OSMCoastline

OSMCoastline extracts the coastline data from an OSM planet file and assembles all the pieces into polygons for use in map renderers etc.

https://osmcode.org/osmcoastline

Prerequisites

Libosmium

https://github.com/osmcode/libosmium
https://osmcode.org/libosmium
At least version 2.16.0 is needed.

Protozero

https://github.com/mapbox/protozero
Debian/Ubuntu: libprotozero-dev
At least version 1.6.1 is needed.

LZ4 (optional)

https://lz4.github.io/lz4/
Debian/Ubuntu: liblz4-dev

Only needed for LZ4 PBF compression.

zlib (for PBF support)

https://www.zlib.net/
Debian/Ubuntu: zlib1g-dev

GDAL (for OGR support)

https://gdal.org/
Debian/Ubuntu: libgdal1-dev
(Must be built with Spatialite and GEOS support which is true for
Debian/Ubuntu packages. You need GDAL 2 or greater.

GEOS

https://trac.osgeo.org/geos/
Debian/Ubuntu: libgeos-dev

Sqlite/Spatialite

https://www.gaia-gis.it/fossil/libspatialite/index
Debian/Ubuntu: sqlite3, spatialite-bin

Pandoc (optional, to build documentation)

https://pandoc.org/
Debian/Ubuntu: pandoc
(If pandoc is found by CMake, the manpages will automatically be built.)

Building

You'll need the prerequisites including libosmium installed.

OSMCoastline uses CMake for building:

mkdir build
cd build
cmake ..
make

Call make doc to build the Doxygen API documentation which will be available in the doc/html directory.

Testing

A few tests are provided that can be run by calling ctest.

The tests themselves are written as shell scripts and can be found in the test/t directory. Some test use the nodegrid2opl helper program found in the src directory, it has some documentation in the source code.

The `runtest` script

Run the script runtest.sh from the directory you built the program in. It will read the supplied testdata.osm and create output in the testdata.db spatialite database.

It is normal for this program to create errors and warnings, because it is testing a rather broken input file. You will get messages such as "Closing ring between node -84 and node -74" and "Warning 1: Self-intersection at or near point 7.48488 53.8169". At the end it should print:

There were 35 warnings.
There were 1 errors.

You can use the supplied coastline_sqlite.qgs QGIS project file to open the output with QGIS.

Call runtest.sh -v to run the tests under Valgrind.

Running

Note that you might want to run osmcoastline_filter first, see below under Filtering.

Run: osmcoastline -o DBFILE PLANET-FILE

For example: osmcoastline -o coastline.db planet.osm.pbf

This will create a spatialite database named DBFILE and write several tables with the output into it.

Use --verbose to see whats going on. Start with --help to see other options.

Output

By default, the output is a spatialite database with the following tables. All tables are always created but depending on the command line options only some of them might contain anything.

error_lines Lines that have errors (for instance not closed rings or self-intersections).
error_points Problematic points such as intersections.
rings Coastline rings as linestrings. The table is not populated by default, because this is only needed for finding errors in the coastline. Use the command line option --output-rings to populate this table.
land_polygons Finished assembled land polygons. Depending on --max-points option this will contain complete or split polygons. Only filled if the option --output-polygons=land (thats the default) or =both has been given.
water_polygons Finished assembled water polygons. Only filled if option --output-polygons=water or =both has been given.
lines Coastlines as linestrings. Depending on --max-points option this will contain complete or split linestrings. Only filled if the option --output-lines has been given.

By default all output is in WGS84. You can use the option --srs=3857 to create output in "Web Mercator". (Other projections are currently not supported.)

By default geometry indexes are created for all tables. This makes the database larger, but faster to use. You can use the option --no-index to suppress this, for instance if you never use the data directly anyway but want to transform it into something else.

Coastlines and polygons are never simplified, but contain the full detail. See the simplify_and_split_spatialite or the simplify_and_split_postgis directories for scripts that help with simplifying and splitting geometries using Spatialite or PostGIS, respectively.

The database tables options and meta contain the command line options used to create the database and some metadata. You can use the script osmcoastline_readmeta to look at them.

By default, OSMCoastline creates a spatialite database. If you need shapefiles use ogr2ogr to convert the data:

ogr2ogr -f "ESRI Shapefile" land_polygons.shp coastline.db land_polygons

Alternatively, OSMCoastline aims to support all geospatial data formats as the output (e.g. Shapefile, by setting GDAL driver as "ESRI Shapefile"). If a data format other than Spatialite database is selected as the output format, the two database tables options and meta will be omitted and geometry indexes will not be created.

