Selecting the maximum value from series of polygons that intersect with line in QGIS

Question

Is there a way to select the cell (single) with the maximum value in a polygon layer using the intersection of a line? The first image is the grid (polygon) layer with the attribute field "VALUE" and a line.

In the second image, the line crosses over cells A, B, C, D and E.

Is there a way to use the line to select the highest "VALUE" from A through E? In some cases, the line could be shorter or longer, so an expression that only took the line and the cells it crosses would be the best scenario.

Answer 1

You can use "Select By Expression" with the following expression (replace 'Line' with the name of your line layer):

array_first(
    array_reverse(
        array_agg(
            $id,
            group_by:=overlay_intersects('Line', $id),
            filter:=array_length(overlay_intersects('Line', $id)) > 0,
            order_by:="VALUE"
            )
        )
    ) = $id

There are quite a few QGIS expression wizards on here (I'm not one of them but I'm trying to learn) so maybe someone will offer a better expression, but I have tested this and it works well.

Reference: I adapted the expression from MrXsquared here.

Answer 2

The can be done through pyQGIS. The script below will need to be updated with your layer names, but the bones of the functionality in there. Hope this helps

lines = QgsProject.instance().getLayersByName("Lines Layer")[0]
poly = QgsProject.instance().getLayersByName("Polygons Layer")[0]

# get all fids for lines features
fids = []
for lineFtr in lines.getFeatures():
    fids.append(ftr.id())

# begin edit session
with edit(lines):
    for fid in fids:
        # set filter to only use one line feature each time
        lines.setSubString(f"$id = {fid}")

        # create virtual layer of all overlapping polys
        res = processing.run("native:extractbylocation", {
            'INPUT': poly,\
            'PREDICATE': [0],\ # 0 = extracting only intersecting polys
            'INTERSECT': line,\
            'OUTPUT': 'TEMPORARY_OUTPUT'
        })
        output = res["OUTPUT"]

        # get max value from overlapping polys
        polyValues = []
        for ftr in output.getFeatures():
            value = ftr["VALUE"]
            polyValues.append(value)
        maxValue = max(polyValues)

        # save max value in current line feature
        for lineFtr in lines.getFeatures():
            lineFtr["MAX_VALUE"] = maxValue
        
        # update line feature
        lines.updateFeature(lineFtr)
        
print("Completed")