[ncl-talk] "Noise" when plotting station data

[David Brown]

It is a "feature" of the triangular mesh contouring method (in effect
whenever you use non-gridded data) that it fills the convex hull
(https://en.wikipedia.org/wiki/Convex_hull) of your data. The boundary
of California is concave in the region depicted, but the contouring
algorithm has no way to know where this boundary is, and thus the
appearance of drawing outside the lines.
The only way to avoid this is to mask around the borders of
California. There are examples of how to do this on the web site. See
http://www.ncl.ucar.edu/Applications/mask.shtml, particularly
mask_8.ncl or mask_11.ncl.
 -dave


On Wed, Jul 12, 2017 at 12:26 PM, Meina Wang <mnawang at ucdavis.edu> wrote:
> Dear Mary,
>
> Thank you for your reply!
>
> I've actually used the setting  res at cnFillMode = "RasterFill" in the plotted
> attached but still got the "streaking" over Nevada.  I just tried with
> smooth contour setting (please see attached below) and still got the noise
> there.  I could map out the rest of the region except California, but still
> curious if this could be fixed.  Please let me know if you have any
> suggestions.  Thank you.
>
>
> Best,
> Meina
>
> On Wed, Jul 12, 2017 at 9:52 AM, Mary Haley <haley at ucar.edu> wrote:
>>
>> Hi Meina,
>>
>> The "streaking" that's occurring I think is because when you plot random
>> data using NCL, it is trying to smooth the data by default, using a
>> non-convex polygon that encloses all your data points.
>>
>> Try drawing raster contours instead, by setting:
>>
>>   res at cnFillMode = "RasterFill"     ; draw raster contours instead of
>> smoothed contours
>>
>> If you want to mask all areas except California, see example mask_8.ncl
>> which is similar:
>>
>> http://www.ncl.ucar.edu/Applications/mask.shtml#ex8
>>
>> I think in your case you could set:
>>
>>   res at mpDataBaseVersion           = "MediumRes"
>>   res at mpFillAreaSpecifiers        = (/"Land", "California","Water"/)
>>   res at mpSpecifiedFillColors       = (/"white","transparent","white"/)
>>   res at cnFillDrawOrder             = "PreDraw"  ; Make sure map fill
>> happens
>>                                                ; last.
>>
>> I will update the station_1 example to reflect the difference b/w smooth
>> and raster fill.
>>
>>
>> --Mary
>>
>>
>>
>>
>> On Tue, Jul 11, 2017 at 9:00 PM, Meina Wang <mnawang at ucdavis.edu> wrote:
>>>
>>> Dear NCL users:
>>>
>>> I ran into a problem when trying to use the function
>>> gsn_csm_contour_map_ce to plot station data (with latitude and longitude).
>>> I followed this example form NCL website and got the figure with "noise"
>>> over region where there is no input data point (over Nevada).  The plot is
>>> attached below, with black dots showing the data points, and color contour
>>> underneath is plotted from gsn_csm_contour_map_ce function, so there is no
>>> data over Nevada, while the function still extrapolate values over there.
>>>
>>> Below are the main res setting for plotting station data:
>>>
>>>   res at sfXArray                    = lons
>>>
>>>   res at sfYArray                    = lats
>>>
>>>
>>>   plot=gsn_csm_contour_map_ce(wks,w,res1)
>>>
>>>
>>> Has any one had this problem before?  Any thoughts or suggestion is much
>>> appreciated!  Thank you.
>>>
>>> Best,
>>> Meina
>>>
>>> --
>>> Meina Wang
>>> PhD candidate in Atmospheric Science
>>> Department of Land, Air and Water Resources
>>> University of California, Davis
>>> Davis, CA 95616
>>>
>>> _______________________________________________
>>> ncl-talk mailing list
>>> ncl-talk at ucar.edu
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>>>
>>
>
>
>
> --
> Meina Wang
> PhD candidate in Atmospheric Science
> Department of Land, Air and Water Resources
> University of California, Davis
> Davis, CA 95616
>
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