On Fri, Aug 14, 2009 at 9:11 PM, Kate
Zinszer<kate.zins...@mail.mcgill.ca> wrote:
> I'm hoping that someone could guide me in how to extract data from
> shapefiles. I want to extract data from a shapefile (classed as
> "SpatialPolygonsDataFrame") and more specifically, the data is contained
> within the slot called "coords" from the class "polygons" within this file
> and despite my best efforts (and much reading!), I'm at a lost.
>
What you're doing is reading the shapefile into an R object of class
SpatialPolygonsDataFrame. And it seems you want the coordinates of the
polygons.
A SpatialPolygonsDataFrame breaks down thus:
f...@polygons is a list of the feature geometries
f...@polygons[[i]] is the i'th feature geometry
f...@polygons[[i]]@Polygons is a list of the simple rings that make up
that feature
f...@polygons[[i]]@Polygons[[j]] is the j'th ring of the i'th feature
f...@polygons[[i]]@polygons[[...@coords is the coords of the j'th ring
of the i'th feature.
Hence for the example in ?"SpatialPolygonsDataFrame-class" you can get
the coordinates of the first ring of the first feature thus:
> ex_...@polygons[[1]]@polygons[[...@coords
x y
s1 0.5 9.5
s1 0.5 10.5
s1 1.5 10.5
s1 1.5 9.5
s1 0.5 9.5
So you probably now want to loop over i and j and do something with
the coordinates.
The structure is complex because features can have multiple rings -
eg some regions have islands or holes. You need to check the @hole
slot of the ....@polygons[[1]] object if you care whether it's a hole
or an island!
Barry
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