MATLAB®

hdfread - Read data from HDF4 or HDF-EOS file

Syntax

data = hdfread(filename, datasetname)
data = hdfread(hinfo.fieldname)
data = hdfread(...,param1,value1,param2,value2,...)
[data,map] = hdfread(...)

Description

data = hdfread(filename, datasetname) returns all the data in the data set specified by datasetname from the HDF4 or HDF-EOS file specified by filename. To determine the name of a data set in an HDF4 file, use the hdfinfo function.

Note   hdfread can be used on Version 4.x HDF files or Version 2.x HDF-EOS files. To read data from and HDF5 file, use hdf5read.

data = hdfread(hinfo.fieldname) returns all the data in the data set specified by hinfo.fieldname, where hinfo is the structure returned by the hdfinfo function and fieldname is the name of a field in the structure that relates to a particular type of data set. For example, to read an HDF scientific data set, specify the SDS field, as in hinfo.SDS. To read HDF V data, specify the Vdata field, as in hinfo.Vdata. hdfread can get the name of the HDF file from these structures.

data = hdfread(...,param1,value1,param2,value2,...) returns subsets of the data according to the specified parameter and value pairs. See the tables below to find the valid parameters and values for different types of data sets.

[data,map] = hdfread(...) returns the image data and the colormap map for an 8-bit raster image.

Subsetting Parameters

The following tables show the subsetting parameters that can be used with the hdfread function for certain types of HDF4 data. These data types are

• HDF Scientific Data (SD)

• HDF Vdata (V)

• HDF-EOS Grid Data

• HDF-EOS Point Data

• HDF-EOS Swath Data

Note the following:

• If a parameter requires multiple values, the values must be stored in a cell array. For example, the 'Index' parameter requires three values: start, stride, and edge. Enclose these values in curly braces as a cell array.

  hdfread(dataset_name, 'Index', {start,stride,edge})

• All values that are indices are 1-based.

Subsetting Parameters for HDF Scientific Data (SD) Data Sets

When you are working with HDF SD files, hdfread supports the parameters listed in this table.

For example, this code reads the data set Example SDS from the HDF file example.hdf. The 'Index' parameter specifies that hdfread start reading data at the beginning of each dimension, read until the end of each dimension, but only read every other data value in the first dimension.

hdfread('example.hdf','Example SDS', ...
                   'Index', {[], [2 1], []})

Subsetting Parameters for HDF Vdata Sets

When you are working with HDF Vdata files, hdfread supports these parameters.

For example, this code reads the Vdata set Example Vdata from the HDF file example.hdf.

hdfread('example.hdf', 'Example Vdata', 'FirstRecord', 400,... 
         'NumRecords', 50)

Subsetting Parameters for HDF-EOS Grid Data

When you are working with HDF-EOS grid data, hdfread supports three types of parameters:

• Required parameters

• Optional parameters

• Mutually exclusive parameters — You can only specify one of these parameters in a call to hdfread, and you cannot use these parameters in combination with any optional parameter.

  Parameter	Description
  Required Parameter
  'Fields'	String naming the data set field to be read. You can specify only one field name for a Grid data set.
  Mutually Exclusive Optional Parameters
  'Index'	Three-element cell array, {start,stride,edge}, specifying the location, range, and values to be read from the data set start — An array specifying the position in the file to begin reading Default: 1, start at the first element of each dimension. The values must not exceed the size of any dimension of the data set. stride — An array specifying the interval between the values to read Default: 1, read every element of the data set. edge — An array specifying the length of each dimension to read Default: An array containing the lengths of the corresponding dimensions
  'Interpolate'	Two-element cell array, {longitude,latitude}, specifying the longitude and latitude points that define a region for bilinear interpolation. Each element is an N-length vector specifying longitude and latitude coordinates.
  'Pixels'	Two-element cell array, {longitude,latitude}, specifying the longitude and latitude coordinates that define a region. Each element is an N-length vector specifying longitude and latitude coordinates. This region is converted into pixel rows and columns with the origin in the upper left corner of the grid. Note: This is the pixel equivalent of reading a 'Box' region.
  'Tile'	Vector specifying the coordinates of the tile to read, for HDF-EOS Grid files that support tiles
  Optional Parameters
  'Box'	Two-element cell array, {longitude,latitude}, specifying the longitude and latitude coordinates that define a region. longitude and latitude are each two-element vectors specifying longitude and latitude coordinates.
  'Time'	Two-element cell array, [start stop], where start and stop are numbers that specify the start and end-point for a period of time
  'Vertical'	Two-element cell array, {dimension, range} dimension — String specifying the name of the data set field to be read from. You can specify only one field name for a Grid data set. range — Two-element array specifying the minimum and maximum range for the subset. If dimension is a dimension name, then range specifies the range of elements to extract. If dimension is a field name, then range specifies the range of values to extract. 'Vertical' subsetting can be used alone or in conjunction with 'Box' or 'Time'. To subset a region along multiple dimensions, vertical subsetting can be used up to eight times in one call to hdfread.

