Product Levels

The Ocean Biology Processing Group collects and processes data from Earth-viewing satellites. These data are organized in various ways reflecting different spatial, temporal, and parameter groupings. Over the years, certain terminology has arisen to describe these organizational conventions. Here follow some explanations of these terms.

Level 0

Level 0 data are unprocessed instrument/payload data at full resolution. Any artifacts of the communication (e.g. synchronization frames, communication headers) of these data from the spacecraft to the ground station have been removed. These data are the most raw format available, and are only provided for a few of the missions that we support.

Level 1A

Level 1A data are reconstructed, unprocessed instrument data at full resolution, time-referenced and annotated with ancillary information including radiometric and geometric calibration coefficients and georeferencing parameters (e.g. platform ephemeris data) computed and appended but not applied to the Level 0 data. It is this last point that makes Level 1A the preferred archival data level. If the sensor calibration changes, the data do not need to be reacquired — no mean feat where large data sets are concerned. The same is not true for Level 1B data (described below) which must be replaced every time the sensor calibration changes.

Level 1B

Level 1B data are Level 1A data that have had instrument/radiometric calibrations applied.

Level 2

Level 2 data consist of derived (by the l2gen program) geophysical variables at the same resolution as the source Level 1 data. These variables are grouped into a few product suites (e.g. OC [ocean color], SST, and SST4 for MODIS).

Level 3

Level 3 data are derived geophysical variables that have been aggregated/projected onto a well-defined spatial grid over a well-defined time period. We archive two types of Level 3 data.

Binned

Each Level 3 binned data product consists of the accumulated data for all L2 products in a product suite, for the specified instrument and resolution, corresponding to a period of time (e.g. daily, 8 days, monthly, etc.) and stored in a global, nearly equal-area, integerized sinusoidal grid.

Mapped

The Level 3 Standard Mapped Image (SMI) products are created from the corresponding Level 3 binned products. Each SMI file contains a Plate Carrée, pixel-registered grid of floating-point values (or scaled integer representations of the values) for a single geophysical parameter. A color look-up table is also provided in each file that may be used to generate an image from the data.

Level 4

Level 4 data are model output or results from analyses of lower level data (e.g., variables derived from multiple measurements). Ocean Primary Productivity is a good example of a Level 4 product.

Product Suites

The numerous products computed from the satellite data that we collect often differ with respect to the flagging or masking criteria used to characterize the quality of each retrieved pixel value. We have found it useful to separate products with differing flagging or masking needs into separate files whose names include a string identifying which product suite the included parameters belong to. At Level-2, this categorization currently separates all the ocean color ("OC") parameters, which share an l2_flags data set, from sea surface temperature products ("SST" and "SST4"), which are classified with quality levels stored in a qual_sst data set.

Further division of the Level-2 products into additional product suites occurs during Level-3 binning of the Level-2 products. Again, this is driven by different masking requirements of the different products during their aggregation into Level-3. Example product suites at Level-3 include: RRS, CHL, KD490, PAR, PIC, POC, FLH, SST, SST4, and NSST.

A detailed description of each product suite is accessible via links provided on the Algorithm Descriptions page.

Format Documents

Note that as part of Reprocessing 2014.0 the file formats were changed from the HDF4 mentioned in the documents below to NetCDF4.

• Level 0
  • MODIS Production Data Set (PDS) format

    We do not have a concise format description for the Level-0 files in our archive, but perhaps this interface control document from NASA's Earth Observing System Data and Operations System can be mined for scraps of information.

• Level 1A
  • CZCS Level 1A format
  • SeaWiFS Level 1A format
  • MODIS Level 1A format
• Level 2
  • Ocean Level 2 format (HDF4)
  • Ocean Level 2 format (netCDF4)
• Level 3
  • Ocean Level 3 Binned data Products Format
  • Ocean Level 3 Standard Mapped Image Products Document