Re: GRAVFFT query/check

  Lester,

There is no difference between GMT4 & GMT5 procedures (except that some 
option syntax did actually change since there never was a gravfft GMT4)
And yes, you need to blank (set to zero) the sea-side of the land grid 
and vice-versa using a mask and add the results at the end.

Joaquim

> Hello
>
> I just wanted to check the use of gravfft for the Bouguer correction 
> (land and Sea). Windows 7 operation.
>
> Under v4 I would split the computation for land/sea and so this would 
> become (under v5):
>
> gravfft test_topo.grd -Gtopo_corr_land.grd -D2670 -E6 -V (use land 
> mask -N0/1)
> gravfft test_bathy.grd -Gtopo_corr_sea.grd -D1640 -E6 -V (use sea mask 
> -N1/0)
>
> grdmath free-air.grd topo_corr_land.grd SUB -V = bouguer_land.grd
> grdmath free-air.grd topo_corr_sea.grd SUB -V = bouguer_sea.grd
> grdmath bouguer_land.grd bouguer_sea.grd ADD -V = Bouguer_land_sea.grd
>
> What I noticed was that, using the land-only grid, the values in the 
> "sea" were say around a near constant -53, when the land mask has the 
> sea set to 0. This would clearly give a wrong value when added to the 
> sea-only data. Clearly, one cannot have NaN values in the grid as the 
> FFT would fail, in fact when tested it caused gravfft to crash.
>
> If it is all land or all sea, there is no problem, but how can this be 
> done for a combined region properly? Do I compute the land and sea 
> density grids over the whole area, and then mask the results - just 
> trying to see the logical path here.
>
> Under GMT 4 I would simply use grdmath and the mask grids to get the 
> separate files to sum, no issues there.
>
> I am sure it is a simple answer (hopefully)
>
> Cheers
> Lester
>
>
>
>
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