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Giovanni-3 Operation Technical Summary: Grid Subsetter

Grid Subsetter

The Grid Subsetter operation performs dimension subsetting on gridded data in HDF files. The special case of regional subsetting is achieved by subsetting according to specified latitude and longitude input. In this case, the dimension constraints are specified by the degree values which define a rectangular bounding box. Dimension constraints other than latitude and longitude are specified in index range in order for the subsetter to work.

Inputs

Input files must be in HDF4.

The inputs to the operation, in Giovanni-3 XML format, are the dimension constraints and a list of files that contain gridded data to be subsetted. For latitude/longitude dimensions, the dimension constraints are specified as the latitude/longitude values, in degrees, which define a bounding box of the subset region. The latitude/longitude values are converted to the corresponding grid point values of the gridded data. Other values (such as pressure levels in the atmosphere) can be supplied as dimension constraints.

Dimension constraints can also be specified directly in terms of the index range of the dimensions; this means that if the index range is 1-180, the dimension constraint minimum and maximum can be given as 5 and 50, specifying the subset range 5-50 in this dimension of the grid.

Note: This operation utilizes a "snap-to-grid" function. Snap-to-grid means that the largest bounding box defined by the grid points which fits inside the bounding box defined by the user input values is the area actually used for the analysis and visualizations. If a map of the exact area defined by the input values was compared to the Giovanni-3 visualization, a slight difference in area would probably be discerned. The difference between the area defined by the input values and the subset area created by Giovanni-3 is dependent on how close the input values are to the grid points, and the resolution of the grid.

Outputs

Outputs of subsetted data are saved into new HDF files. The output HDF files have the same structure as the input HDF, except that the dimension information in the outputs is modified to reflect the subsetted data contents.

Algorithm

Dimension subsetting is a process that extracts a sub-array, in one or more dimensions, of regularly gridded data and writes it to an output file. The algorithm first determines the index ranges of the dimensions to be subsetted. The sub-array of data is then extracted and written to an output file, in the same HDF format as the input.

For most dimensions, the index ranges are directly provided from input, and they are referred to as index-based dimension constraints. In the common case of latitude/longitude dimensions, a bounding box latitude/longitude range in degrees and the range is then mapped to an array index range. For example, a longitude range from -120° W to 120° E is mapped 60-300 in the index range of 1° x 1°global data. These type of dimension constraints are called value-based.

For the longitude dimension, the situation is more complex when a region crosses the international dateline , which is where the data's dimension starts. There will actually be two sub-ranges in this case, one from the start to the dateline and the other from the dateline to the end of the range. Data in the two sub-ranges must be extracted separately and then stitched back together.

 

This Technical Summary page is a draft version -- for information on this operation while
the document is being refined, please send a message to GES DISC Help.

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Last updated: Nov 10, 2011 01:08 PM ET
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