QGIS – Mean Coordinates Yielding Displaced Point from Centroid

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I need to get the weighted centroid from a grid layer (according to an attribute of the polygons on this layer). For that, I am using the "mean coordinates" tool, but I realized that the yielded points are displaced from the place they should be located. This is clear when I apply a mean coordinate to just one feature, like in the image:

The red point is the one yielded by the mean coordinates, while the orange one appears to be the logic centroid for the selected feature (yellow piece from grid). Distance between both points is around 20m, but this distance it's not always the same for other cases (even though it's around that value).

Can this be due to having a different CRS for the grid from the project one?

Best Answer

Can this be due to having a different CRS for the grid from the project one?

I don't think so. This is more likely caused by that the polygon starts and ends in the lower left corner. (that means there are two points there), or there are other points (vertex) there. A polygon having n vertices has n+1 points in QGIS.

You can test it by creating polygons starting from different corner:

Related Solutions

QGIS Analysis – Find Mean Distance Between Cluster Center and Points

On the layer with the center point, you can use QGIS expressions. Get an array of the points from the same cluster using array_foreach and then for each of these points create create a line to the center and get it's length. You get an array of lengths. With array_mean, calculate the mean. This is how the expression looks like:

array_mean (
    array_foreach (
        overlay_nearest('points', $geometry, limit:=-1),
        length (
            make_line (
                $geometry,
                @element
            )
        )
    )
)

Based on the data you have and how it is structured, you have to include a condition so that only points belonging to the same center are considered.

From the red center point, the expression generates a line to each of the white dots from the points layer, measures the length and gets the mean. The lines itself that you see here are generated with an expression based on this one with Geometry generator, see on the right side:

QGIS – How to Calculate Mean Coordinates from Several Columns

You can use the following script. It adds all results to one layer.

Add CSV to QGIS.
Select the layer and run the script.

layer = iface.activeLayer()

crs = layer.crs().authid()
uri = "Point?crs=" + crs + "&field=field:string&field=MEAN_X:double&field=MEAN_Y:double&index=yes"
mean_layer = QgsVectorLayer(uri, 'TEST', 'memory')

fields = layer.fields().names()[2:]
features = list(layer.getFeatures())

for field in fields:
    weighted_xs = []
    weighted_ys = []
    weights = []
    for f in features:
        weight = f[field] if f[field] != NULL else 1.0
        p = f.geometry().asPoint()
        weighted_xs.append(p.x() * weight)
        weighted_ys.append(p.y() * weight)
        weights.append(weight)
    
    feat = QgsFeature(mean_layer.fields())
    feat["field"] = field
    mx = sum(weighted_xs) / sum(weights)
    my = sum(weighted_ys) / sum(weights)
    feat["MEAN_X"] = mx
    feat["MEAN_Y"] = my
    
    geom = QgsGeometry.fromPointXY(QgsPointXY(mx, my))
    feat.setGeometry(geom)
    
    mean_layer.dataProvider().addFeatures([feat])

QgsProject.instance().addMapLayer(mean_layer)

Result:

Attribute Table: (Field names are added as field value)

Best Answer

Related Solutions

QGIS Analysis – Find Mean Distance Between Cluster Center and Points

QGIS – How to Calculate Mean Coordinates from Several Columns

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