Default values of fitGLS_opt() allow for negative nugget.

[morrowcj]

Problem

By default, no transformations or limits are placed on the parameters for fitGLS_opt(). This allows the nugget to be any value that returns a valid likelihood - including negative values. Negative values of the nugget are nonsensical, though.

Solution

The most straight forward solution would be to limit the nugget to be between 0 and 1 by default, since this makes the most sense from the semivariogram perspective. This is easily achieved through the trans and backtrans arguments and the logit function.

fitGLS_opt(formula, data, coords,
    trans = list(nugget = function(p){log(p/(1-p))}),
    backtrans = list(nugget = function(l){1/(1+exp(-l))}),
    ...
) 

Note that this solution is already presented in the examples for the function, but should be implemented by default to prevent nonsensical results.

[arives]

Running 15min late

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---

From: Clay Morrow @.> Sent: Monday, November 6, 2023 4:22:47 PM To: morrowcj/remotePARTS @.> Cc: Subscribed @.***> Subject: [morrowcj/remotePARTS] Default values of fitGLS_opt() allow for negative nugget. (Issue #29)

Problem

By default, no transformations or limits are placed on the parameters for fitGLS_opt(). This allows the nugget to be any value that returns a valid likelihood - including negative values. Negative values of the nugget are nonsensical, though.

Solution

The most straight forward solution would be to limit the nugget to be between 0 and 1 by default, since this makes the most sense from the semivariogram perspective. This is easily achieved through the trans and backtrans arguments and the logit function.

fitGLS_opt(formula, data, coords, trans = list(nugget = function(p){log(p/(1-p))}), backtrans = list(nugget = function(l){1/(1+exp(-l))}), ... )

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