[R-sig-Geo] Finding the county shapefile polygon closest to along-lat position

[Roger Bivand]

On Thu, 21 May 2009, Pilar Tugores Ferra wrote:

>
>
> Dear Joe,
> Maybe you could use a similar process that Marcelino suggested to me 
> yesterday in order to compute the shortest distance from points to a 
> polyline. You need to 1)convert your point data to ppp object in 
> spatstat, 2)coerce your polygon data to a psp object in spatstat. 
> 3)nncross between the two.
> You'll get "dist" - the nearest neighbour distance between the two 
> patterns and "which" the nearest neighbour index of the second pattern. 
> Maybe it won't be so straightforward to know which ids of the coerced 
> polyline correspond to each polygon but I am not sure about this point.

Thanks, Pilar!

The only possible weakness in this might be that the input points and 
polygon boundaries are in geographical, not projected (planar) 
coordinates, so if there are differences in distances on the plane and 
great circle distances, they might lead to the "outside" points being 
assigned to the wrong county.

Depending on how many there are, you might consider two alternatives, one 
visual inspection in Google Earth or similar (export the "outside" points 
with IDs as one KML, and the county boundaries you are using with 
writeOGR() in rgdal (or use those built into GE)); the other would be to 
use the closeness to the label points of the counties of the "outside" 
points - coordinates() of the SpatialPolygons* object retrieves these, so 
looping over the "outside" points in spDistsN1() in sp with longlat=TRUE 
would give the great circle distances.

Of course, the underlying issue is why they are outside - is this a datum 
shift problem (counties in NAD27 and points in NAD83/WGS84)? If you fixed 
that, they would match better.

Hope this helps,

Roger

> Best regards,
> Pilar
>
>
> -----Mensaje original-----
> De: r-sig-geo-bounces at stat.math.ethz.ch en nombre de BRWIN338 at aol.com
> Enviado el: jue 21/05/2009 21:14
> Para: r-sig-geo at stat.math.ethz.ch
> Asunto: [R-sig-Geo] Finding the county shapefile polygon closest to along-lat position
>
> Greetings
> I have a large number of long-lat locations dispersed over the  US and need
>
> to identify which US county that each point is located in  (or nearest to).
>
> After reading the past posts  and Roger's book, I have been able to use the
> overlay function
> to  identify the appropriate counties for the set of  locations
> with  long-lats lying within or on a polygon boundary.   However, due to
> longlat precision errors (I am assuming), some of the  points lie  outside
> all of
> my shapefile's county polygon  boundaries.
>
> Is there an R function similar to "overlay" that I could use to find  which
>
> county polygon is closest to each of my longlat points that do  not lie
> within the shapefile's polygons?  I have spent quite a  bit of time
> searching
> and browsing past list discussions and can seem  to find my  answer.
>
> My apologies if I have missed an obvious  answer.
>
> Joe
>
>
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-- 
Roger Bivand
Economic Geography Section, Department of Economics, Norwegian School of
Economics and Business Administration, Helleveien 30, N-5045 Bergen,
Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43
e-mail: Roger.Bivand at nhh.no