Is there a way to render lines with a smooth (bezier) curves in QGIS?

Question

I am moving from creating maps with Illustrator to making them with QGIS and actual data. One of the things I haven't been able to re-create in QGIS are the smooth curved lines I can get in Illustrator. I am not talking about Join and Cap Styles, but how an entire line is rendered.

I am looking for an answer that doesn't include exporting as SVG to Illustrator and finishing the map there.

Also, I realize they could be considered an inaccurate representation but, for the most part, these maps are for giving riders an idea of where they are and don't necessarily have to be an exact representation.

Here is an example of what I mean:

Answer 1

Check out the Generalizer plugin, it should do what you want. The plugin has smoothing options which work quite well.

It doesn't change how your lines are rendered but instead makes a new shapefile with smoothing (or simplification/generalization) applied.

Answer 2

Using QGIS + GRASS plugin:

Add the Grass toolbar from the "Manage Plugins" window.

Your lines will need to be a grass vector, so convert them into a new or existing GRASS mapset.

From the GRASS toolbar, select GRASS Tools...

Look for the module: v.generalize (WIKI tutorial here)

You can then choose several different generalizing algorithms:

Note the algorithms: Chaiken, Hermite or even Sliding Average. There are several others as well!

The differences can be seen in the wiki pictures:

from Wiki: "Note, that a difference between "Chaiken" and "Hermite" is that the lines produced by "Chaiken" "inscribe" the orginal lines whereas the "Hermite" lines "circumscribe" the original lines as can be seen in the picture [above]. (Black line is original line, green line is "Chaiken" and blue is "Hermite")

Using uDig + jgrasstools or grass:

If you have difficulty getting desirable results from GRASS's v.generalize, try the latest version of uDig with jgrasstools. You can also grab the grass jar from the same place as jgrasstools.

Once uDig is installed and up and running, enable the Spatial Toolbox: Window > Show View > Other...

Un-dock the toolbar for easier readability (right-click Spatial Toolbox tab > Detach...):

Load the required jgrasstools and grass (optional) modules:

Don't forget to set your grass parameters (point to grass executable or bat file on windows, or grass command in linux ie, /usr/bin/grass) if you load the grass jar file as well:

Finally! Using the tools from jgrasstools (not grass) you can generalize the line easily with Vector Processing > LineSmootherJaiTools

Be sure to set the input and output properly (works on Shapefiles only, so no need to convert your data to mapsets/locations for grass):

Once done setting the smoothing tolerance (0.0 - 1.0) and input/output, press the Play button on the Spatial Toolbox window (top right):

Results:

Answer 3

You can use geometry generator to smooth without changing the original data:

Answer 4

There is a Smooth geoalghoritm under Processing Toolbox -> Vector geometry -> Smooth.

This algorithm smooths the geometries in a line or polygon layer. It creates a new layer with the same features as the ones in the input layer, but with geometries containing a higher number of vertices and corners in the geometries smoothed out.

The iterations parameter dictates how many smoothing iterations will be applied to each geometry. A higher number of iterations results in smoother geometries with the cost of greater number of nodes in the geometries.

The offset parameter controls how "tightly" the smoothed geometries follow the original geometries. Smaller values results in a tighter fit, and larger values will create a looser fit.

The maximum angle parameter can be used to prevent smoothing of nodes with large angles. Any node where the angle of the segments to either side is larger than this will not be smoothed. For example, setting the maximum angle to 90 degrees or lower would preserve right angles in the geometry.

If input geometries contain Z or M values, these will also be smoothed and the output geometry will retain the same dimensionality as the input geometry.

Answer 5

There is another way, possibly this way only since version QGIS 2.0:

Under Processing>Toolbox you can access GRASS and SAGA Tools. For smoothing, there you find v.generalize, but also line simplification from SAGA. These tools work on regular shapefiles, so no grass vector is needed:

Batch mode is not that good solved yet (right click in the right-hand menue and choose "Execute as batch", since one has to choose every single file manually. The generalizer plug-in mentioned further above lets one choose many files more simply, but this only works right now if one does not specify a file to save to, otherwise the plug in crashes.