Mapshaper is a program for simplifying cartographic lines while preserving the topological relationships between adjacent polygons and intersecting polyline paths. It can read and write Shapefile and GeoJSON files and can also export TopoJSON files. The current version can be found at mapshaper.org.

This software is loosely based on the original MapShaper program that I wrote at the University of Wisconsin, Madison in 2006-2007. That version is still available here.

The new mapshaper was rewritten from scratch in JavaScript and improves on the original in a few ways. The earlier program sent data to a server to be processed; the new version does all its processing locally, so the program can be used offline and user data stays private. The new version has a better algorithm for topology processing. There is also a command line script that runs in Node.js.

To set up mapshaper's web interface for online use, copy the files in www/ to a web server. All processing is done in the browser; there is no backend service to run.

Browser compatibility: The web interface works well in recent versions of Chrome and Firefox. Chrome seems more likely to run out of memory when loading large .shp files (say, >200MB). Exporting doesn't work in Safari. Opera and Internet Explorer are not supported.

To run the mapshaper gui directly from the filesystem, open www/index.html in a web browser. Firefox works well in this mode; offline exporting is not fully supported in Chrome.

bin/mapshaper is a Node.js script. It was developed on OS X and has also been used successfully on Ubuntu 13.04 and Windows 8.

$ mapshaper -p 0.1 counties.shp Retain 10% of removable vertices using default simplification.

$ mapshaper -i 100 states.shp --dp Remove features smaller than ~100 meters using Douglas-Peucker simplification.

$ mapshaper -h Read a help message.

You will need to regenerate mapshaper.js if you edit any of the files in the src/ or lib/ directories. Run $ build to update mapshaper.js (used by the command line tool); run $ build gui to update www/mapshaper.js (used by the web interface). The build script requires Node.js.

$ build [gui] -f continuously monitors source files and regenerates mapshaper.js whenever a source file is modified.

Run $ mocha in the project directory to run mapshaper's unit tests.

This software is licensed under the MPL 2.0

According to Mozilla's FAQ, "The MPL's "file-level" copyleft is designed to encourage contributors to share modifications they make to your code, while still allowing them to combine your code with code under other licenses (open or proprietary) with minimal restrictions."

To suggest additions to this list, add an issue.

• Import Shapefile attribute table (.dbf file)
• Selection and aggregation, using attribute queries and mouse selection
• Ability to heal minor topological problems in source data
• Ability to prevent or remove polygon overlaps and self-intersections
• Update the original "modified Visvalingam" method, add control over degree of smoothing.
• Experiment with new simplification and smoothing methods.