QGIS Expressions – Numbering Points per Polygon Group with QGIS

expressionoverlaypointpolygonqgis

In QGIS, I have a point and a polygon layer. I want to number the points ascending from left to right (from smallest to highest x-coordinate value – West to East). However, counting should be made separately for each group of polygons with a common attribute value.

The screenshot shows the setting: all points inside polygons with value = 1 are considered together and should be counted from 1 to N, starting with the leftmost and proceeding to the right. Points inside polygons with value = 2 should also start from 1 to N, left to right, points in polygons = 3 as well and so on.

How to achieve that using QGIS expressions?

Best Answer

It is not a solution using the expression, it may seem a bit lamer.

Let's assume there are two layers: a point 'random_points_test' and a polygon 'poly_test' respectively, see image below.

Step 1. Use the "Join attributes by location" to get "id"s from polygons

Step 2. Proceed with the "Add autoincremental field" geoalgorithm for the result from the Step 1 with the following settings:

"id"s from polygons was used as 'Group values by [optional]'

and get the output

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[GIS] In Qgis, how to move a group of point/attributes with an offset of 100 metres

The v.transform tool in the GRASS provider tools lets you do just that.

The tool actually lets you perform affine transformations, but setting the scale to 1 means it is basically a linear translation, i.e. a shift.

Set your values in the X and Y, and remember that it's cartesian values:

+X = East
-X = West
+Y = North
-Y = South

And the units depend on your CRS.

You can combine axes to shift your points (or any other vector type) diagonally. In this example I moved the green origin points some 300 meters east and 50 meters north. The resulting red points can be seen.

QGIS Expressions – How to Get Attribute Values of Another Layer

Instead of collecting the values and selecting back the geometry using this value, you can directly collect the geometries.

collect_geometries (
    array_foreach (
        aggregate (
            'points',
            'array_agg',
            $geometry,
            "value">500 
        ) ,
    make_line(start_point($geometry),@element)
    )
)

Best Answer

Related Solutions

[GIS] In Qgis, how to move a group of point/attributes with an offset of 100 metres

QGIS Expressions – How to Get Attribute Values of Another Layer

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