One can import and plot shapefiles in R using the spdep package.
`library(spdep)`
`ccShape=readShapePoly('cbg00barncnty') # here cbg00barncnty
is an ArcGIS shapefile`
`plot(ccShape,add=T,lwd=lwd,border='grey') # ,col='lightgrey')
# this assumes you've already plotted some data based on coordinates
in the projection of the shapefile`

Or, to plot the regions in a new plot:
`plot(ccShape)` # plot the regions

One can then manipulate the polygon information to extract neighborhood
structure for use in CAR and SAR models
`ccNb=poly2nb(ccShape) # produces neighbor structure based
on shared borders`
`plot(ccNb,centroidCoords,add=TRUE)` # add neighbor structure;
for this you would need the centroid coordinates (not necessarily
in the shapefile)
`ccListw=nb2listw(ccNb,style='B')` # convert neighbor structure
to neighbor list for use in spautolm() (be careful to make sure to
specify the weights argument (the heteroscedasticity in the variances)
if needed for legitimate CAR model
`plot(ccListw,centroidCoords,add=TRUE)` # add neighbor structure;
for this you would need the centroid coordinates (not necessarily
in the shapefile)
`ccMat=nb2mat(ccNb,style='B')` # can examine to see what the
weight matrix looks like
`ccNumNeighbors=rowSums(ccMat)` # calculate number of neighbors
for each region

Keywords: shapefile, GIS, CAR, SAR, neighbor

Last modified: 12/20/09.