About isostatic computation and -N+l (gravfft)

Hello,

I have a doubt concerning the isostatic calculation done by gravfft (-T). Using, for example, Te=0 and a compensation depth (-Z) of 35000 m, I would expect that all topography had its root bellow 35000. Nevertheless, since -N+a is set, the response is a fluctuation around it. 
Is there a way to avoid the demeaning? Setting -N+l does nothing, so I don't know whether this is a bug or if -N+a is really needed.

Thanks in advance,
Gabriel

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