For example,

hdfread(grid_dataset, 'Fields', fieldname, ...
              'Vertical', {dimension, [min, max]})

Subsetting Parameters for HDF-EOS Point Data

When you are working with HDF-EOS Point data, hdfread has two required parameters and three optional parameters.

For example,

hdfread(point_dataset, 'Fields', {field1, field2}, ...
         'Level', level, 'RecordNumbers', [1:50, 200:250])

Subsetting Parameters for HDF-EOS Swath Data

When you are working with HDF-EOS Swath data, hdfread supports three types of parameters:

• Required parameters

• Optional parameters

• Mutually exclusive

You can only use one of the mutually exclusive parameters in a call to hdfread, and you cannot use these parameters in combination with any optional parameter.

For example,

hdfread('example.hdf', swath_dataset, 'Fields', fieldname, ...
         'Time', {start, stop, 'midpoint'})

Examples

Example 1

Specify the name of the HDF file and the name of the data set. This example reads a data set named 'Example SDS' from a sample HDF file.

data = hdfread('example.hdf', 'Example SDS')

Example 2

Use data returned by hdfinfo to specify the data set to read.

1. Call hdfinfo to retrieve information about the contents of the HDF file.

   fileinfo = hdfinfo('example.hdf')
   fileinfo = 
   
       Filename: 'N:\toolbox\matlab\demos\example.hdf'
            SDS: [1x1 struct]
          Vdata: [1x1 struct]

2. Extract the structure containing information about the particular data set you want to import from the data returned by hdfinfo. The example uses the structure in the SDS field to retrieve a scientific data set.

   sds_info = fileinfo.SDS
   sds_info = 
   
          Filename: 'N:\toolbox\matlab\demos\example.hdf'
              Type: 'Scientific Data Set'
              Name: 'Example SDS'
              Rank: 2
          DataType: 'int16'
        Attributes: []
              Dims: [2x1 struct]
             Label: {}
       Description: {}
             Index: 0

3. You can pass this structure to hdfread to import the data in the data set.

   data = hdfread(sds_info)

Example 3

You can use the information returned by hdfinfo to check the size of the data set.

sds_info.Dims.Size
ans =
    16
ans =
     5

Using the 'index' parameter with hdfread, you can read a subset of the data in the data set. This example specifies a starting index of [3 3], an interval of 1 between values ([] meaning the default value of 1), and a length of 10 rows and 2 columns.

data = hdfread(sds_info, 'Index', {[3 3],[],[10 2]});

data(:,1)
ans =
     7
     8
     9
    10
    11
    12
    13
    14
    15
    16

data(:,2)
ans =
     8
     9
    10
    11
    12
    13
    14
    15
    16
    17

Example 4

This example uses the Vdata field from the information returned by hdfinfo to read two fields of the data, Idx and Temp.

info = hdfinfo('example.hdf');

data = hdfread(info.Vdata,...
   'Fields',{'Idx','Temp'})

data = 
    [1x10 int16]
    [1x10 int16]

index = data{1,1};
temp = data{2,1};

temp(1:6)
ans =
     0    12     3     5    10    -1

See Also

hdfinfo, hdf

hdfinfo

hdftool