I have two line-layers: TLM-layer and SEGMENT-layer The TLM-layer has much more segments than the SEGMENT-layer (example: x – z – x in TLM but only red in SEGMENT-layer), but they have the same vertices (except segment a which is only in the SEGMENT-layer)

I'd like to add a new attribute to the SEGMENT-layer called object_kat (integer) which should be defined as:
blue = "object_kat" = 1
yellow = "object_kat" = 2
red = "object_kat" = 3
violet = "object_kat" = NULL

If a segment in the SEGMENT-layer contains more than one segment of the TLM-layer (as example red in SEGMENT-layer and x – z – x in the TLM-layer), the object_type of the longest segment should be used (e.g. x -> red -> "object_kat" = 3 and z which would be yellow and "object_kat" = 2 should be ignored because it's shorter)

I think I should use following steps:

  1. which object_type's (y -> "object_type" = '3', z -> "object_type" = '2', x -> "object_type" = '1') of the TLM-layer are within a segment of the SEGMENT-layer (blue, yellow, red, violet)
  2. which of these is longest
  3. add the value of it (blue -> 1, yellow -> 2, red -> 3, in the new "object_kat" attribute (integer) of the SEGMENT-layer

I'd like to have this as easy as possible using an expression in the Field Calculator in QGIS

enter image description here

  • the size of the dataset can be different, as it is a process I have to do over and over again in different regions. the perpendicular lines should symbolise the start and end of the segments. and the segments are different road types.
    – mymymy
    Commented Feb 18, 2022 at 12:54
  • For x-z-x, "object_kat" = 3. Because the object_type of the longest segment which is x. OK. So, is it 3, because "total x" is the longest (I mean length(x+x) > length(z)), or because any of segment is bigger than the others regardless of total length of any letter? I mean this: i.sstatic.net/dQ4LA.png Commented Feb 18, 2022 at 16:27
  • that's a very good question @KadirŞahbaz I think it should be the sum which makes it the longest. There is also a threshold which we use before, so segments longer than a number are splitted before manually.
    – mymymy
    Commented Feb 25, 2022 at 9:12

1 Answer 1


You can use this Python script.

tlm_layer_name = "tlm_layer"
segment_layer_name = "segment_layer"

tlm_layer = QgsProject.instance().mapLayersByName(tlm_layer_name)[0]
seg_layer = QgsProject.instance().mapLayersByName(segment_layer_name)[0]

# add object_kat field if not exist
field_index = seg_layer.fields().indexFromName("object_kat")
if field_index==-1:
    seg_layer.dataProvider().addAttributes([QgsField("object_kat", QVariant.String, len=3)])
    field_index = seg_layer.fields().indexFromName("object_kat")

buffer_distance = 0.1 # 0.1 unit 
# if CRS unit is degree, use 0.000001

attr_map = {}
for seg in seg_layer.getFeatures():    
    buffer = seg.geometry().buffer(buffer_distance, 5)    
    intersected_tlms = [t for t in tlm_layer.getFeatures() if t.geometry().within(buffer)]
    len_ = len(intersected_tlms)
    if len_ == 1:
        longest = intersected_tlms[0]
    elif len_ > 1 :
        longest = sorted(intersected_tlms, key=lambda x: x.geometry().length(), reverse=True)[0]
        print(f"no intersecting tlm for segment id: {seg.id()}")
    attr_map.update({seg.id(): {field_index: longest["object_type"]}})

enter image description here

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