Creating buffer in QGIS without including surface of original polygon?

Question

I have to have a layer (shapefile) with a polygon that has a 100-m buffer zone around each polygon of my original file. With the QGIS Buffer (fixed distance) tool, I get polygons that include the original polygon. But I only want the outside for each polygon.
If the buffer for polygon A intersects the buffer for polygon B, I don't want to lose that part of the "outside" buffer (so I can't use the différenciation symétrique tool (sorry, I'm on a French version of software and I don't know it's equivalent in English). Alternative would be a smart intersection that is polygon-to-polygon based on an ID to get rid of the inner part.

My original datafile is the green and the buffers are underneath in blue.

Just the buffer layer that I tried in blue.

Answer 1

Let me suggest an approach by a Virtual Layer.

SELECT st_difference(st_buffer(Your_layer.geometry, 100), Your_layer.geometry)
FROM Your_layer

Please save this Virtual Layer as another vector file, such as ESRI Shapefile, before you proceed to next steps.

Answer 2

If you acquire the Multi Ring Buffer plugin, it will enable you to create "doughnut" buffers which exclude the original area being buffered.

If, for some reason, you'd like to do it yourself:

Use the 'Split vector layer' tool, found under 'QGIS geoalgorithms>Vector general tools' in the toolbox, or under 'Vector>Data Management Tools' in the main toolbar.

Once you've got individual layers for each polygon ID, create your 100m buffer and then run the 'Symmetrical Difference' tool. The result will by the buffer of polygon A, minus the original area, and unaffected by any other polygon/buffer.

I would strongly advise using Graphical Modeler to create a single tool to handle the buffer/difference process. Doing so would enable you to batch process your polygon layers once they are split, saving you lots of clicks and potential human error.

Answer 3

Geometry By Expression:

single_sided_buffer(geometry:=boundary( force_rhr( $geometry)), distance:=30)

Force right hand rule

Forces a geometry to respect the Right-Hand-Rule, in which the area that is bounded by a polygon is to the right of the boundary. In particular, the exterior ring is oriented in a clockwise direction and the interior rings in a counter-clockwise direction

Which ensures that a positive distance in single sided buffer is always outwards: