Simplify a Shapefile with Mapshaper on OSX
Yesterday I needed to simplify a shapefile quite substantially to get the size down enough that it could be loaded into CartoDB. Using QGIS this tended to leave sliver or gaps between polygons, but I came across Mapshaper. This is primarily a command line tool and is built on node.js. However, a web version also exists. There are a load really useful GIS functions such as simplifying, clipping, dissolve, joins and merges.
Using Mapshaper on OSX
From install to use (including node.js) was about two minutes…
The first step is to install node.js; visit the website and download the package by clicking the install button on the homepage. Run the install and follow through the instructions. This will install node.js on your computer – this is a platform built on Chrome’s JavaScript runtime for developing applications. Node.js is cropping up in lots of new applications – for example, the new blogging system Ghost.
After install of node.js, you need to install Mapshaper which can be done by running the following on the terminal:
npm install -g mapshaper
If you get an error about permissions when running the above, you might have to preface the command with sudo (which will ask you for a password):
sudo npm install -g mapshaper
After this you are done. In my case, I was interested in simplifying a shapefile (located in my Dropbox) which I could complete with the following command (the % are the percentage of removable points to retain). The first shapefile listed is the input, and the second the desired output.
mapshaper /Users/alex/Dropbox/US_tract_clusters_new.shp -simplify 1% -o /Users/alex/Dropbox/US_tract_clusters_new_05pct.shp
Examples of the output:
