QGIS操作：将多边形ID赋予重叠多边形

Hey there! Let's dig into why your "Join attributes by location" tool spit out all zeros, and get you a working solution—including an automated workflow you can reuse.

Your all-zero result almost certainly comes from misconfigured parameters, especially the geometric predicate and join type. Here's the correct step-by-step for QGIS:

• Load both Shapefile A (the one with your target IDs) and Shapefile B (the polygons you want to tag) into QGIS.
• Open the Join attributes by location tool—you can find it under Vector > Data Management Tools or search for it directly in the Processing Toolbox.
• Configure these critical parameters:
  • Target layer: Select Shapefile B (the layer you want to add IDs to)
  • Join layer: Select Shapefile A (the layer with the source IDs)
  • Geometric predicate: Choose Within (this tells QGIS to match B polygons that lie entirely inside A polygons; if partial overlaps count too, pick Intersects instead)
  • Join settings:
    • Join type: Pick Keep all target features and matching join features (a left join—this ensures every B polygon stays in the output, even if it doesn't overlap any A polygon)
    • Merge rule: If a B polygon overlaps multiple A polygons (unlikely for your use case), pick Take first matching value (adjust if you need a different logic like max/min)
    • Fields to join: Select the ID field from Shapefile A (e.g., ID)
• Run the tool. The output layer will have your A-layer ID attached to every B polygon that overlaps it; non-overlapping ones will show NULL (not zero—your earlier zeros were likely a default field value from misconfigured matching).

If you need to run this task repeatedly or integrate it into a larger workflow, a PyQGIS script will automate the process entirely. Here's a ready-to-use script:

from qgis.core import (
    QgsVectorLayer,
    QgsProcessingFeedback,
    QgsProcessingContext
)
from qgis import processing

# Update these paths and field names to match your data
SHAPEFILE_A_PATH = "/your/path/to/ShapefileA.shp"
SHAPEFILE_B_PATH = "/your/path/to/ShapefileB.shp"
OUTPUT_PATH = "/your/path/to/TaggedShapefileB.shp"
A_ID_FIELD_NAME = "ID"  # Replace with your actual ID field name in Shapefile A

# Load the input layers
layer_a = QgsVectorLayer(SHAPEFILE_A_PATH, "ID_Source_Layer", "ogr")
layer_b = QgsVectorLayer(SHAPEFILE_B_PATH, "Target_Layer", "ogr")

# Validate layer loading
if not layer_a.isValid() or not layer_b.isValid():
    print("Error: Failed to load one or more shapefiles! Check your paths.")
else:
    # Define processing parameters
    join_params = {
        "INPUT": layer_b,
        "JOIN": layer_a,
        "PREDICATE": [6],  # 6 = "Within" predicate; use [0] for "Intersects" if needed
        "JOIN_FIELDS": [A_ID_FIELD_NAME],
        "METHOD": 1,  # 1 = Left join (keep all target features)
        "DISCARD_NONMATCHING": False,
        "PREFIX": "A_",  # Adds a prefix to the joined ID field to avoid conflicts
        "OUTPUT": OUTPUT_PATH
    }

    # Execute the join
    feedback = QgsProcessingFeedback()
    context = QgsProcessingContext()
    processing.run("qgis:joinattributesbylocation", join_params, feedback=feedback, context=context)

    print(f"Automated tagging complete! Output saved to: {OUTPUT_PATH}")

How to use this script: #

• Replace the file paths and A_ID_FIELD_NAME with your actual data details
• Run it directly in QGIS's Python Console (Plugins > Python Console)
• For batch processing, wrap the core logic in a loop over a list of shapefile pairs

The all-zero outcome was probably due to:

Choosing the wrong geometric predicate (e.g., Contains instead of Within—which would look for A polygons inside B, not the other way around) OR selecting a join type that only kept matching features, with no matches found, leading to default zero values in the output field.

内容的提问来源于stack exchange，提问作者SnoruntIce