IBM Visualization Data Explorer User's Guide

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B.3 CDF Files

CDF is a data abstraction for self-describing multidimensional Arrays. It represents a simpler data model than that of Data Explorer, one similar to that of the Array Object. Data are accessed in CDF through an applications programming interface, available as C and FORTRAN libraries from the National Space Science Data Center, NASA/Goddard Space Flight Center, Greenbelt, MD. Data in CDF may be stored in a number of physical formats (e.g., native or portable binary, single or multiple files, row or column majority), but the interface is the same. Hence, data in a CDF written in a format "foreign" to the workstation on which Data Explorer is running are converted automatically during the Import process.

Data Explorer provides support for importing Fields stored as CDF r-variables. To import data from a CDF, specify the CDF name as the name parameter in the Import Configuration dialog box (not the file name, since the CDF may be in multiple-file format). If the CDF has more than one variable, which is typical, Data Explorer categorizes each variable as positions, series, or data as appropriate. Variables that vary in one dimension only and are not record-variant are considered positions, and become the positions component in a Field Object. In many cases, these variables may have the CDF variable mnemonics of LATITUDE and LONGITUD, which are mapped to the first (x) and second (y) components of the positions vector, if they exist. This mapping permits direct use of these data with cartographic and other tools for the earth and space sciences that are publicly available for use with Data Explorer. Otherwise, the first n variables categorized as positions (where n is the dimensionality of the CDF dimensions) are used to form the positions component. Any additional such variables are treated as data variables. If there are no positions-type variables, the positions component will be a regular grid with origin of 0 and increments of 1 in each axis, where the number of axes corresponds to the dimensionality of the imported CDF r-variable.

If there are records in the CDF, each record is imported as a series member. In many cases there is a variable with the mnemonic EPOCH, which corresponds to a time stamp for each record in the CDF. If so, the double representing msec since 0 AD in each value of EPOCH is stored as the series position attribute. If not, the first variable that is record-variant and nondimensional-variant is considered the series variable. This variable is imported as the series position attribute. If there is no time variable, the series position starts at 1 and increments by 1 per series member, so that there is one member for each record in the CDF. The series position attribute, containing the time stamp, may be accessed with the Attribute module.

You can specify the name or names of the data variable in the variable parameter of the Import tool and the corresponding variable(s) will be imported. In the same way, you can use start, end, and delta to import a subset of CDF records.

Variable and global attributes present in the CDF are imported as Object attributes. These attributes may be accessed through the Attribute and Inquire modules (e.g., to build metadata-driven applications).

Variables that vary in all dimensions and are record-variant are considered data variables. Any variable that is not a position or time variable is also considered a data variable, allowing every variable to be imported. If you want the positions to be a variable other than the one chosen by Data Explorer, you can use Replace or Rename to switch the components (e.g., two or more sets of positions information are stored for different coordinate systems). Each data variable becomes a data component in a Field Object. Hence, there is one Field for each data variable in the Group imported. Since Data Explorer can handle data more flexibly than CDF, some assumptions are imposed upon certain classes of data that may be imported:


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