Computes hessian to be used for variance-covariance matrix. The hessian is the outer product of the vector of first partials (see pg 62 of Buckland et al 2002).
distance sampling object
width-transect width (W); int.range-integration range for observations; showit-0 to 3 controls level of iteration printing; integral.numeric-if TRUE integral is computed numerically rather than analytically
variance-covariance matrix of parameters in the detection function
This is an internal function used by
ddf.ds to fit
distance sampling detection functions. It is not intended for the user to
invoke this function but it is documented here for completeness.
Jeff Laake and David L Miller
Buckland et al. 2002
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