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11.2 The DFSD Scientific Data Set Data Model

The scientific data set, or SDS, data model supports four primary data objects: arrays, dimensions, dimension scales, and dimension attributes. As in the multifile SD SDS model, the fundamental object of the data model is the SDS array. Unlike the SD multifile SDS model the DFSD SDS model has, in addition to dimension attributes, attributes that refer to the SDS array itself.

FIGURE 11a The Contents of a Three-Dimensional DFSD Scientific Data Set

11.2.1 Required DFSD SDS Objects

The only required objects in the DFSD SDS model are the array and the number type of the array data. Without this information, the data set is inaccessible. Required objects are created by the library using the information supplied at the time the SDS is defined.

Descriptions of these objects in Chapter 3, titled Scientific Data Sets (SD API).

11.2.1.1 Dimensions

Unlimited dimensions, supported in the multifile SD SDS model, aren't supported in the single-file DFSD SDS model.

11.2.2 Optional DFSD SDS Objects

There are two types of optional objects available for inclusion in an SDS: dimension scales and attributes. Optional objects are only created when specified by the calling program.

11.2.2.1 Dimension Scales

Conceptually, adimension scale is a series of numbers placed along a dimension to demarcate intervals in a data set. They are assigned one per dimension. Structurally, each dimension scale is a one-dimensional array with size and name equal to its assigned dimension name and size.

11.2.2.2 Predefined Attributes

Predefined attributes are attributes that have reserved labels and in some cases predefined number types. They are described in Chapter 3, titled Scientific Data Sets (SD API).

11.3 The Single-File Scientific Data Set Interface

The HDF library currently contains several routines for storing scientific data sets in the HDF format. DFSDadddata, DFSDputdata, and DFSDgetdata perform data I/O and by default assume that all scientific data is uncompressed 32-bit floating-point data stored in row-major order. DFSD library routines also read and write subsets and slabs of data, set defaults, determine the number of data sets in a file, and inquire about or assigning reference numbers before reading or writing data.

11.3.1 DFSD Library Routines

The names of the C routines in the DFSD library are prefaced by "DFSD" and the names of the equivalent Fortran-77 functions are prefaced by "ds". They are categorized as follows:

DFSD library routines are more explicitly defined in Table 11A and on their respective pages in the HDF Reference Guide.

TABLE 11A DFSD Library Routines
Category

Routine Name

Description

C

Fortran-77

Write

DFSDadddata

dsadata

Appends a data set to a file.

DFSDclear

dsclear

Clears all possible set values.

DFSDputdata

dspdata

Overwrites new data to a file.

DFSDsetdims

dssdims

Sets the rank and dimension for succeeding data sets.

DFSDsetNT

dssnt

Sets the number type for the data set.

DFSDwriteref

dswref

Assigns a reference number to the next data set written.

Read

DFSDgetdata

dsgdata

Retrieves the next data set in the file.

DFSDgetdims

dsgdims

Returns the number and dimensions for the next data set.

DFSDgetNT

dsgnt

Determines the number type for the data in the data set.

DFSDlastref

dslref

Returns the reference number of last data set accessed.

DFSDndatasets

dsnum

Returns the number of data sets in a file.

DFSDpre32sdg

dsp32sd

Determines if the data set was created before HDF version 3.2.

DFSDreadref

dsrref

Locates a data set with the specified reference number.

DFSDrestart

dsfirst

Sets the location of the next access operation to be the first data set in the file.

Slabs

DFSDendslab

dssslab

Terminates a read or write slab operation.

DFSDreadslab

dsrslab

Reads a slab of data from a data set.

DFSDstartslab

dssslab

Begins a read or write slab operation.

DFSDwriteslab

dswslab

Writes a slab of data to a data set.

Data Set Attribute

DFSDgetcal

dsgcal

Retrieves the calibration information for the data se.t

DFSDgetdatalen

dsgdaln

Retrieves the length of the attributes assigned to the data.

DFSDgetdatastrs

dsgdast

Returns the label, unit, format and coordinate system for data.

DFSDgetfillvalue

dsgfill

Retrieves the fill value used to complete the data set.

DFSDgetrange

dsgrang

Retrieves the range of values for the data set.

DFSDsetcal

dsscal

Sets the calibration information for the data set.

DFSDsetdatastrs

dssdast

Sets label, unit, format and coordinate system for data.

DFSDsetfillvalue

dssfill

Sets the fill value to use when completing a data set.

DFSDsetlengths

dsslens

Sets the length for the data set and dimension attributes.

DFSDsetrange

dssrang

Sets the range of values for the data set.

Dimension Attribute

DFSDgetdimlen

dsgdiln

Retrieves the length of the attributes assigned to the dimension.

DFSDgetdimscale

dsgdisc

Returns the scale for a dimension.

DFSDgetdimstrs

dsgdist

Returns the label, unit, and format for a dimension.

DFSDsetdimscale

dssdisc

Sets the scale for a dimension.

DFSDsetdimstrs

dssdist

Sets the label, unit and format for the dimension.

11.3.2 File Identifiers in the DFSD Interface

File identifiers are handled internally by each routine and access to a file is granted simply by providing a filename. As the file identifier is handled by the function call, the calling program need not keep track of how to open and close files.



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HDF User's Guide - 06/04/97, NCSA HDF Development Group.