[ncl-talk] Errors using get_isolines and area_poly_sphere together

[Rashed Mahmood]

Hi Philippe,
I think, it makes sense to expect two same lat or lon for a circle since at
two different longitudes there could be possibility of getting two
latitudes and vice versa. To get unique values, you can use the following
function:
For example:

 y:= get_unique_values(y)

https://www.ncl.ucar.edu/Document/Functions/Built-in/get_unique_values.shtml

Cheers,
Rashed










On Fri, Apr 26, 2019 at 2:51 PM Philippe Papin <pppapin at gmail.com> wrote:

> Hi All,
>
> I've been running into an issue from the combined usage of the
> "get_isolines" and "area_poly_sphere" functions. First, I'm using
> get_isolines to isolate a contour that completes loop around some location
> on the globe. Then I want to find the area within that contour.
>
> What I've noticed is that occasionally (not always) the get_isolines
> function will find a contour with multiple identical lat,lon pairs for the
> given contour. This is problematic for the area_poly_sphere code which
> requires unique vertex points as far as I can tell.
>
> Is there anyway to avoid get_isolines from providing identical lat,lon
> points for a unique contour? If not, is there a function I could use to
> remove non-unique lat,lon pairs?
>
> Here is a print out snippet of the relevant get_isolines output of what
> I'm trying to avoid (see index values 212,213).
>
> Variable: x
>
> Type: float
>
> Total Size: 2452 bytes
>
>             613 values
>
> Number of Dimensions: 1
>
> Dimensions and sizes: [points | 613]
>
> Coordinates:
>
> Number Of Attributes: 5
>
>   n_points : ( 4, 613, 29, 7, 21, 41, 38, 45, 29 )
>
>   start_point : ( 0, 4, 617, 646, 653, 674, 715, 753, 798 )
>
>   segment_count : 9
>
>   level : 2
>
>   _FillValue : 9.96921e+36
>
>
> (211) 17.06323
>
> (212) 16
>
> (213) 16
>
> (214) 14.94309
>
> (215) 13.88631
>
> Variable: y
>
> Type: float
>
> Total Size: 2452 bytes
>
>             613 values
>
> Number of Dimensions: 1
>
> Dimensions and sizes: [points | 613]
>
> Coordinates:
>
> Number Of Attributes: 5
>
>   n_points : ( 4, 613, 29, 7, 21, 41, 38, 45, 29 )
>
>   start_point : ( 0, 4, 617, 646, 653, 674, 715, 753, 798 )
>
>   segment_count : 9
>
>   level : 2
>
>   _FillValue : 9.96921e+36
>
>
> (211) 69.65783
>
> (212) 69.83716
>
> (213) 69.83716
>
> (214) 70.0269
>
> (215) 70.21735
>
> It seems like the code is outputting an unnecessary extra lat,lon pair at
> 69.83716 N, 16E. Why does this happen, and how can I avoid this for
> accurate spherical area calculations?
>
> Thanks,
> Philippe
> --
>
> -------------------------------------
> Philippe P. Papin, Ph.D.
> NRC Research Associate
> Global Modeling Division
> Naval Research Laboratory - Monterey, CA
> http://www.atmos.albany.edu/student/ppapin/
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