How to create border around multiple polygons

Question

Spent hours on a search. No luck. I need to create a border around selected adjacent polygons. Concave Hull and Convex Hull don't give the desired results. It seems to be a simple request. The problem is when you deal with thousands of them. Please see before and after image attached.

Answer 1

If you need it only for visualization purposes, you can use the first soution. Below you find a second solution that works for visualization purpose as well as in the case you want to create actual geometries (polygon or line):

Solution 1:

1. Duplicate the layer

2. Set the layer style to Inverted polygons > Simple line

3. Check the box next to Merge polygons before rendering (slow)

Screenshot: yellow - original polygon layer; blue: duplicated layer:

Solution 2:

You can use QGIS expressions to create a buffer around the outer boundary of the polygons. You can use either Geometry generator or Geometry by Expression (see here for details) with the following expression (replace 'my_layer' with the name of your layer and adapt the size of the buffer - 800 in my example). You get a polygon - if you want to get a line instead, look to version B below:

difference( 
    buffer ( 
        aggregate( 
            'my_layer', 
            'collect', 
            $geometry
        ), 
    800),
    aggregate( 
            'my_layer', 
            'collect', 
            $geometry
        )
)

Screenshot: demonstrating the solution using Geometry generator:

Version B

To get a line instead of a polygon, simply add a boundary () function and set the buffer value to something close to 0 (in my case: 1):