Programming

Using the HDF4 Import Tool Subsetting Options

When you select a data set, the Importing and Subsetting pane displays the subsetting options available for that type of data set. For data sets that support multiple, mutually exclusive subsetting options, like HDF-EOS Grid data, the contents of the Importing and Subsetting pane change when you select one of the options. The following sections describe these subsetting options for all supported data set types. For general information about the tool, see Using the HDF Import Tool.

• HDF Scientific Data (SD)
• HDF Vdata
• HDF-EOS Grid Data
• HDF-EOS Point Data
• HDF-EOS Swath Data
• HDF Raster Image Data

Note    To use these data subsetting options effectively, you must understand the HDF and HDF-EOS data formats. Use this documentation with the HDF documentation available at the NCSA Web site (hdf.ncsa.uiuc.edu) and the HDF-EOS documentation at the NASA Web site (hdfeos.gsfc.nasa.gov/hdfeos/index.cfm).

HDF Scientific Data (SD)

HDF Scientific Data (SD) data sets are multidimensional arrays. You can import a subset of an HDF SD data set by specifying the location, range, and values to be read from the data set.

The HDF Import Tool displays the subsetting options available, where each row represents a dimension in the data set and each column represents these subsetting parameters:

• Start -- Specifies the position on the dimension to begin reading. The default value is 1, which starts reading at the first element of each dimension. The values specified must not exceed the size of any dimension of the data set.
• Increment -- Specifies the interval between the values to read. The default value is 1, which reads every element of the data set.
• Length -- Specifies how much data to read along each dimension. The default value is the length of the dimension, which causes all the data to be read. The Length parameter automatically updates when you change the value of Start or Increment.

HDF Vdata

HDF Vdata data sets are tables. You can import a subset of an HDF Vdata data set in two ways:

• By field name
• By record

Fields.   Select a specific field you want to import.

Records.   Specify the range of records you want to import.

HDF-EOS Grid Data

In HDF-EOS Grid data, a rectilinear grid overlays a map. The map uses a known map projection. The HDF Import Tool supports the following mutually exclusive subsetting options for Grid data:

• Direct Index
• Geographic Box
• Interpolation
• Pixels
• Tile
• Time
• User-Defined

Direct Index.   You can import a subset of an HDF-EOS Grid data set by specifying the location, range, and values to be read along each dimension.

Each row represents a dimension in the data set and each column represents these subsetting parameters:

• Start -- Specifies the position on the dimension to begin reading. The default value is 1, which starts reading at the first element of each dimension. The values specified must not exceed the size of any dimension of the data set.
• Increment -- Specifies the interval between the values to read. The default value is 1, which reads every element of the data set.
• Length -- Specifies how much data to read along each dimension. The default value is the length of the dimension, which causes all the data to be read.

Geographic Box.   You can import a subset of an HDF-EOS Grid data set by specifying the rectangular area of the grid that you are interested in.

You define the rectangular area of interest by specifying two points that are two corners of the box:

• Corner 1 -- Specify longitude and latitude values in decimal degrees. Typically, Corner 1 is the upper-left corner of the box.
• Corner 2 -- Specify longitude and latitude values in decimal degrees. Typically, Corner 2 is the lower-right corner of the box.

When specifying geographic box subsetting, you can optionally further define the subset of data you are interested in by using Time parameters (see Time) and by specifying other User-Defined subsetting parameters (see User-Defined).

Interpolation.   Interpolation is the process of estimating a pixel value at a location in between other pixels. In interpolation, the value of a particular pixel is determined by computing the weighted average of some set of pixels in the vicinity of the pixel.

You define the region used for bilinear interpolation by specifying two points that are two corners of the interpolation area:

• Corner 1 -- Specify longitude and latitude values in decimal degrees. Typically, Corner 1 is the upper-left corner of the box.
• Corner 2 -- Specify longitude and latitude values in decimal degrees. Typically, Corner 2 is the lower-right corner of the box.

Pixels.   You can import a subset of the pixels in a Grid data set by defining a rectangular area over the grid.

You define the box by specifying two points that define two corners of the box:

• Corner 1 -- Specify longitude and latitude values in decimal degrees. Typically, Corner 1 is the upper-left corner of the box.
• Corner 2 -- Specify longitude and latitude values in decimal degrees. Typically, Corner 2 is the lower-right corner of the box.

Tile.   In HDF-EOS Grid data, a rectilinear grid overlays a map. Each rectangle defined by the horizontal and vertical lines of the grid is referred to as a tile. HDF-EOS Grid data can be stored as tiles. If it is, you can import a subset of the Grid data set by specifying the coordinates of the tile you are interested in.

Tile coordinates are 1-based, with the upper-left corner of a two-dimensional data set identified as 1,1. In a three-dimensional data set, this tile would be referenced as 1,1,1.

Time.   You can import a subset of the Grid data set by specifying a time period.

Specify these values:

• Start -- Specifies the start time.
• Stop -- Specifies the endpoint in the time span.

