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

[Roger Bivand]

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
>
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>
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-- 
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
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http://depsy.org/person/434412