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• Coefficients may be generated one latitude at a time at run-time in a program array.
• They may be generated for all latitudes at run-time in a dynamically allocated memory array and written to a new file.
• They may be generated for all latitudes at run-time in a shared memory segment.
• They may already exist in a file and can be accessed in a number of ways.
  • The file may be part of a memory resident file system.
  • The file may be read into a dynamically allocated memory array.
  • The file may be memory-mapped.
  • The file may be read one latitude at a time into a program array.
  The size of coefficent coefficient files depends on the spectral resolution and the grid spacing; they may be tens or hundreds of megabytes.

The way in which the coefficients are handled can be configured using the environment variable CONFIG_INTERP; the default method for handling coefficients depends on the computer platform. CONFIG_INTERP can take the following values (entries start in column one):

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• FILEIO to read coefficents coefficients from a file one latitude at a time.
• MEMORY to read all coefficents coefficients from a file into memory.
• SHARED to read coefficents coefficients from a shared memory segment.
• ON_FLY to generate coefficents coefficients for one latitude at a time - 'on the fly'.
• MAPPED to map a file into memory.

The environment variable PPDIR gives the pathname of a directory to hold interpolation coefficients. If PPDIR is not defined, coefficient files are created in the application program's current working directory.

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For common transformations, files often already exist, having been generated by other users of the interpolation software (such as MarsMARS, Metview, ...).