[gradsusr] GrADS shapefile rings caveat

[Michael Mase]

This is very helpful. Good ideas. Thanks a ton!
On Sat, Sep 16, 2017 at 9:13 PM Jennifer M Adams <jadams21 at gmu.edu> wrote:

> You want to modify the GrADS-generated shapefiles so they have inner
> rings? If so, then I have no real idea how do that. Maybe there are GIS
> tools smart enough to open a GrADS shapefile and merge all those small
> not-smooth polygons (which look normal if you draw them filled in without
> the outlines) into one big polygon and then export that big polygon with
> inner rings? That seems like something the GIS tools should be able to
> manage…
>
> What if you export a shapefile from GrADS with contour lines instead of
> polygons — can you get the GIS tool to merge those lines into inner/outer
> rings of a polygon shape file?
>
> I don’t know if this will help you, but there are some smoothing features
> in GrADS — ‘set csmooth on’ and also the smth9() function, which can be
> used recursively — e.g. smth9(smth9(smth9(smth9(var)))).
>
> —Jennifer
>
> > On Sep 16, 2017, at 12:33 PM, Michael Mase <masester at gmail.com> wrote:
> >
> > Just to clarify - the main reason that this is a problem is it prohibits
> applying algorithms like basis spline to modify/smooth the geometries, so
> it would be fantastic to find a workaround.
> >
> > On Sat, Sep 16, 2017 at 12:32 PM Michael Mase <masester at gmail.com>
> wrote:
> > Thanks, Jennifer. That makes sense. I'm wondering if there is a
> practical way outside of GrADS to transform the exported shapefile to this
> desired result. If you have any ideas, I would love to hear them. I may dig
> in to see if I can find a practical way to do this and perhaps it could be
> added as a hint to the documentation if you so wish.
> >
> > Thanks again!
> >
> > Mike
> >
> > On Sat, Sep 16, 2017 at 11:24 AM Jennifer M Adams <jadams21 at gmu.edu>
> wrote:
> > Hi, Mike —
> > It is likely that the caveat will remain a feature in GrADS — even if we
> did not draw the inner rings, filling in the outer ring will always cover
> the entire area, and the area inside the inner rings could not be undrawn.
> A corrollary of the caveat is that shapefiles exported from GrADS will
> never have inner rings — the shading algorithm is specifically designed to
> avoid this problem. It would certainly have been easier to export closed
> contours as polygons with inner rings, but then GrADS would not have been
> able to display its own shapefiles properly.
> >
> > The workaround for the caveat is to rewrite the shapefile so that the
> polygons do not have inner rings; the boundary points would include the
> inner and outer rings with an overlapping line that connects the two rings.
> > —Jennifer
> >
> >
> > > On Sep 15, 2017, at 11:10 AM, Michael Mase <masester at gmail.com> wrote:
> > >
> > > Hi there,
> > >
> > > Just wondering if the latest GrADS version supports exporting
> shapefiles with inner rings now or if there are plans to support this in
> the future?
> > >
> > > Thanks,
> > > Mike
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> >
> > --
> > Jennifer Miletta Adams
> > Center for Ocean-Land-Atmosphere Studies (COLA)
> > George Mason University
> >
> >
> >
> >
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> --
> Jennifer Miletta Adams
> Center for Ocean-Land-Atmosphere Studies (COLA)
> George Mason University
>
>
>
>
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