developer.book.apds.cdf
From autoplot.org
Contents |
1. Introduction
CDF files are used in space physics, invented and maintained at the Goddard Space Physics Data Facility. They are a binary format which stores data in blocks of records, each parameter having a short name, with associated metadata. CDF has a mature metadata standard, and parameters can be related, such as asserting that each index of "Density" is associated with an "Epoch" time when the data was collected.
2. Using CDF Files
Point Autoplot at a CDF file, and a list of the parameters found is shown. A parameter name is selected and "Plot" will show this parameter. A little metadata about the parameter is displayed in the panel below, such as its description found in the CDF, and how many records are found. Note too that the dependencies are reflected as well, so BGSM[Epoch=1440,3] indicates that the first index changes with time. Note that when the data contains named components (e.g. BGSM has Bx, By, and Bz) then either individual components or the set of all components can be plotted.
CDF is able to store time in three ways, with Epoch (double milliseconds since 0000-00-00T00:00), Epoch16 which was used on the Cluster and Themis missions, and finally TT2000, which should be used in all new CDF files.
3. Advanced Options
Advanced options include subsetting, where only a portion of the records are loaded. Example entries here include [0:10] to load the first ten records, [::2] to load every other record, and [:-100] to load the last 100 records.
"Only Load Data Where" allows filtering based on another parameter. Sometimes a quality flag is included in the file, and this can be used to load only the records having a high-quality rating.
Initially these are the "data" parameters, and the "support data" parameters are hidden. The data parameters are what is measured (e.g. Density), and the support data is when and where it is measured (e.g. Time). The checkbox "show all" will update the list of parameters to include the support data as well.
"no dependencies" and "no ISTP" are useful when developing CDF files, where a malformed CDF file cannot be loaded (for example the number of density and time records is different). These will ignore metadata so that the raw numbers can be examined. This is also useful for odd CDF files which were created without any attempt to use CDF conventions.
4. Quirks
4.1. Single-Record Parameters may be Interpreted as Non-Record-Varying
A parameter can be identified as non-record-varying (NRV), meaning the parameter is constant for any time. So for example, suppose you have the three parameters Flux, Time, and Energy. For each record of Flux, there is a record of Time. Energy is marked as NRV, so it is understood that the one record of Energy applies to all records of Flux.
The problem is that many CDF files don't mark the parameter as non-record-varying. Autoplot must then guess what CDF author's the intent was, and the logic is that a single record is always interpreted as NRV.
5. Glossary
In an effort to establish standard names for things:
- parameter is a science quantity stored in a CDF.
- "variable" is used as an alias for parameter.
- "parameter id" is the name identifying the parameter.
