Generate beautiful images as PNG or vector PDF files from PostGIS, ideally suited for rendering rivers and streams such as the National Hydrography Dataset.
QGIS and Mapbox are great tools for styling map data. For quick visualizations, however, they may be too complicated. GDAL rasterize and built-in PostGIS functions such as ST_AsPNG provide good capabilities to render images from a database, but lack some of the styling options provided by Apple's CoreGraphics.
The gen_image tool was purpose-built to run efficiently and generate large images with little effort.
Note: The current version of gen_image only supports geometries of type LINESTRING.
Libgeos is required. Download the latest version, build it and install it using the default settings. The Makefile looks for the library in /usr/local/lib.
The commandline tool gen_image requires a connection to a PostgreSQL database.
Generate an image from a table nhdflowline with a 1-pixel width:
./gen_image -pg "host=localhost user=admin dbname=mydb password=XXX" -query "select st_astext(shape), 1 from nhdflowline where id = 2" -f output.png The query must contain two columns: the first must be a geography in WKT format (the st_astext function converts it to a WKT format). The second column must be a number corresponding to the width of the line that will be rendered.
The following example show how to generate an image from a table centered around New York with a width calculated using the length. The ST_Simplify function improves rendering performance for large geometries.
./gen_image -pg "host=localhost user=admin dbname=mydb password=XXX" -query \
"select st_astext(st_simplify(shape,0.005)), \
case \
when stream_length < 10000 then 0.5 \
when stream_length < 100000 then 2 \
else 3 \
end as length \
from nhdflowline s join tl_2018_us_state us on s.shape && us.wkb_geometry \
where us.stusps = 'NY'" \
-progress -f foo.pngBuild gen_image using XCode or the included Makefile
John Robokos
This project is licensed under the MIT License - see the LICENSE file for details