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Re: [ferret_users] Problem with Longitude too
Hi Siang and Han,
The point is that, while defining an X-axis with
"E"/"W" with the numerical values or explicit units like /units=longitudes
or /units=degrees, Ferret will issue a modulo attribute BY DEFAULT ONLY IF
the difference between first x-point and last x-point is DELTA_X
here is an example :
define axis/x=0E:360E:1.875 xlon ! first point = last point
show axis xlon
name axis # pts start end
XLON LONGITUDE 193 r 0E 0E(360)
Axis span (to cell edges) = 361.875
here "193 r" --indicates the axis is not modulo, since we have violated
above rule.
Now,
define axis/x=0E:358.125E:1.875 xlon
show axis xlon
name axis # pts start end
XLON LONGITUDE 192mr 0E 1.875W
Axis span (to cell edges) = 360 (modulo length = axis span)
it is modulo by default (indicated by "192mr"),
since (take 0E=360E) 360-358.125 = 1.875
A similar exercise will show you that the X-axis you have defined was
not modulo by default. Now, you can force the modulo behaviour by
adding a /modulo attribute to the 'define axis' command. A similar
step with a NetCDF file will be
yes? set axis/modulo `variable_name,return=xaxis`
Please note that you have to use GRAVE ACCENTS instead of DOUBLE QUOTES
(this was the mistake with your attempt).
However, forcing a modulo nature will not make sense if your axis is not
wrapping around with respect you x_start, x_end and x_delta. You may see
unexpected results if the above mentioned condition is not satisfied.
If you still have questions, please mail back with the following details
1. Precise values of X_start, X_end and X_delta
2. Same for Y-axis too.
3. How is your ascii file oriented --> howmany rows and columns?
Then i will be able to offer a better help.
Regards,
Jaison
On Fri, 29 Jun 2007 nguyen@xxxxxxxxxx wrote:
> Hi Siang Chye
> when define your lat and lon axes, add the qualifier /unit=degrees like that:
> DEF AXIS/y=87.9S:87.9N/npoints=64/unit=degrees y1
> DEF AXIS/x=0E:2.8W/npoints=128/unit=degrees x1
> your xaxis will inherently "modulo", indicating that the axis wraps
> around?the first point immediately following the last
>
> Or you can also specify your xaxis with the qualifier /modulo
> cheers
>
> Hanh
>
>
> > Dear all,
> >
> > I am trying to plot a graph of Europe i.e from 31.7W to 33.7E, but the
> > longitude
> > stops at greenwich. I had read the data from an ascii file and I am not
> > sure if
> > this might be the problem as I do not have a problem plotting Europe with
> > a
> > netCDF file I obtained from MeteoFrance.
> >
> > The Ferret commands I had used to read from the ascii file are as follows:
> >
> > DEF AXIS/y=87.9S:87.9N/npoints=64 y1
> > DEF AXIS/x=0E:2.8W/npoints=128 x1
> > DEF GRID/x=x1/y=y1 g1
> > FILE/VAR="alpha"/GRID=g1 alpha.dat
> >
> > I also tried converting the ascii file to a netCDF file using the
> > following
> > command:
> >
> > SAVE/FILE="alpha.nc" alpha
> >
> > but I still had problems with the longitudes. When I typed
> >
> > SET AXIS/modulo "alpha,return=xaxis",
> >
> > a solution I found from one of the archived mails, I was returned the
> > following
> > error message:
> >
> > invalid command: unknown axis: alpha,return=xaxis
> >
> > I would greatly appreciate if someone could advice me how to overcome the
> > problem. Thank you.
> >
> > Best Regards,
> > Siang Chye
> >
>
>
>
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