[R-sig-Geo] dotsInPolys sometimes not generating correct number of points

[Nick Eubank]

Thanks Roger -- that makes sense. I'll look into it!

Is maptools on git? Seems easiest fix would be to add a `max.iter`
parameter to the function.

On Sat, Sep 24, 2016 at 11:13 AM Roger Bivand <Roger.Bivand at nhh.no> wrote:

> On Sat, 24 Sep 2016, Nick Eubank wrote:
>
> > Hi Roger,
> >
> > Thanks for explaining. This isn't for maps -- it's for statistical
> analyses.
> > My interest is in converting my polygons to points so I can do some
> > point-statistic calculations on the distribution of people (the points
> I'm
> > adding in each polygon). I have a lot of small polygons (with large
> > variation in the number of points in each), so I think the point pattern
> > will be relatively well preserved (at least on scales I care about) if I
> > uniformly distribute points within each polygon.  Thus the magnitudes of
> > points that don't make sense for visualization.
> >
> > Is there a magnitude of points where the function would perform
> reasonably?
> > i.e. is this an integer overflow problem or something where, if I stay
> > below a given number of points I can count on a set number of truly
> > randomly distributed points? Or do I need a different tool?
>
> Try to use spsample directly on a small subset of polygons to see whether
> you need to increase the iter= argument radically. If you have lots of
> oddish polygons that are very different in shape to
> vertically-oriented rectangles, you are going to need lots of tries. Find
> the answer for the small subset of polygons and scale up from there.
> sp::spsample() takes a sample in the bounding box of the polygon, drops
> points falling outside the polygon, and goes on trying until the effort
> feels pointless - you'd need to tell it to carry on to the bitter end.
> Look at the relevant code for the methods, and work out where it is
> bailing out - probably not scale, more likely oddish polygons.
>
> Roger
>
> >
> > Relatedly, might I suggested that this is something that should be made
> > more clear in the documentation of the function? Right now, there's
> nothing
> > in the docs suggesting this only appropriate for visualization, or that
> it
> > only *approximately* creates a set of randomly distributed points, and
> > can't handle large N.
> >
> > Thanks again,
> >
> > Nick
> >
> > On Sat, Sep 24, 2016 at 2:39 AM Roger Bivand <Roger.Bivand at nhh.no>
> wrote:
> >
> >> On Sat, 24 Sep 2016, Nick Eubank wrote:
> >>
> >>> Hi All,
> >>>
> >>> Trying to run dotsInPolys from maptools on a set of polygons. I have
> one
> >>> set of polygons for each US state, along with a vector of desired
> points
> >>> for each polygon.
> >>>
> >>> However, when I check the number of dots created after running the
> code,
> >> I
> >>> find that for some states -- but not all -- the number of points
> doesn't
> >>> match the number of dots that were specified. In particular, I seem to
> >> have
> >>> fewer dots than expected.
> >>
> >> maptools::dotsinpolys() was written for visualization only (so the
> number
> >> of points could be apprpoximate), a long time ago, and wraps looped
> calls
> >> to sp::spsample() methods. Look at sp::spsample() to see that it tries
> to
> >> sub-divide the number of random sample points among rings, but when a
> >> Polygons object has many member Polygon objects, and they have awkward
> >> geometries, it will struggle to get to the right number, iterating by
> >> default 4 times to increase the density within the bounding box of the
> >> Polygon object. maptools::dotsinpolys() doesn't pass through the tuning
> >> arguments to sp::spsample(), but you could. I'm really uncertain how
> >> visiulizing thousands of polygons works, same for millions of random
> >> points.
> >>
> >> Hope this clarifies,
> >>
> >> Roger
> >>
> >>>
> >>> Paraphrasing somewhat (haven't come up with a minimal replicating
> >> example I
> >>> can post, sorry), I have a polygon file called `my.polygons`, and I'm
> >>> effectively doing the following:
> >>>
> >>>    my.dots <- dotsInPolys(my.polygons, my.polygons[,'count.vector'])
> >>>
> >>> I've confirmed that `typeof(my.polygons[,'count.vector'])` is
> >> `"integer"`,
> >>> and get no warnings running dotsInPolys. Nevertheless, for many
> states, I
> >>> get FALSE from:
> >>>
> >>>    length(my.dots) == sum(my.polygons$count.vector)
> >>>
> >>> There are no NAs in the count vector.
> >>>
> >>> No zero-area polygons. using rgeos, one state that is having problems
> >>> reports areas of:
> >>>
> >>>    summary(gArea(my.polygons, byid=TRUE))
> >>>        Min.   1st Qu.    Median      Mean   3rd Qu.      Max.
> >>>        1.474e+04 9.858e+05 4.264e+06 3.189e+07 3.159e+07 1.144e+09
> >>>
> >>> I also get `gIsValid(my.polygons)` is TRUE, so it's not that...
> >>>
> >>> It seems like I get more problems in states with more polygons /
> counts,
> >> if
> >>> that's at all informative. For example, one problem state has 3,671
> >>> polygons, and should have 634,397 points, but only ends up with
> 632,474.
> >>> Another with 6,983 polygons should have 3,942,589 points, but has
> >>> only 3,840,180. By contrast, no problems in a state with 343 polygons
> >>> and 120,397 points.
> >>>
> >>> Any suggestions on possible cause?
> >>>
> >>> Thanks,
> >>>
> >>> Nick
> >>>
> >>>       [[alternative HTML version deleted]]
> >>>
> >>> _______________________________________________
> >>> R-sig-Geo mailing list
> >>> R-sig-Geo at r-project.org
> >>> https://stat.ethz.ch/mailman/listinfo/r-sig-geo
> >>>
> >>
> >> --
> >> Roger Bivand
> >> Department of Economics, Norwegian School of Economics,
> >> Helleveien 30, N-5045 Bergen, Norway.
> >> voice: +47 55 95 93 55; fax +47 55 95 91 00
> >> e-mail: Roger.Bivand at nhh.no
> >> http://orcid.org/0000-0003-2392-6140
> >> https://scholar.google.no/citations?user=AWeghB0AAAAJ&hl=en
> >> http://depsy.org/person/434412
> >
>
> --
> Roger Bivand
> Department of Economics, Norwegian School of Economics,
> Helleveien 30, N-5045 Bergen, Norway.
> voice: +47 55 95 93 55; fax +47 55 95 91 00
> e-mail: Roger.Bivand at nhh.no
> http://orcid.org/0000-0003-2392-6140
> https://scholar.google.no/citations?user=AWeghB0AAAAJ&hl=en
> http://depsy.org/person/434412
>

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