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How to Write EPIC Data Files

EPIC is a system for management, display and analysis of oceanographic in-situ data. If your data is written into EPIC format, then EPIC programs can be used to diaplay and analyze your data, or presesnt it on the Web. Please read the EPIC System Overview and other EPIC documentations for a basic understanding of the architecture of the EPIC system before proceeding. The EPIC system supports multiple data file formats, but the recommended format is netCDF. Many characteristics of EPIC netCDF files are general and apply to all the types of data stored in EPIC, whereas others are specific to the type of data stored in the data file. Supported data types include:

Profile data (i.e., CTD, XBT, Bottle data)

Profile data is stored with a single profile in a single datafile. This is a 1-dimensional data, indexed by (generally) pressure or depth. Since profile data is taken at a single point, the time, latitude, and longitude axes each consist of a single value, whereas the depth/pressure axis will contain many points. The profile data file includes all variables measured or calculated for the profile. An example is a CTD data file with measured variables of pressure, temperature, conductivity and oxygen and calculated variables of salinity, density, potential temperature, and potential density.

Time series data is stored with a single 1-dimensional time series in a single datafile. This 1-dimensional data is indexed by time. Since time series is taken at a single point, the depth, latitude, and longitude axes each consist of a single value, whereas the time axis will contain many points. The time seies data file includes all variables measured or calculated by the instrument. An example is a current meter data file with measured variables of zonal and meridianal velocity and calculated variables of speed and direction.

Track data (i.e., shipboard ADCP data, or other data collected along the track of a moving platform)

Shipboard ADCP data (in the scope of this project) consists of processed acoustic data collected along the shiptrack. The shipboard instrumentation records average measurements, derived from individual acoustic ping "events". The time interval over which an "ensemble" of individual pings are processed is fixed. Current velocities (and some other measurements) are calculated for some number of depth bins during each ensemble interval, representative of the time between the start and end of the interval, and therefore of the section that spans the ship's initial and final positions. Thus, the resulting data is a mix of 1-D fields spanned by time (e.g. position of one ensemble) and 2-D fields spanned by time and depth (e.g. velocities). This structure needs to be reflected by the netCDF data files. Please refer to this commented frame of a shipboard ADCP file for detailed information. The original netCDF file can be found here. For further information look on PMEL's anonymous ftp server: ftp://ftp.epic.noaa.gov/epic/adcp/.

Steps to write your data file

1. Assemble your data, and decide which data type it is (profile, time series or track).

2. Decide which variable codes match the variables or parameters in your data file. See Variables and the EPIC key file.

3. Look at the Attributes to see which must be used to include required metadata in your data file, and which you may choose to use, both general, and those specific to your data type.

4. Look at the example programs for writing EPIC data files. Select one to use as a template, and customize it for your own use.

5. Obtain the EPIC I/O system library (epslib) and install it. Compile, link and run your program.

6. Refer to the manual for the EPIC System I/O Library (epslib), as needed.

7. If you have further questions, please contact the EPIC team at epic@noaa.gov.