) through the High Resolution Picture Transmission (HRPT) satellite dish operated by the Biological Oceanography section of Bedford Institute of Oceanography. Under the contract with the National Aeronautics and Space Administration (NASA), the data were received free of charge in encrypted format and were subject to a two-week embargo. After each two-week period, a decryption key was made available for the purpose of decrypting the data so they could be processed.

The data were processed using the SeaWiFS Data Analysis System (SeaDAS, available for download from NASA) version 4.5 and 4.8. The difference between these two versions is not in the chlorophyll algorithm itself. The processed chlorophyll (chl_oc4) data were then remapped at a 1.5 km x 1.5 km resolution to the standard region using a Mercator projection. This slightly coarser resolution had been chosen for disk space considerations. Composites were created using SeaDAS/Interface definition language (IDL), by taking the average of all valid chlorophyll values for a given latitude/longitude grid point from all remapped files corresponding to the given semi-monthly period.

Unfortunately, the contract with NASA expired on December 24, 2004, so we were no longer able to receive these high resolution data free of charge. Since that time, these high resolution SeaWiFS data can still be received under a restricted contract with GeoEye, but it is not legal to make them available to the public.

SeaWiFS GAC (Global Area Coverage) Processing Overview:

These data represent weekly composites of SeaWiFS chlorophyll estimates for the North Atlantic (39N - 62.5N, 42W to 71W), from the second half of September 1997 to the present. Since the end of 2004, the low resolution (4 x 4 km) SeaWiFS data are available for download from NASA free of charge, because NASA has purchased these data from GeoEye. The SeaWiFS GAC data represent a systematic subsampling of one out of every four pixels of the original high resolution (1 km * 1 km) SeaWiFS LAC data. These data are commonly referred to as SeaWiFS GAC, because they are available for the entire globe, rather than being confined to regions surrounding HRPT receivers.

This product was processed at the Bedford Institute of Oceanography using SeaDAS (available for download from NASA) version 5.05. For detailed information about the algorithm changes (from previous versions of SeaDAS), see http://oceancolor.gsfc.nasa.gov/REPROCESSING/SeaWiFS/R5/. The processed chlorophyll (chlor_a) data were then remapped to the standard region using a Mercator projection, while retaining the 4 km * 4 km resolution. Composites were created using SeaDAS/IDL, by taking the average of all valid chlorophyll values for a given lat/lon grid point from all remapped files corresponding to the given weekly period.

The Ocean Colour Database (OCDB) Application

This application extracts information from the Ocean Sciences OCDB database according to user specified spatial and temporal criteria. Output results can be either statistical summaries of the data or the actual data stored in the database (for input into your own analyses). The query is performed off-line. You will be contacted by email when your results become available. Results should normally be available within a few hours (depending upon the size of your query and the number of requests ahead of you). If you haven't had a reply within 24 hours, contact us and we will try to determine what happened.

A Brief Tour

(A) Query Identification

All of your queries are assigned a unique query number and saved under your username. You can re-run existing queries or edit them and submit them as new queries. You can assign a name under TITLE. This name is saved internally in the results files for your reference.

(B) Area Selection Type

You may define a geographical area in one of three ways;

  • Choose from a list of pre-defined polygons (multiple selections are permitted).
  • Provide your own polygon definition by latitude/longitude co-ordinates. To do this, select the Define Area button at the top of your screen, follow the instructions to create a new polygon, and then select it from the list of predefined polygons.
  • Define a rectangle by latitude/longitude coordinates. The blocks parameter permits you to subdivide the entire rectangle into x by y blocks. For example, specifying latitude from 42° to 45° and longitude from -62° to -65° with 1° blocks in both latitude and longitude would result in defining nine separate 1° grid squares for which statistics would be generated.

Note that the convention for longitude is positive East. Latitude and longitude must be specified as decimal degrees, but we provide a converter if you prefer degrees, minutes, seconds.

(C) Time Specification

Specify a continuous time period from (month/day/year) to (month/day/year) and/or a seasonal window. Months are inclusive. Months 6 to 9 means June to September. Months 12 to 2 would select only December to February. Default is all months for the entire time period for which there are data.

(D) Chlorophyll Filter

SeaWifs can contain very high chlorophyll values even though they have passed the quality tests. This option permits the user to set a range of acceptable values. Any value outside this range is excluded from any further computations.

(E) Data Source

Select the desired data source.

(F) Result Information Set

The result set files returned to you depend upon what options you request; The standard deviation is calculated using the "nonbiased" or "n-1" method using the following formula:

s = ( 1 n - 1 i = 1 n ( x i - x ¯ ) 2 ) 1 2

Seasonal Cycle, Time Series Will get both the Times series of monthly statistics (average, minimum, maximum and count of observations for each year and month and depth level for which there are data) and Seasonal Cycle (average, minimum. maximum, standard deviation, count of observations and count of months in average). The values are determined by an un-weighted average over all months from the time series statistics.

Complete Profile Data extracts every value of chlorophyll referenced to latitude, longitude and date. These files can be very large. Many people request it simply because they can. Make sure it is what you really require before requesting it. There are also some file size restrictions. See Caveats.

The complete result set can consist of up to four ascii text files. All files are of the form qry_xx.txt, where xx is your unique query identifier. Files are comma delimited with a header label for easy import into a spreadsheet or database application. (see detailed file description)

(G) Run

After the query specification is complete, clicking the Run button will submit the query.