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

[Nick Eubank]

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?

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

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