Re: masking the CCSM T42 output with a shape file

Thank you Mary.

This new example is really helpful.

You may also want to upload the "mrb.shp" file with the official
applications example. I used my mrb.shp file and it worked for me.

To answer your earlier question: You were right, i wanted to mask out all
the points that were falling outside the Mississippi river basin. In other
words i wanted to use only the data points falling inside the Mississippi
river basin.

Thanks a lot once again.

sanjiv

On Sat, Mar 27, 2010 at 9:02 AM, Mary Haley <haley@ucar.edu> wrote:

> Grr. Argh. I misspelled "Mississippi" in this script. :-)
>
> I made an official applications example out of this one (and hopefully
> spelled Mississippi correctly). See example 9 at:
>
> http://www.ncl.ucar.edu/Applications/mask.shtml#ex9
>
> This script allows you to indicate whether you want to mask the area inside
> or outside the given geographical outline.
>
> --Mary
>
> On Mar 26, 2010, at 3:44 PM, Mary Haley wrote:
>
>
> I need to point something out here. The lat/lon arrays I'm generating get
> large
> in a hurry, because there are 88565 points in the MRB outline. This may
> cause
> memory problems if your data array is large to start with. I'm only using
> one that is
> 16 x 32.
>
> You may have to modify the script to loop across each lat/lon point in your
> data
> grid and call gc_inout for each one, rather than calling gc_inout with big
> data arrays.
>
> This will slow your script down considerably, but it won't require as much
> memory.
>
> You would remove the lines:
>
> > mrb_lat3d = conform_dims((/nlat,nlon,nmrb/),mrb_lat,2)
> > mrb_lon3d = conform_dims((/nlat,nlon,nmrb/),mrb_lon,2)
> > lat2d = conform_dims((/nlat,nlon/),lat1d,0)
> > lon2d = conform_dims((/nlat,nlon/),lon1d,1)
>
>
> and do this instead:
>
> ;---Create masked data array.
> data_mask = data
>
> do ilt=0,nlat-1
> xlat = lat1d(ilt)
> do iln=0,nlon-1
> xlon = lon1d(iln)
> if(.not.gc_inout(xlat,xlon,mrb_lat,mrb_lon)) then
> data_mask(ilt,iln) = data_mask@_FillValue
> end if
> end do
> end do
>
> This can be slow!
>
> --Mary
>
>
>
> On Mar 26, 2010, at 3:27 PM, Mary Haley wrote:
>
> Sanjiv,
>
> I'm not sure what you mean by "mask the CCSM3 T43 outputs". Do you mean you
> only want to
> do *calculations* over areas of your grid that are not part of the
> Missippippi River Basin, or do
> you want to *graphically* mask out the area that is the Missippippi River
> Basin?
>
> There's no way in NCL to provide a lat/lon boundary and say "do the
> calculations only on the part of
> the grid that falls within this boundary". In a situation like this, you
> would generally need a separate
> "mask" array that conrtains 0s and 1s indicating where your data is inside
> or outside a particular area.
>
> One thing people have done is to set the areas in the grid that they are
> not interested in to
> data@_FillValue (missing value). This will only work if the functions you
> plan to use know
> how to handle missing data.
>
> To get your data grid filled with missing values in the area you're not
> interested in, see the
> "gc_inout" function:
>
> http://www.ncl.ucar.edu/Document/Functions/Built-in/gc_inout.shtml
>
> Note that this can be slow if you have a lot of points in your basin, which
> you do. And,
> this will provide an approximate area only.
>
> I've attached a sample NCL script that creates a dummy (small) array. I
> then use gc_inout
> to mask out the dummy array that is *not* the MRB (maybe you want the
> opposite of this).
>
> I draw the full data contours on the first frame, the masked data contours
> on the second,
> and the lat/lon grid over the masked area in the third.
>
> [Note: I'm used the new "png" option that will be available in V5.2.0 to
> gsn_open_wks to create these png images.]
> <mrb.000001.png>
> <mrb.000002.png>
> <mrb.000003.png>
> <mrb2.ncl>
> --Mary
>
>
> On Mar 26, 2010, at 1:35 PM, Sanjiv Kumar wrote:
>
> Mary,
>
> Thank you for the reply.
>
> I did go through that example. This example does not seem sufficient for my
> problem.
>
> I will try to make my question more clear:
>
> I want to mask the CCSM3 T42 outputs with the Mississippi River Basin shape
> (MRB) file. I am attaching the MRB shape file here.
>
> So the question is: processing the climate model output for the given
> shape.
>
> Sanjiv
>
>
>
> On Thu, Mar 25, 2010 at 10:13 AM, Mary Haley <haley@ucar.edu> wrote:
>
>> Sanjiv,
>>
>> If you have V5.1.1, we have support for reading shapefiles via "addfile".
>>
>> See:
>>
>> http://www.ncl.ucar.edu/Applications/shapefiles.shtml
>>
>> for some examples.
>>
>> Let me know if this is not enough information.
>>
>> --Mary
>>
>>
>> On Mar 25, 2010, at 7:16 AM, Sanjiv Kumar wrote:
>>
>> Hello,
>>>
>>> I want to process the output from CCSM T42 runs for the Mississippi River
>>> Basin (MRB). I have a shape file for the MRB.
>>>
>>> I was wondering whether this can be done with ncl or not.
>>>
>>> Thanks
>>>
>>> Sanjiv
>>> _______________________________________________
>>> ncl-talk mailing list
>>> List instructions, subscriber options, unsubscribe:
>>> http://mailman.ucar.edu/mailman/listinfo/ncl-talk
>>>
>>
>>
> <mrb.zip>
>
>
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Received on Mon Mar 29 06:59:43 2010