gshhs

Read Global Self-Consistent Hierarchical High-Resolution Geography

Syntax

S = gshhs(filename)
S = gshhs(filename, latlim, lonlim)
indexfilename = gshhs(filename, 'createindex')


Description

S = gshhs(filename) reads GSHHG (formerly GSHHS) vector data for the entire world from filename. GSHHG files must have names of the form gshhs_x.b, wdb_borders_x.b, or wdb_rivers_x.b, where x is one of the letters c, l, i, h or f, corresponding to increasing resolution (and file size). The result returned in S is a polygon or line geographic data structure array (a geostruct, with 'Lat' and 'Lon' coordinate fields).

S = gshhs(filename, latlim, lonlim) reads a subset of the vector data from filename. The limits of the desired data are specified as two-element vectors of latitude, latlim, and longitude, lonlim, in degrees. The elements of latlim and lonlim must be in ascending order. Longitude limits range from [-180 195]. If latlim is empty, the latitude limits are [-90 90]. If lonlim is empty, the longitude limits are [-180 195].

indexfilename = gshhs(filename, 'createindex') creates an index file for faster data access when requesting a subset of a larger dataset. The index file has the same name as the GSHHG data file, but with the extension 'i', instead of 'b' and is written in the same folder as filename. The name of the index file is returned, but no coastline data are read. A call using this option should be followed by an additional call to gshhs to import actual data. On that and subsequent calls, gshhs detects the presence of the index file and uses it to access records by location much faster than it would without an index.

Output Structure

The output structure S contains the following fields. All latitude and longitude values are in degrees.

Field Name	Field Contents
'Geometry'	'Line' or 'Polygon'
'BoundingBox'	[minLon minLat; maxLon maxLat]
'Lon'	Coordinate vector
'Lat'	Coordinate vector
'South'	Southern latitude boundary
'North'	Northern latitude boundary
'West'	Western longitude boundary
'East'	Eastern longitude boundary
'Area'	Area of polygon in square kilometers
'Level'	Scalar value ranging from 1 to 4, indicates level in topological hierarchy
'LevelString'	'land', 'lake', 'island_in_lake', 'pond_in_island_in_lake', or ''
'NumPoints'	Number of points in the polygon
'FormatVersion'	Format version of data file. Positive integer for versions 3 and later; empty for versions 1 and 2.
'Source'	Source of data: 'WDBII' or 'WVS'
'CrossesGreenwich'	Scalar flag: true if the polygon crosses the prime meridian; false otherwise
'GSHHS_ID'	Unique polygon scalar id number, starting at 0

For releases 2.0 and higher (FormatVersion 7 and higher), the following additional fields are included in the output structure:

Field Name	Field Contents
'RiverLake'	Scalar flag: true if the polygon is the fat part of a major river and the Level value is set to 2; false otherwise.
'AreaFull'	Area of original full-resolution polygon in units $$\frac{1}{10}k{m}^2$$.
'Container'	ID of container polygon that encloses this polygon. Set to -1 to indicate none.
'Ancestor'	ID of ancestor polygon in the full resolution set that was the source of this polygon. Set to -1 to indicate none.

For Release 2.2 and higher (FormatVersion 9 and higher) the following additional field is included in the output structure:

Field Name	Field Contents
'CrossesDateline'	Scalar flag: true if the polygon crosses the dateline; false otherwise.

Background

The Global Self-Consistent Hierarchical High-Resolution Geography (formerly the Global Self-Consistent Hierarchical High-Resolution Shoreline) was created by Paul Wessel of the University of Hawaii and Walter H.F. Smith of the NOAA Geosciences Lab. At the full resolution, the data requires 85 MB uncompressed, but lower resolution versions are also provided. This database includes coastlines, major rivers, and lakes. The GSHHG data in various resolutions is available over the Internet from the National Oceanic and Atmospheric Administration, National Geophysical Data Center website.

Version 3 (Release 1.3) of the gshhs_c.b (coarse) data set ships with the toolbox in the matlabroot/examples/map/data folder. For details, type

type gshhs_c.txt

at the MATLAB^® command prompt. The gshhs function has been qualified on GSHHG releases 1.1 through 2.3.6 (version 15). It should also be able to read newer versions, if they adhere to the same header format as releases 2.0 and 2.1.

Examples

Read Entire Coarse Data Set and Display Levels

Read the entire coarse data set. You can use similar code to read intermediate, full resolution, or high resolution GSHHG data sets.

filename = gunzip('gshhs_c.b.gz', tempdir);
shorelines = gshhs(filename{1});
delete(filename{1})

Display data as a coastline.

figure
worldmap world
geoshow([shorelines.Lat],[shorelines.Lon])

Display each level using a different color.

levels = [shorelines.Level];
land = (levels == 1);
lake = (levels == 2);
island = (levels == 3);
figure
worldmap world
geoshow(shorelines(land),  'FaceColor',[0 1 0])
geoshow(shorelines(lake),  'FaceColor',[0 0 1])
geoshow(shorelines(island),'FaceColor',[1 1 0])

Read GSHHG Dataset Using an Index

Read the entire coarse data set, creating an index. You can use similar code to read intermediate, full-resolution, or high resolution GSHHG data sets.

filename = gunzip('gshhs_c.b.gz', tempdir);
indexname = gshhs(filename{1}, 'createindex');

Display Africa as a green polygon. Note that gshhs detects and uses the index file automatically.

figure
worldmap Africa

projection = gcm;
latlim = projection.maplatlimit;
lonlim = projection.maplonlimit;
africa = gshhs(filename{1}, latlim, lonlim);
delete(filename{1})
delete(indexname)

Sort by descending level to keep smaller level 2 and level 3 features on top.

[~,ix] = sort([africa.Level],'descend');
africa = africa(ix);
geoshow(africa, 'FaceColor', 'green')
setm(gca, 'FFaceColor', 'cyan')

Tips

• If you are extracting data within specified geographic limits and using data other than coarse resolution, consider creating an index file first. Also, to speed rendering when mapping very large amounts of data, you might want to plot the data as NaN-clipped lines rather than as patches.

• When you specify latitude-longitude limits, polygons that completely fall outside those limits are excluded, but no trimming of features that partially traverse the region is performed. If you want to eliminate data outside of a rectangular region of interest, you can use maptrimp with the Lat and Lon fields of the geostruct returned by gshhs to clip the data to your region and still maintain polygon topology.

• You can read the WDB rivers and borders datasets but the LevelString field will be empty. The Level values vary from feature to feature but the interpretations of these values are not documented as part of the GSHHG distribution and are therefore not converted to character vectors.