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, 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)? 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
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