.. _gshhg-information:

GSHHG Information
-----------------

The coastline database is GSHHG (formerly GSHHS) which is compiled from
three sources:  World Vector Shorelines (WVS, not including Antarctica),
CIA World Data Bank II (WDBII), and Atlas of the Cryosphere (AC, for Antarctica only).
Apart from Antarctica, 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.
The Antarctica coastlines come in two flavors: ice-front or grounding line,
selectable via the **-A** option.
Much processing has taken place to convert WVS, WDBII, and AC 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 Global Self-consistent, Hierarchical, High-resolution Geography Database (GSHHG)
<https://github.com/GenericMappingTools/gshhg-gmt#readme>`_ for further details.
