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INTERP-Interpolates points of a 'spiketrum' to form a spectrum

Description:
INTERP interpolates between the points of a 'spiketrum' - an array where most of the data is zero except for a few elements whose values are set - to produce a spectrum. Interpolation is by spline fitting, except for those cases where there are not enough points available; in these cases a global polynomial will be used.

Parameters:
SPIKETRUM
The name of the spiketrum - an array where only a few elements are set to the correct values, the other elements all being zero - which is to be used to produce a spectrum by interpolation between those specified elements. SPIKETRUM has probably been produced by GSPIKE from a table of values.
SPECTRUM
The name of the spectrum to be produced by interpolation between the points of the spiketrum.
LOG
It doesn't usually make much difference whether you fit to the log of the data or to the data itself, but if the data is changing very rapidly, particularly at the ends, the fit may be better constrained if the log is used.
LINEND
A spline fit tends to go a little wild at the ends of the fitted spectrum - before the first and after the last point defined by the spiketrum. Using linear interpolation in these extreme ranges prevents this, at the expense of the smoothness of the resulting curve. Usually, it doesn't make much difference.

``` I N T E R P    /     S P I F I T    /   L I N T E R P

Interpolates between the points of a 'spiketrum' to
generate a spectrum.   The INTERP command does this by
spline interpolation, the SPIFIT command uses global polynomial
fitting, and the LINTERP command uses linear interpolation.

Command parameters -

SPIKETRUM  The name of the structure containing the spiketrum

ORDER      The order for the global polynomial to be fitted
(SPIFIT only).

SPECTRUM   The name of the result of the operation.  This can
be the same as for SPIKE.  If not, a new structure
is created, with everything but the data a direct
copy of the input.

Command keywords  -

LOG        (INTERP & SPIFIT only) Perform the interpolation on
the log of the data

LINEND     (INTERP only) Use a linear interpolation for the
ends of the data - spline fits can go wild outside the
range of the defined points.

User variables used - None
KS / CIT 6th July 1984
```

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FIGARO A general data reduction system