[R-sig-Geo] sp::disaggregate() does not seem to add the necessary "comment" attribute to identify holes if Polygons does not contain exactly 1 polygon with 0 or 1 holes

[Roger Bivand]

I've pushed the fix to my fork:

https://github.com/rsbivand/sp

and created a pull request:

https://github.com/edzer/sp/pull/28

Only one part of a complicated set if nested if() in disaggregate() was 
adding comments, but in some settings the existing comments survived the 
disaggregation. Now the Polygons object comment attribute is re-created 
for all Polygons objects. This is version 1.2-6, also including code 
changes that internally affect rgdal and rgeos - you may like to 
re-install them from source after installing this sp version (shouldn't 
matter).

Roger

On Fri, 28 Apr 2017, Anne C. Hanna wrote:

> Hello.  I first posted this issue report on the sp GitHub repo
> (https://github.com/edzer/sp/issues/27) and it was suggested that I redirect
> it to here.
>
> I am working with a geographic dataset with complex borders. The data is
> stored as a SpatialPolygonsDataFrame.  Each Polygons object in the data frame
> may be composed of multiple disjoint polygons, and each polygon may have
> multiple holes.  I want to disaggregate each of the Polygons objects into its
> individual disjoint polygons and construct an adjacency matrix for all the
> disjoint components, and I was using disaggregate() to do this.  However, when
> I run gTouches() on the disaggregated data, in order to compute the
> adjacencies, I get a number of warnings like this:
>
> Error in RGEOSBinPredFunc(spgeom1, spgeom2, byid, "rgeos_touches") : Polygons
> object missing comment attribute ignoring hole(s).  See function createSPComment.
>
> Looking at the Polygons "comment" attributes in the SpatialPolygonsDataFrame
> output by disaggregate(), I see that the only comment values are "0"
> (indicating a single polygon with no holes), "0 1" (indicating a single
> polygon with a single hole), and NULL (apparently no comment was written).
> Since I know my dataset contains several Polygons objects which are composed
> of multiple disjoint regions, and also several Polygons which contain more
> than one hole, this is not the expected result.  In reading the disaggregate()
> code in the sp GitHub repository (specifically, explodePolygons()), I also
> can't see anywhere the comment is being added for the cases where a Polygons
> object has more than two parts or more than two holes.  It actually seems like
> it's getting carried along almost accidentally in the few cases that do get
> comments, and neglected otherwise.
>
> Assuming I'm not failing to understand the code and the desired behavior
> (entirely possible, as I am new at working with this software!), this seems
> suboptimal to me.  My dataset is pretty well-behaved (despite its complexity),
> so I should be able to fix my issues with judicious application of
> createPolygonsComment.  But I had a heck of a time figuring out what was going
> wrong with gTouches, since Polygons comment management appears to be a pretty
> obscure field (and createSPComment wasn't working for me, for whatever
> reason).  So it seems like it might be better if disaggregate() just parses
> and passes along the comments from its input correctly, or, if it's absolutely
> necessary to not create comments, passes nothing and warns clearly in the
> manual that comments and associated hole information are being lost.  Passing
> along comments in some cases while silently dropping them in others seems like
> kind of the worst of both worlds.
>
> I've attached a set of tests I wrote to demonstrate desired/undesired
> behavior: disaggregate_comment_tests.R.  My R version is 3.4.0, my sp version
> is 1.2-4, my rgeos version is 0.3-23 (SVN revision 546), and my GEOS runtime
> version is 3.5.1-CAPI-1.9.1 r4246.  I am using Debian Release 9.0 with kernel
> version 4.9.0-2-amd64.  I hope this is useful; please let me know if you need
> more info or if there is a better place to post this issue.
>
> - Anne
>

-- 
Roger Bivand
Department of Economics, Norwegian School of Economics,
Helleveien 30, N-5045 Bergen, Norway.
voice: +47 55 95 93 55; e-mail: Roger.Bivand at nhh.no
Editor-in-Chief of The R Journal, https://journal.r-project.org/index.html
http://orcid.org/0000-0003-2392-6140
https://scholar.google.no/citations?user=AWeghB0AAAAJ&hl=en