GIS: The Essentials - Sampling Spatial Units for Agricultural Surveys - page 60

analysis procedures described in Sect. 3.3 , the reader can refer to Neteler and
Mitasova ( 2008 ).
The main facility for performing spatial analysis operations with reference to
polygons is represented by the command v.overlay. 3 This procedure allows the
user to overlay two vector area maps. The resulting output map has a merged
attribute table. The operations differ for the logical operators (AND, OR, XOR,
NOT) used to combine the two areas, as indicated by the operator parameter for
the v.overlay command.
The intersection procedure can be used, for instance, to define the school
districts that are contained in urban areas. This example creates a new area that is
the intersection of two original areas using operator ¼ and. The new intersec-
tion area has the attributes of both input areas. If we want to combine two polygons,
we can use the parameter operator ¼ or. This command forms a new area by
merging the two original areas. Three different areas have been created; each of
them has a different category and the attribute tables of the original areas. If we
require the union of two polygons without overlaps, we can use the parameter
operator ¼ xor. Areas in the first or second maps are preserved, but not those
existing in both maps. The result is complementary to the intersection. The
operator ¼ not generates a new polygon by subtracting the second area from
the first. In this situation, areas in the first map not overlapping any area in the
second map are maintained.
Another possible operation in GIS is the combination of areas and points. We
may be interested in extracting areas that satisfy a certain condition. For example,
we may want to select all the census tracts (i.e., areas) with more than 500 inhab-
itants. In this case, the new vector map contains only the census tracts with more
than 500 inhabitants. This is done using the v.extract command on the popu-
lation column of the table associated with the census tracts map. If the aim is to
extract points (i.e., schools) from a particular area that has greater than 500 inhab-
itants, we can use the v.select command.
The vector overlay operations between points and lines can refer to the selection
of lines overlapping areas, or to the clipping of lines overlapping areas. Consider a
case study where the lines are roads, and the areas are the census tracts. Using v.
extract it is possible to select the roads passing through the census tracts with
more than 500 inhabitants. With v.overlay we can create a map containing only
the parts of the roads inside the census tracts.
Furthermore, GRASS can convert between raster and vector models, and can
transfer attributes. The command v.to.rast converts vector maps to raster
maps. The module r.to.vect does the opposite.
When considering the spatial statistics described in Sect. 1.4 , the analysis
capabilities of GRASS can be substantially improved by combining it with R .
To integrate R into GRASS, we run R from the GRASS shell environment. The
interface dynamically loads compiled GIS library functions into the R executable
3 The GRASS commands are written in italic to emphasize.