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Re: [ferret_users] accuracy of axis in ferret

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


Martin Schmidt wrote:
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`

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`

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?


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