[R-sig-Geo] Extracting "lines" from ArcGIS file

[Barry Rowlingson]

On Fri, Jan 28, 2011 at 1:03 PM, Christian Paroissin
<christian.paroissin at univ-pau.fr> wrote:
> Hello,
>
> I have downloaded an Arcgis file here:
> http://services.sandre.eaufrance.fr/data/zonage/Hydrographie2010/arcgis/Bassins/AG/COURS_D_EAU.SHP.zip
>
> I would like to extract informations from these dataset. It gives an S4
> object but I know nothing about it. Here is the code I wrote:
>
> [code]
> object1.shp <- readShapeSpatial("Cours-AG/COURS_D_EAU.shp")
> index= which(object1.shp$TOPONYME=="L'Adour")
> object1.shp[index,]
> [/code]
>
> I would like to extract the 'information' called "lines" but I don't known
> how do it. I find how to extract the 'information' "bbox"
> [quote]
> object1.shp[index,]@bbox
> object1.shp[index,]@lines
> [/quote]
>
> Could you help me please?
>
> Do "lines" correspond to the coordinates of the different points defining
> the river ("L'Adour" in my case)?

 Yes. but rivers are never simple. The data structure is therefore complex :)

 object1.shp is a SpatialLinesDataFrame. Its a bit like a dataframe,
but with a geometry for each row. Each row is a 'feature' - not
necessarily a single wiggly line, it could be several lines. In your
case it seems there is one of everything, so you can do:

object1.shp[index,]@lines[[1]]@Lines[[1]]@coords

to get a 2-column matrix of the coordinates of that feature.

See the help for SpatialLinesDataFrame and the vignette for the sp
package and the ASDAR book for more.

Barry