This will be a short demonstration of the code I use to convert Natural Earth SQLite package into .gmt format for plotting in GMT. The version of GRASS GIS in this demonstration is grass78.
Introduction: Natural Earth is a public domain map dataset provides a wide range of physical, cultural and raster features for users to create well-crafted maps with cartography and GIS tools. Physical vector features, such as coastline, land,rivers, lakes, and cultural vector features, such as countries, boundaries, roads are available at 1:10m, 1:50m and 1:110m scales in the format of SHP, SQLite, or GeoPackage.
GDAL is a translator library for raster and vector geospatial data. GRASS GIS is a free and open source software for geographic resources analysis. Documentation and installation guide can be found in the GRASS Wiki page.
1. Convert discrete and simple geographic features (e.g. rivers, lakes, coastlines, boundaries) into GMT format vectors with GDAL
Assumed that we have downloaded a SQLite package from the Natural earth data download page, we can view the data info with the command ogrinfo:
ogrinfo packages/natural_earth_vector.sqlite
To convert simple features data between file formats, we can use the command ogr2ogr.
ogr2ogr -f OGR_GMT ne_110m_rivers_lake_centerlines.gmt packages/natural_earth_vector.sqlite ne_110m_rivers_lake_centerlines
Where OGR_GMT is the desire format, ne_110m_rivers_lake_centerlines.gmt is the outfile name, packages/natural_earth_vector.sqlite is the infile name and ne_110m_rivers_lake_centerlines is the layer name.
2. Convert continues and complex geographic features (e.g. ice sheets, ocean) into GMT format vectors with GRASS GIS
Open GRASS GIS.
grass78
Import vector data into a GRASS vector map with the command v.in.ogr.
v.in.ogr input=packages/natural_earth_vector.sqlite layer=ne_50m_ocean output=ne_50m_ocean
Set region of interest. You can view region info with g.region -p.
g.region e=180e w=180w n=90n s=90s
Creates a vector map of a user-defined grid. We would like to create a 6x12 grid (30 arc-min or 0.5 degree resulotion) and name it grid30min. For details of making grid, see manual.
v.mkgrid map=grid30min grid=6,12
View the vector map you have just made.
g.list type=vect
Overlays two vector maps and clip the ocean ploygon within the 30 arc-min grid vector with the and operator.
v.overlay ainput=ne_50m_ocean binput=grid30deg operator=and output=ne_50m_ocean_blocks
Export the output vector in GMT format.
v.out.ogr input=ne_50m_ocean_blocks output=ne_50m_ocean_blocks.gmt format=OGR_GMT
Exit program.
From my expereience, if GMT can not plot the polygon properly, it may because it doesn't like the points in "90" degree (the real reason is unknown to me). Try to open .gmt file in text editor and replace "90.0" to "89.999" or avioding plotting "90" in your GMT script.