Boundary of SpatialGridDataFrame ?

Is it possible to get the boundary polygon
of a SpatialGridDataFrame object resulting from
a geotif image imported through rgdal?

My goal is to divide that rectangle
into two polygons defined by a
"SpatialLinesDataFrame" object (shp imported through
readOGR) and then overlay on the SpatialGridDataFrame object
to extract raster values from each polygon and compare.

Thanks!

Agus

Dr. Agustin Lobo
Institut de Ciencies de la Terra "Jaume Almera" (CSIC)
LLuis Sole Sabaris s/n
08028 Barcelona
Spain
Tel. 34 934095410
Fax. 34 934110012
email: Agustin.Lobo at ija.csic.es
http://www.ija.csic.es/gt/obster