expressionqgis
I want to select points 2 and 3 with a 'selection with expression' in QGIS 3.4. I want to do this by using the buffer function in the expression builder.
I already managed to create a separate buffer and then selecting points that are overlapped by the buffers.
I expect that the expression will select points 2 & 3. The expression buffer( $geometry, 5000) is not giving the result i expect.
This can be done using the Processing Graphical Modeler to set up a custom chain. Here's a pic:
From left to right we have three inputs - the tree layer (a Vector Layer Input Object), the buffer size (a Numeric Input) and the tree height (a String Input). The tree layer is then selected by its height attribute. The selected trees are then buffered by the value of the buffer_width input - this is saved as a layer treebuff (remove that output if you don't need it, I just used it for checking). Then the Select By Location process sets the selection to those trees inside the buffer region (set the first input to the trees, the second to the buffer, and the predicate to "contains")
Here's the resulting output:
Large trees are drawn large, and you can see the buffer zone and the selection now includes the large trees and their neighbours within the buffer.
The expression works correctly.
The problem is related to the circle variable which creates a circle for each point.
When we use within (@selected_pt, @circle), QGIS selects all the circles that contain the selected point.
Below a screenshot that demonstrates the above, the selected point is contained only in 5 circles.
Best Answer
Thanks to the suggestion by @she_weeds, I found a much better solution. This method can be performed entirely in the Select by Expression tool, without creating any additional layers.
Use the
collectfunction to combine the church points into a multipoint geometry. Use this multipoint geometry in theintersectsfunction. Because we're usingcollecton a different layer, we have to put it inside theaggregate()function, like so: