9 Working with vector geometry

In GRASS an area polygon is defined by a boundary + a centroid. Lines can be a (poly-)line or a boundary.

Various tools are provided to report on or to manipulate vector geometry:

Figure 17: Native directions in GRASS vector data used for network analysis

• v.build: generate topology (automatically done), write erratic vectors to a new error map for later inspection

• v.build.polylines: make polylines of connected vector lines

• v.category: report vector IDs (called ``categories'' or ``cats'' in GRASS), automatically assign new cats to vectors, add missing centroids

• v.clean: cleans topological problems, snap nodes, remove dangles, small areas, eliminate slivers, prune etc.

• v.to.db: report area side IDs of boundaries (left, right)

• v.type: convert vector geometry types (point vs. centroid; 3D point vs. kernel (3D centroid); line vs. boundary; 3D area vs. face)

• d.vect: display directions of vector lines (indicated by small arrow, see figure 17)