gshhg − Extract ASCII listings from binary GSHHG data files
gshhg binaryfile.b [ −Aarea ] [ −G ] [ −Iid ] [ −L ] [ −M ] [ −Nlevel ] [ −Qe|i ] > asciifile.txt
gshhg
reads the binary coastline (GSHHS) or political boundary or
river (WDBII) files and extracts an ASCII listing. It
automatically handles byte-swabbing between different
architectures. Optionally, only segment header info can be
displayed. The output header information has the format
ID npoints hierarchical-level source area f_area west
east south north container ancestor, where hierarchical
levels for coastline polygons go from 1 (shoreline) to 4
(lake inside island inside lake inside land). Source is
either W (World Vector Shoreline) or C (CIA World Data Bank
II); lower case is used if a lake is a river-lake (a portion
of a river that is so wide it is better represented by a
closed polygon). The west east south north is the
enclosing rectangle, area is the polygon area in km^2
while f_area is the actual area of the ancestor polygon (at
full resolution), container is the ID of the polygon
that contains this polygon (-1 if none), and ancestor
is the ID of the polygon in the full resolution set that was
reduced to yield this polygon (-1 if full resolution since
there is no ancestor). For river and border data the header
is simply ID npoints hierarchical-level source west east
south north. For more information about the file
formats, see TECHNICAL INFORMATION below.
binaryfile.b
GSHHS or WDBII binary data file as distributed with the GSHHS data supplement. Any of the 5 standard resolutions (full, high, intermediate, low, crude) can be used.
|
−A |
Only output polygons whose area equals or exceeds the area value in km^2 [Default outputs all polygons]. |
||
|
−G |
Write output that can be imported into GNU Octave or Matlab by ending each segment with a NaN-record. |
||
|
−I |
Only output information for the polygon that matches id. Use −Ic to get all the continents only [Default outputs all polygons]. |
||
|
−L |
Only output a listing of polygon or line segment headers [Default outputs headers and data records]. |
||
|
−M |
Start all header records with the GMT multiple segment indicator ’>’ [Default uses P for polygons and L for lines]. |
||
|
−N |
Only output features whose level matches the given level [Default will output all levels]. |
||
|
−Q |
Control what to do with river-lakes (river sections large enough to be stored as closed polygons). Use −Qe to exclude them and −Qi to exclude everything else instead [Default outputs all polygons]. |
To convert the entire intermediate GSHHS binary data to ASCII files for Octave/Mathlab, run
gshhg gshhs_i.b -G > gshhs_i.txt
To only get a listing of the headers for the river data set at full resolution, try
gshhg wdb_rivers_f.b -L > riverlisting.txt
To only extract lakes, excluding river-lakes, from the high resolution file, try
gshhg gshhs_h.b -Ee -N2 > all_lakes.txt
Users who wish
to access the GSHHG (GSHHS and WDBII) data directly from
their custom programs should consult the gshhg.c and gshhg.h
source code and familiarize themselves with the data format
and how various information flags are packed into a single
4-byte integer. While we do not maintain any Octave/Matlab
code to read these files we are aware that both Mathworks
and IDL have made such tools available to their users.
However, they tend not to update their code and our file
structure has evolved considerably over time, breaking their
code. Here, some general technical comments on the binary
data files are given.
GSHHS: These files contain completely closed polygons of
continents and islands (level 1), lakes (level 2),
islands-in-lakes (level 3) and ponds-in-islands-in-lakes
(level 4); a particular level can be extracted using the
−N option. Continents are identified as the
first 6 polygons and can be extracted via the
−Ic option. The IDs for the continents are
Eurasia (0), Africa (1), North America (2), South America
(3), Antarctica (4), and Australia (5). Files are sorted on
area from large to small. There are two sub-groups for level
2: Regular lakes and the so-called "river-lakes",
the latter being sections of a river that are so wide to
warrant a polygon representation. These river-lakes are
flagged in the header (also see −Q). All five
resolutions are free of self-intersections. Areas of all
features have been computed using a Lambert azimuthal
equal-area projection centered on the polygon centroids,
using WGS-84 as the ellipsoid. GMT use the GSHHS as a
starting point but then partition the polygons into pieces
using a resolution-dependent binning system; parts of the
world are then rebuilt into closed polygons on the fly as
needed. For more information on GSHHS processing, see Wessel
and Smith (1996).
WDBII. These files contain sets of line segments not
necessarily in any particular order. Thus, it is not
possible to extract information pertaining to just one river
or one country. Furthermore, the 4 lower resolutions derive
directly from the full resolution by application of the
Douglas-Peucker algorithm (see gshhg_dp), hence
self-intersections are increasingly likely as the resolution
is degraded. Note that the river-lakes included in GSHHS are
also duplicated in the WDBII river files so that each data
set can be a stand-alone representation. Users who wish to
access both data sets can recognize the river-lakes features
by examining the header structure (see the source code for
details); they are also the only closed polygons in the
WDBII river file. There are many levels (classes) in the
river file: River-lakes (0), Permanent major rivers (1),
Additional major rivers (2), Additional rivers (3), Minor
rivers (4), Intermittent rivers -- major (6), Intermittent
rivers -- additional (7), Intermittent rivers -- minor (8),
Major canals (10), Canals of lesser importance (11), and
Canals -- irrigation type (12). For the border file there
are three levels: National boundaries (1), Internal domestic
boundaries (2), and international maritime boundaries (3).
Individual levels or classes may be extracted via
−N.
Douglas, D. H.,
and T. K. Peucker, 1973, Algorithms for the reduction of the
number of points required to represent a digitized line of
its caricature, Can. Cartogr., 10, 112-122.
Gorny, A. J., 1977, World Data Bank II General User
GuideRep. PB 271869, 10pp, Central Intelligence Agency,
Washington, DC.
Soluri, E. A., and V. A. Woodson, 1990, World Vector
Shoreline, Int. Hydrograph. Rev., LXVII(1), 27-35.
Wessel, P., and W. H. F. Smith, 1996, A global,
self-consistent, hierarchical, high-resolution shoreline
database, J. Geophys. Res., 101(B4), 8741-8743.
GMT(1), gshhg_dp(1) gshhgtograss(1)