QGIS – How to Count Points in Percentage Within an Extension Defined by Coordinates Pairs

labelingqgisqgis-expressionselect-by-location

Starting from a points geometry layer, my goal is to count points in percentage that are within an extension defined by two pairs of coordinates corresponding to the corners:

The initial situation is as shown in the following screen shot:

The goal would be to get to labeling the percentage of points that are within the extension and the percentage of points that are outside the extent. The idea behind this issue need is to use a labeling expression that avoids using a physical layer of polygons. I add a recreation of what I am looking for:

Solution implementation:

I have tried the eurojam user solution and the correction from Xeppit user.

As you can see I have a layer of 10 points. 5 points are within the rectangle extension and 5 points are outside rectangle. Therefore the result should be 50%.

The rectangle has the following coordinates:

'lowerleft': '-0.00,-4.49'

'upperright': '5.40,4.45'

And the expression I use is:

round(with_variable(
'lowerleft',
'-0.00,-4.49',
with_variable(
'upperright',
'5.40,4.45',
with_variable('rect',
      make_rectangle_3points( 
         make_point(to_real(string_to_array(@lowerleft,',')[0]),to_real(string_to_array(@lowerleft,',')[1])),
         make_point(to_real(string_to_array(@upperright,',')[0]),to_real(string_to_array(@lowerleft,',')[1])),
         make_point(to_real(string_to_array(@upperright,',')[1]),to_real(string_to_array(@upperright,',')[1]))),
      aggregate(layer:='points',
         aggregate:='count',
         expression:=$id,
         filter:=intersects($geometry, @rect)))))
    / count($id) * 100,2) || ' %'

Best Answer

There is a mistake in make_rectangle_3points geometry:

It should be

 make_rectangle_3points( 
         make_point(to_real(string_to_array(@lowerleft,',')[0]),to_real(string_to_array(@lowerleft,',')[1])),
         make_point(to_real(string_to_array(@upperright,',')[0]),to_real(string_to_array(@lowerleft,',')[1])),
         make_point(to_real(string_to_array(@upperright,',')[1]),to_real(string_to_array(@upperright,',')[1])

--> Last line !

Check your rect geometry with geometry generator in symbology of a seperate layer that you can see the selecting rectangle and adapt it to your needs.

Solution 1: copy/paste selected features

Use Select by expression to select the points inside the polygons using overlay_within ('polygons') (use the name of your polygon layer).

Then copy/paste the selected features to a new layer. Add the name of the polygon layer they are within using the expression in step 1 of solution 3 below.

Solution 2: virtual layer

Create a virtual layer with this query:

select *, pt.geometry 
from "area_and_points(1) — points" as pt, 
"area_and_points(1) — polygons" as poly
where st_within ( pt.geometry, poly.geometry)

Using QGIS expressions

Create a new attribute name on the point layer with this expression, using Field calculator:
```
overlay_within(
    'polygons_e2b7c2ad_6eec_4a0a_b6cc_2b580b12acef',
    name
)[0]
```
Extract the points that are within polygons to a new layer with Geometry by expression and the following expression:
```
case 
when 
    overlay_within(
        'polygons_e2b7c2ad_6eec_4a0a_b6cc_2b580b12acef'
    )
then $geometry
end
```
However, you will get a layer with all features from the initial layer, but the points outside polygons will come without features (only attributes) - so you probably want to delete these using Remove null geometries

QGIS Labeling – How to Label Number of Points in Concave Hull Without Duplicates

As you want to number the points per hull and the hull is based on the value in the class field, you can base your expression on this and don't have to complicate yourself with the hull's geometries:

round (count (class,group_by:=class) / count (class) * 100, 0)

By the way, better construct your hull with a simpler expression:

concave_hull(
    collect(
    $geometry,
    group_by:="class"),
    0.2
)

Best Answer

Related Solutions

qgis – Selecting Points Inside a Polygon Layer Using Aggregate Function in QGIS

Solution 1: copy/paste selected features

Solution 2: virtual layer

Using QGIS expressions

QGIS Labeling – How to Label Number of Points in Concave Hull Without Duplicates

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