Merge polygons and plot using spplot()

Merge polygons and plot using spplot()

References

Reference NO 1

http://stackoverflow.com/questions/15201875/merge-polygons-and-plot-using-spplot

Reference NO 2

http://stackoverflow.com/questions/12873591/overplotting-two-spatialpolygonsdataframes-with-spplot

Reference NO 3

http://sites.google.com/site/spatialr/plottingmaps

Reference NO 4

http://r-sig-geo.2731867.n2.nabble.com/Merging-SpatialPolygonsDataFrame-and-SpatialLinesDataFrame-td2766608.html

Reference NO 5

https://stat.ethz.ch/pipermail/r-sig-geo/2010-November/009842.html

Reference NO 6

http://cran.r-project.org/web/packages/sp/sp.pdf

Reference NO 7

http://casoilresource.lawr.ucdavis.edu/drupal/book/export/html/519

Reference NO 8

http://www.inside-r.org/packages/cran/sp/docs/spplot