Re: [ferret_users] FILL_XY and modulo operatinos

[Ansley Manke <ansley.b.manke@xxxxxxxx>]

Hi Martin,
The exernal function code needs to explicitly do modulo operations. When 
writing an external function, the author can call the utility function 
ef_get_axis_info to find out whether the X axis is defined as a modulo 
axis, and if so the calculations may use that fact and do whatever is 
needed to carry out operations across the branch cut.

Since you're the original author of this function (10 years ago), would 
you like to take a look at this?  If not I imagine I can see what is the 
right thing to do.

Ansley

On 3/17/2011 11:11 AM, Martin Schmidt wrote:
> Hi ferreters,
>
> processing satellite data with ferret, I found strange behaviour of 
> the fill_xy - function.
> Earlier, I had processed data from a region 20W:22E, now the data sets 
> are global. I notice different results from steps using
> fill_xy near the 0 meridian compared with the earlier regional analysis.
>
> To track this down the following example shows the problem:
>
> !define a global grid:
> define axis/x=10:350:20/unit=degrees_east/modulo xax
> define axis/y=-50:50:10/unit=degrees_north yax
>
> !define a variable and blank a stripe near the 0-meridian
> let dummy=x[gx=xax]+0*y[gy=yax]
> let d1= if i[gx=dummy] gt 3 then dummy else 1/0
> let d2= if i[gx=d1] lt 17 then d1 else 1/0
>
> This leaves two stripes undefined. Note that there are 3 cells empty 
> in the east and 2 cells empty in the west.
>
> Now save to a file otherwise results are really strange - but this is 
> not the point here.
>
> save/clobber/file=zw.nc dummy, d1, d2
>
> ncdump shows a modula axis
>
> variables:
>         double XAX(XAX) ;
>                 XAX:units = "DEGREES_EAST" ;
>                 XAX:point_spacing = "even" ;
>                 XAX:axis = "X" ;
>                 XAX:modulo = 360. ;
>
> Start ferret again
>
> use zw.nc
>
> Now fill d2 using fill_xy
>
> shade fill_xy(d2,dummy,1)
> shade fill_xy(d2,dummy,2)
> shade fill_xy(d2,dummy,3)
>
> The last example gives a wrong figure. Averaging should happen when  
> filling near the 0 meridian, hence the modulo attribute is ignored
> by fill_xy
>
> Now define a region
> set region/x=-60:100
> shade fill_xy(d2,dummy,1)
> shade fill_xy(d2,dummy,2)
> shade fill_xy(d2,dummy,3)
>
> This gives correct figures - in the last example, shade 
> fill_xy(d2,dummy,3), averaging over right and left
> neighbours of cells is carried out correctly.
>
> Now consider the same for the @fnr transormation. Without any region
> can region
> shade d2[x=@fnr:1]
> shade d2[x=@fnr:2]
> shade d2[x=@fnr:3]
>
> In the last example two neighbours are found and the average is 
> displayed - should be correct.
>
> But defining the region
>
> set region/x=-60:100
>
> gives a different result
>
> shade d2[x=@fnr:1]
>
> fills all missing cells and averages over both the neighbours. I would 
> not expect this.
>
> So both the filling methods show different behaviour if either a 
> region around the 0-meridian is specified or not.
>
> I would expect the same result as long as all points used for filling 
> are within the region.
> This should be achieved in future releases of ferret.
>
> Greetings,
> Martin