Extending several lines at once using polygon reference layer in QGIS

Question

I have transverse shapefile lines (blue line) with a fixed length and I would like to extend them (orange line) using another reference layer (green polygon) that has a variable shape.

The 'extend' command built into QGIS 3.22 does this function, however it stretches the line with uniform sizes, however in my case each cross line will have a different size according to the reference layer (green polygon).

In QGIS 3.22 (or other versions) is there something that solves this problem?

Update: i add my files problem

https://github.com/wesleysc352/wesleysc352.github.io/blob/master/files.zip

Answer 1

You could use the said extend lines function, use a huge number for extending the lines to ensure they are all completely within the polygon and then clip the too long lines to the polygon. That's just two processing steps.

In case you end up with lines having two or more parts in the polygon you can run multiparts to singleparts on the lines and delete the parts that disjoin the small red line from your screenshot, e.g. by using extract by location, extracting the parts that intersect with the red line.

Answer 2

Some small python script you can use:

1. Access the layers
2. Set a threshold (your source layers need to be in the same unit). The value needs to be big enough.
3. Extend lines using native:extendlines
4. Clip the overlapping using native:clip
5. Add the layer to the map
6. Filter all multipart segments which may intersect a second time with the polygon

from qgis import processing

# Load layers
blue_lines = QgsProject.instance().mapLayersByName("blue_lines")[0]
green_polygon = QgsProject.instance().mapLayersByName("green_polygon")[0]

# Set distance value
distance = 5000

# Extend Lines 
temp_layer1 = processing.run("native:extendlines", {
    'INPUT' : blue_lines, 
    'OUTPUT' : 'TEMPORARY_OUTPUT', 
    'START_DISTANCE' : distance,
    'END_DISTANCE' : distance
})['OUTPUT']

# Clip
resultLayer = processing.run("native:clip", {
    'INPUT' : temp_layer1, 
    'OUTPUT' : 'TEMPORARY_OUTPUT', 
    'OVERLAY' : green_polygon
})['OUTPUT']

# Iterate over all features and remove multiparts
resultLayer.startEditing()
for feature in resultLayer.getFeatures():
    
    # Create a multi_polyline from the geometry of each feature
    multi_polyline = feature.geometry().asMultiPolyline()
    
    # Iterate over each multi polyline segment so we can find the correct ones
    for polyline in multi_polyline:
        
        # Create a temorary geometry
        geom = QgsGeometry.fromPolyline([QgsPoint(x) for x in polyline])
        
        # Iterate over all blue line features 
        for blue_lines_feature in blue_lines.getFeatures():
            
            # Create a small buffer around the segment 
            # and see if any of the blue line geometries is totaly within
            if blue_lines_feature.geometry().within(geom.buffer(1,5)):
                
                # If it is within update the geometry
                resultLayer.dataProvider().changeGeometryValues({feature.id(): geom}) 
                
                
resultLayer.commitChanges()

QgsProject.instance().addMapLayer(resultLayer)

Update: If your polygon has sharp turns it's better to identify the correct multipart element by creating a small buffer around the extended lines so we can check which blue lines are 100% within those buffers. Those segments are kept within the result layer.