On Sat, 29 Apr 2017, Anne C. Hanna wrote:
Roger,
This looks great, and I will try it out ASAP. I do have one reservation
though --- it seems you are using createSPComment() to reconstruct the
comments, and I have seen some discussion that that may not be reliable
in all cases (e.g. if the initial polygons are wonky in some way). I
don't know a lot about this, but is it possible that it would be
preferable to parse and appropriately disaggregate the original comments
strings (if they exist), so as to deal slightly more smoothly with such
cases (i.e., the polygons would still be wonky, but at least the
hole/polygon matching would track with whatever was in the original
data)?
Contributions very welcome - this was code moved from the raster package,
so I really have little feeling for why it is necessary.
Please provide references and use cases. GEOS is strict in its treatments
of geometries, but does work in most cases. People just expressing
opinions doesn't help, and may well be misleading. It is possible to clean
geometries too, see the cleangeo package. createPolygonsComment probably
isn't foolproof, but specific reports help, because then it is possible to
do something about it. sp classes didn't use simple features when written
because sf was not widely used then - introduced in 2004, when sp was
being developed. Using sf helps, and rgeos::createPolygonsComment was a
work-around from 2010 when rgeos was written.
Even when you use sf in the sf package, you will still run into invalid
geometries because the geometries are in fact invalid, the sf package
also uses GEOS.
Roger
I also had no success using createSPComment to fix disaggregate()'s
output previously, even though my polygons are perfectly non-wonky, so I
am perhaps a little more untrusting of it than I should be. But I'll
let you know how this version works with my data. Thanks for addressing
this so quickly!
- Anne
On 04/28/2017 12:11 PM, Roger Bivand wrote:
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.biv...@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
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