.help gsder Aug85 "Gsurfit Package"
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NAME
gsder -- evaluate the derivatives of the fitted surface
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SYNOPSIS
gsder (sf, x, y, zfit, npts, nxder, nyder)

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pointer	sf		# surface descriptor
real	x[npts]		# x array, xmin <= x[i] <= xmax
real	y[npts]		# y array, ymin <= x[i] <= ymax
real	zfit[npts]	# data values
int	npts		# number of data points
int	nxder		# order of x derivative, 0 = function
int	nyder		# order of y derivative, 0 = function
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ARGUMENTS
.ls sf    
Pointer to the surface descriptor structure.
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.ls x, y
Array of x and y values.
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.ls zfit
Array of fitted values.
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.ls npts
The number of points to be fit.
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.ls nxder, nyder
The order of derivative to be fit. GSDER is the same as GSVECTOR if nxder = 0
and nyder = 0. If nxder = 1 and nyder = 0 GSDER calculates the first
derivatives of the surface with respect to x.
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DESCRIPTION
Evaluate the derivatives of a surface at a set of data points.
GSDER uses the coefficients stored in the surface descriptor structure.
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NOTES
Checking for out of bounds x and y values is the responsibility of the
calling program.
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SEE ALSO
gseval, gsvector
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