[gradsusr] GrADS shapefile rings caveat

[Jennifer M Adams]

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