`Gshhs Information <#toc11>`_
-----------------------------

The coastline database is GSHHG (formerly GSHHS) which is compiled from two sources:
World Vector Shorelines (WVS) and CIA World Data Bank II (WDBII). In
particular, all level-1 polygons (ocean-land boundary) are derived from
the more accurate WVS while all higher level polygons (level 2-4,
representing land/lake, lake/island-in-lake, and
island-in-lake/lake-in-island-in-lake boundaries) are taken from WDBII.
Much processing has taken place to convert WVS and WDBII data into
usable form for GMT: assembling closed polygons from line segments,
checking for duplicates, and correcting for crossings between polygons.
The area of each polygon has been determined so that the user may choose
not to draw features smaller than a minimum area (see **-A**); one may
also limit the highest hierarchical level of polygons to be included (4
is the maximum). The 4 lower-resolution databases were derived from the
full resolution database using the Douglas-Peucker line-simplification
algorithm. The classification of rivers and borders follow that of the
WDBII. See the GMT Cookbook and Technical Reference Appendix K for
further details.
