Re: [ferret_users] accuracy of axis in ferret

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

Hi -
Is the GEBCO topography data available on an OPeNDAP server, or is it 
otherwise available on the web? Is there documentation that indicates 
whether the x_range and  y_range are intended as the edges of the grid 
cells or the first and last coordinate values?

Depending on the answers to these questions, it looks as if one 
alternative might be to define the axes using the dimension variable 
that you show:

  define 
axis/x=`X_RANGE[i=2]`:`X_RANGE[i=1]`/npoints=`dimension[i=1]`/edges 
/units="degrees_east" xax
  define 
axis/y=`Y_RANGE[i=2]`:`Y_RANGE[i=1]`/npoints=`dimension[i=2]`/edges 
/units="degrees_north" yax
  define grid/x=xax/y=yax xygrid

Ansley

Martin Schmidt wrote:
> Hi,
> I am trying to read some large part of the GEBCO topography with 
> ferret. The file does not have axes as usual but start-end-space 
> definition.
>
> With ncdump one sees
> x_range = -60, 50 ;
>
> y_range = -50, 20 ;
>
> z_range = -7861, 5778 ;
>
> spacing = 0.00833333333333333, 0.00833333333333333 ;
>
> dimension = 13200, 8400 ;
>
> So I try to set up a grid from this definition:
>
> define axis/x=`X_RANGE[i=2]`:`X_RANGE[i=1]`:`SPACING[i=2]`/edges 
> /units="degrees_east" xax
> define axis/y=`Y_RANGE[i=2]`:`Y_RANGE[i=1]`:`SPACING[i=1]`/edges 
> /units="degrees_north" yax
> define grid/x=xax/y=yax xygrid
>
> Now to see the result:
> say `x[gx=xygrid],return=iend`
> !-> MESSAGE/CONTINUE 13201
> 13201
>
> Hence the resulting axis has wrong size. The same happens for the 
> y-axis. Mapping topography to this grid gives a strange looking piece 
> of the earth.
>
> However, the varables x_range and so on are defined as doubles. Why is 
> this ignored by ferret?
>
> Playing around with the axis definition gives better results:
> define axis/x=`X_RANGE[i=1]`:`X_RANGE[i=2]`:`0.0083334`/edges 
> /units="degrees_east" xax
> say `x[gx=xygrid],return=iend`
> !-> MESSAGE/CONTINUE 13200
> 13200
>
> However, I want to process several pieces of the topography and I 
> would prefer a clean solution: reading data from file and writing the 
> results without any adjustment of scripts.
> Is this possible? What I am doing wrong here?
>
> Greetings,
> Martin