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-boun...@stat.math.ethz.ch en nombre de brwin...@aol.com
Enviado el: jue 21/05/2009 21:14
Para: r-sig-geo@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.biv...@nhh.no
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