Steps

OSMCoastline runs in several steps, each can optionally create some output. In most cases you will only be interested in the end result but preliminary results are supplied for debugging or other special uses.

Step 1: Filter out all nodes and ways tagged natural=coastline and all nodes needed by those ways. (This can also be done with the osmcoastline_filter program, see below)

Step 2: Assemble all coastline ways into rings. Rings that are not closed in the OSM data will be closed depending on the --close-distance option.

Step 3: Assemble polygons from the rings, possibly including holes for water areas.

Step 4: Split up large polygons into smaller ones. The options --max-points and --bbox-overlap are used here.

Step 5: Create water polygons as the "inverse" of the land polygons.

The errors encountered in each step are written to the error_points and error_lines tables.

Options

-c, --close-distance=DISTANCE

OSMCoastline assembles ways tagged natural=coastline into rings. Sometimes there is a gap in the coastline in the OSM data. OSMCoastline will close this gap if it is smaller than DISTANCE. Use 0 to disable this feature.

-b, --bbox-overlap=OVERLAP

Polygons that are too large are split into two halves (recursively if need be). Where the polygons touch the OVERLAP is added, because two polygons just touching often lead to rendering problems. The value is given in the units used for the projection (for WGS84 (4326) this is in degrees, for Mercator (3857) this is in meters). If this is set too small you might get rendering artefacts where polygons touch. The larger you set this the larger the output polygons will be. The best values depend on the map scale or zoom level you are preparing the data for. Disable the overlap by setting it to 0. Default is 0.0001 for WGS84 and 10 for Mercator.

-m, --max-points=NUM

Set this to 0 to prevent splitting of large polygons and linestrings. If set to any other positive integer OSMCoastline will try to split polygons/linestrings to not have more than this many points. Depending on the overlap defined with -b and the shape of the polygons it is sometimes not possible to get the polygons small enough. OSMCoastline will warn you on stderr if this is the case. Default is 1000.

-s, --srs=EPSGCODE

Set spatial reference system/projection. Use 4326 for WGS84 or 3857 for "Web Mercator". If you want to use the data for the usual tiled web maps, 3857 is probably right. For other uses, especially if you want to re-project to some other projection, 4326 is probably right. Other projections are currently not supported. Default is 4326.

-v, --verbose

Gives you detailed information on what osmcoastline is doing, including timing.

Run osmcoastline --help to see all options.

Return codes

osmcoastline uses the following return codes:

0 - OK
1 - Warning
2 - Error
3 - Fatal error (output file could not be opened etc.)
4 - Error parsing command line arguments

The difference between warnings and errors is somewhat muddy. Warnings are geometry problems that have either been fixed automatically or seem to be small. Errors are larger problems that couldn't be fixed. If there were errors you probably do not want to use the generated data but fix the OSM data first. If there were warnings the data might be okay, but there still could be data missing or geometry problems such as self-intersections in the coastline. But the classification of problems into warnings and errors is difficult, so to be on the safe side you might only want to use the data if there are no warnings and no errors at all.

Antarctica

OSMCoastline has special code for the coastline of Antarctica. This is the only coastline that can remain open. The coastline starts somewhere around 180° East, 77° South and ends around 180° West and 77° South. OSMCoastline will find those open ends and connect them by adding several "nodes" forming a proper polygon. Depending on the output projection (EPSG:4326 or EPSG:3857) this polygon will either extend to the South Pole or to the 85.0511° line.

Filtering

The program osmcoastline_filter can be used to filter from an OSM planet file all nodes and ways needed for building the coastlines and write them out in OSM format. The resulting file will be a lot smaller (less than 1%) than the original planet file, but it contains everything needed to assemble the coastline polygons.

If you are playing around or want to run osmcoastline several times with different parameters, run osmcoastline_filter once first and use its output as the input for osmcoastline.

Run it as follows: osmcoastline_filter -o OUTFILE.osm.pbf INFILE.osm.pbf

osmcoastline_filter can read PBF and XML files, but write only PBF files. PBF files are much smaller and faster to read and write.

Extracts

Generally you can not run OSMCoastline on extracts. OSMCoastline assembles ways into continuous coastline linestrings and then into land and water polygons. But if the coastline is not closed, it can't do that. This might work if your extract only contains an island, but in most cases this will not work.

License

OSMCoastline is available under the GNU GPL version 3 or later.

Authors

Jochen Topf (jochen@topf.org)

osmcode / osmcoastline

readme