developer.cdf

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Purpose: Show how CDF Metadata appears on the Autoplot display.

Audience: Data product developers

See also: http://autoplot.org/developer.cdfguide which also talks about the CDF format.

Contents

  1. Introduction
  2. Attributes
  3. Global Attributes
  4. Where things appear on Autoplot
  5. Filename Conventions

1. Introduction

CDF is a data format created at NASA/Goddard to store data produced in space physics. Each file is like a small database, and conventionally files for each interval together form a long time series. This is intended to serve as a primer to develop these files. These files are rarely read and written directly, instead NASA provides libraries for working with these files.

For example, the CDF file data_20060101.cdf contains data collected on 2006/01/01. It contains named parameters "Magnitude" "Epoch" and BGSM. Each of these parameters contains 24 records. Magnitude is the magnitude of the symbol, and Epoch contains the time tags for each measurement. BGSM is a vector collected at the same time.

Each variable has metadata attached. For example BGSM has an attribute "CATDESC" that is a title for the data "Magnetic field vector in GSM coordinates (1 hr)" and it also has units specified in an attribute "UNITS" which is "nT." Note attributes can be any data type, for example FILLVAL is the value that marks missing data.

Metadata also identifies relationships between variables. For example BGSM and Magnitude both have DEPEND_0="Epoch" indicating they are dependent parameters on the independent parameter Epoch. LABL_PTR_1 is used with BGSM to a variable containing three string labels for each component.

2. Attributes

http://spdf.gsfc.nasa.gov/istp_guide/vattributes.html

3. Global Attributes

There are attributes for the file itself, called global attributes. Note when data is combined from files to make a time series, implicitly the Global Attributes should be the same for each file.

4. Where things appear on Autoplot

  • The source_name is derived from Global Attribute "Source_name", removing the characters after the ">". So "AC>Advanced..." becomes "AC"
  • The descriptor is derived from Global Attributes "Descriptor" the same way, so "Descriptor=MAG>ACE Magnetic Field Instrument" becomes "MAG"
  • Autoplot's plot title is derived from Attribute CATDESC. If source_name, and descriptor are available, then the title is "source_name/descriptor CATDESC." In the example, the title used is "AC/MAG Magnetic field vector in GSM coordinates (1 hr)"
  • The y-label is LABLAXIS for line plots.
  • The colorbar label, for spectrograms, is comes from LABLAXIS and the DEPEND_1 parameter's LABLAXIS is used to label the axis.
  • UNITS identify the units. Autoplot allocates new Units objects to represent new unit types. A small set of units have conversions defined as well.
  • VALIDMIN is the minimum valid value.
  • VALIDMAX is the maximum valid value.
  • SCALEMIN is a suggested axis minimum. If this does not seem consistent with data, Autoplot performs the autoranging.
  • SCALEMAX is a suggested axis maximum. If this does not seem consistent with data, Autoplot performs the autoranging.
  • DISPLAY_TYPE is either "time_series" or "spectrogram" "time_series" hints that the plot should be Autoplot's "series" mode. "spectrogram" hints that it should be "spectrogram" mode.

5. Filename Conventions

Daily files is typical. Target less than 600MB per file. For example, use 10 minute files when each file would have been 1.2G. data_$Y$M$D_$H$M_v01.cdf.

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