

arc - GRASS \vector maps\ are stored in an \arc-node\ representation
consisting of non-intersecting curves called arcs.  An arc (a.k.a.,
\vector\) is stored as a series of x,y coordinate pairs.  The two
end-points of an arc are called \nodes\.  Two consecutive x,y pairs
define an arc segment (a.k.a., line segment).  Arcs, either singly
or in combination with other arcs, form higher-level \map features\:
line features (e.g., roads or streams) and \area\ features (e.g.,
soil types or timber cutting units).  Areas are also called \polygons\.
GRASS vector format does not store the polygons explicitly; they are
reconstructed by finding the particular arcs which form the polygon
perimeter.  Arcs that form linear features are sometimes called
\lines\ and arcs that outline areas are called area edges or area
lines.
