[ncl-talk] Visualizing shapefiles with multiple polygons

[Rick Brownrigg]

Hi Adam,

Firstly, much thanks for providing the concise, nicely illustrated example
of the problem!

After looking through the code, I can believe this is a bug, and
unfortunately, not one easily fixed. The underlying code properly knows how
to decode shapefile "features" that are comprised of multiple "segments" or
closed loops. But in drawing, it treats each sub-polygon independently.
This works well, for example, for a coastal nation with a mainland and many
islands, but it does not work for the case you provided, one with an outer
hull containing interior islands/regions. Alas, the code that
gsn_add_shapefile_polygons() is based upon does not provide interface to
specify multiple nested interior loops. We can file this as a github issue,
but with our current prioritization, I can't promise a timely fix.

I wish I had a better response...

Rick


On Wed, May 29, 2019 at 11:15 AM Adam Sisco <aws0006 at uah.edu> wrote:

> Hello NCL users,
>
> I have a question regarding the visualization of shapefiles using NCL. For
> the sake of this help request, I have constructed a sample shapefile with a
> simplified geometry to illustrate the issue I am having. The shapefile
> contains a polygon within the opening of larger polygon, but I'm not sure
> NCL's gsn_add_shapefile_polygons function is capturing the
> polygon-within-polygon nature of this shapefile. So that my issue can be
> reproduced, I have attached:
>
>    1. The sample shapefile I am attempting to plot
>    2. The desired visualization of the shapefile
>    3. The visualization that results from the gsn_add_shapefile_polygons
>    function
>    4. The NCL script used to plot the shapefile
>
> I suspect that a more nuanced approach may be needed to produce the proper
> visualization. If this is within the capabilities of NCL, I would
> appreciate any assistance you can offer. I am using NCL 6.4.0 on a Windows
> machine.
>
> ********************
> begin
>   wks  = gsn_open_wks("x11","shapefile_testing")
>
>   res = True
>   res at gsnDraw = False
>   res at gsnFrame = False
>   res at gsnMaximize = True
>
>   res at mpMinLonF = -110
>   res at mpMaxLonF = -75
>   res at mpMinLatF = 25
>   res at mpMaxLatF = 55
>
>   plot = gsn_csm_map(wks,res)
>
>   polyRes = True
>   polyRes at gsEdgesOn = True
>
>   poly = gsn_add_shapefile_polygons(wks,plot,"***PATH TO
> SHAPEFILE***/test_shapefile.shp",polyRes)
>
>   draw(plot)
>   frame(wks)
> end
> ********************
>
> Regards,
>
> *Adam Sisco*
> Research Associate
> NASA MSFC IMPACT
> University of Alabama in Huntsville
>
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