Note    The units used (hours, minutes, seconds) to specify the time are defined by the data set.

Along with these time parameters, you can optionally further define the subset of data to import by supplying user-defined parameters (see User-Defined).

User-Defined.   You can import a subset of the Grid data set by specifying user-defined parameters.

Specify these values:

• Dimension or Field Name -- Specifies the name of the dimension or field to be read from. Dimension names are prefixed with the characters DIM:.
• Min -- Specifies the start of a range. For dimensions, Min represents the start of a range of elements to extract. For fields, Min represents the start of a range of values to extract.
• Max -- Specifies the endpoint of a range. For dimensions, Max represents the end of a range of elements to extract. For fields, Max represents the end of a range of values to extract.

HDF-EOS Point Data

HDF-EOS Point data sets are tables. You can import a subset of an HDF-EOS Point data set by specifying any of these parameters:

• Field name
• Rectangular area of interest
• Record
• Time

Fields.   Select a specific field you want to import.

Rectangular Area.   You can import a subset of an HDF-EOS Point data set by specifying the rectangular area that you are interested in.

You define the rectangular area of interest by specifying two points that are two corners of the box:

• Corner 1 -- Specify longitude and latitude values in decimal degrees. Typically, Corner 1 is the upper-left corner of the box.
• Corner 2 -- Specify longitude and latitude values in decimal degrees.Typically, Corner 2 is the lower-right corner of the box.

Records.   Specify the range of records you want to import.

Time.   You can import a subset of the HDF-EOS Point data set by specifying a time period.

Specify these values:

• Start -- Specifies the start time.
• Stop -- Specifies the endpoint in the time span.

Note    The units used (hours, minutes, seconds) to specify the time are defined by the data set.

HDF-EOS Swath Data

HDF-EOS Swath data is data that is produced by a satellite as it traces a path over the earth. This path is called its ground track. The sensor aboard the satellite takes a series of scans perpendicular to the ground track. Swath data can also include a vertical measure as a third dimension. For example, this vertical dimension can represent the height above the Earth of the sensor.

The HDF Import Tool supports the following mutually exclusive subsetting options for Swath data:

• Direct indexing
• Geographic region
• Time
• User-Defined

Direct Index.   You can import a subset of an HDF-EOS Swath data set by specifying the location, range, and values to be read along each dimension.

Each row represents a dimension in the data set and each column represents these subsetting parameters:

• Start -- Specifies the position on the dimension to begin reading. The default value is 1, which starts reading at the first element of each dimension. The values specified must not exceed the size of any dimension of the data set.
• Increment -- Specifies the interval between the values to read. The default value is 1, which reads every element of the data set.
• Length -- Specifies how much data to read along each dimension. The default value is the length of the dimension, which causes all the data to be read.

Geographic Box.   You can import a subset of an HDF-EOS Swath data set by specifying the rectangular area of the grid that you are interested in. When you use this subsetting method, you can also specify the Cross Track Inclusion Mode and the Geolocation Mode.

Define the area by specifying two points that specify two corners of the box:

• Corner 1 -- Specify longitude and latitude values in decimal degrees. Typically, Corner 1 is the upper-left corner of the box.
• Corner 2 -- Specify longitude and latitude values in decimal degrees. Typically, Corner 2 is the lower-right corner of the box.

For Swath data, you must also specify the Cross Track Inclusion Mode. This determines how much of the area of the geographic box that you define must fall within the boundaries of the swath.

Select from these values:

• AnyPoint -- Any part of the box overlaps with the swath.
• Midpoint -- At least half of the box overlaps with the swath. This is the default.
• Endpoint -- All of the area defined by the box overlaps with the swath.

For Swath data, you must also specify Geolocation Mode. This specifies whether geolocation fields and data must be in the same swath.

Select from these values:

• Internal -- Geolocation fields and data fields must be in the same swath.
• External -- Geolocation fields and data fields can be in different swaths.

Time.   You can import a subset of the Swath data set by specifying a time period.

Specify these values:

• Start -- Specifies the start time.
• Stop -- Specifies the endpoint in the time span.

Note    The units used (hours, minutes, seconds) to specify the time are defined by the data set.

When you use this subsetting method, you must also specify the Cross Track Inclusion Mode and the Geolocation Mode. You can optionally also specify user-defined subsetting options.

User-Defined.   You can import a subset of the Swath data set by specifying user-defined parameters.

Specify these values:

• Dimension or Field Name -- Specifies the name of the dimension or field to be read from. Dimension names are prefixed with the characters DIM:.
• Min -- Specifies the start of a range. For dimensions, Min represents the start of a range of elements to extract. For fields, Min represents the start of a range of values to extract.
• Max -- Specifies the endpoint of a range. For dimensions, Max represents the end of a range of elements to extract. For fields, Max represents the end of a range of values to extract.

HDF Raster Image Data

No subsetting options are available for HDF raster image data.

Using the HDF Import Tool

Using the MATLAB hdfread Function

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