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network_analysis.py

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+#Libraries To Import For Doing Network Analysis using QGis
+#Using The Core QGIS functions and we're using PyQT classes for graph building and layer manipulation to learn 2 points to another
+import sys
+import os
+from qgis.analysis import QgsGraphBuilder #Used To analyze two points
+from qgis.analysis import QgsGraphAnalyzer #Uses Djikstra's Algorithm to find the most optimal path between 2 points
+
+from qgis.core import (
+	QgsApplication,
+	QgsWkbTypes,
+	QgsProject, #Project Manager
+	QgsVectorLayer, #To Load Shapefiles and geopackages
+	QgsFeature, #Single row in a vector layer
+	QgsGeometry, #For mathematical geometry operations
+	QgsPointXY, #Makes a 2D point
+	QgsPoint,
+	QgsFields,
+	QgsField, #For multple attribute schemas and individual attribute field
+	QgsVectorFileWriter #For Package Export
+)
+
+from qgis.PyQt.QtCore import QVariant  # Used to define field data types
+
+#Now we load the Philippine roads dataset from a non-profit organization called Humanitarian Open Street map team
+#This will give us an open dataset about roads in NCR and give us the shortest path between two points
+#Setup QGIS Paths
+QGIS_PREFIX_PATH = "/usr"  # Common for Ubuntu QGIS installs
+QGIS_PLUGIN_PATH = "/usr/lib/qgis/plugins"
+
+qgs = QgsApplication([], False)
+qgs.setPrefixPath(QGIS_PREFIX_PATH, True)
+qgs.initQgis()
+
+
+layer = QgsVectorLayer("phl_roads_lines.gpkg", "roads", "ogr")
+
+#print("Layer loaded:", layer.name()) 
+
+#We are going to find shortest path for Quezon city
+start_point = QgsPointXY(121.0365, 14.6760)
+end_point = QgsPointXY(121.0000, 14.5550)
+
+tolerance = 0.001 #to snap nearest vortex
+builder = QgsGraphBuilder(layer.sourceCrs())
+
+points = [start_point, end_point]
+
+graph = builder.graph()
+
+#getting the feature graphs
+for feature in layer.getFeatures():
+	geom = feature.geometry()
+	if geom.isMultipart():
+		lines = geom.asMultiPolyline()
+	else:
+		lines = [geom.asPolyline()]
+	for line in lines:
+		for i in range(len(line) -  1):
+			#Then we convert it to a QgisPoint
+			p1 = QgsPointXY(line[i].x(), line[i].y())
+			p2 = QgsPointXY(line[i + 1].x(), line[i + 1].y())
+
+			builder.addVertex(-1, p1)
+			builder.addVertex(-1, p2)
+
+			id1 = builder.graph().findVertex(p1)
+			id2 = builder.graph().findVertex(p2)
+
+			distance = p1.distance(p2)
+
+			builder.addEdge(id1, p1, id2, p2, [distance])
+
+graph = builder.graph()
+
+
+# Convert the coordinates (start_point and end_point) into node indexes in the graph
+start_idx = graph.findVertex(start_point)
+end_idx = graph.findVertex(end_point)
+
+# Use Dijkstra's algorithm to compute the shortest path tree from the start node
+# 0 is the index for cost (in our case: distance)
+tree, cost = QgsGraphAnalyzer.dijkstra(graph, start_idx, 0)
+
+# If cost is infinite, it means there’s no possible path between start and end
+if cost[end_idx] == float('inf'):
+    print("No Path Found")
+    sys.exit(1)
+
+# Backtrack from end point to start to extract the shortest path
+route = []
+cur_pos = end_idx
+while cur_pos != start_idx:
+    incoming_edge = tree[cur_pos]  # The edge that leads to current vertex
+    if incoming_edge == -1:
+        print("No Route!")
+        sys.exit(1)
+    
+    # Get the source vertex of this edge, and retrieve its point
+    from_vertex = graph.edge(incoming_edge).fromVertex()
+    point = graph.vertex(from_vertex).point()
+    route.append(point)
+
+    # Move to the previous vertex
+    cur_pos = from_vertex
+
+# Add the final starting point to complete the route
+route.append(start_point)
+route.reverse()  # Reverse because we built the route from end to start
+
+# Define fields for the output layer
+fields = QgsFields()
+fields.append(QgsField("id", QVariant.Int))  # We'll assign an ID = 1 to this feature
+
+# Define the GeoPackage file path for saving the shortest path result
+output_path = "shortest_path.gpkg"
+
+# Create the output vector layer writer (LineString geometry, same CRS as roads layer)
+output_layer = QgsVectorFileWriter.create(
+    output_path,         # File to save
+    fields,              # Attribute schema
+    QgsWkbTypes.LineString,  # Geometry type
+    layer.sourceCrs(),   # Coordinate reference system
+    "GPKG"               # Format
+)
+
+# Create a feature from the route geometry
+feat = QgsFeature()
+feat.setFields(fields)
+feat.setAttribute("id", 1)
+feat.setGeometry(QgsGeometry.fromPolylineXY(route))
+
+# Add the feature to the output file
+output_layer.addFeature(feat)
+
+# Close and save the layer to disk
+del output_layer
+
+print(f"✅ Shortest path successfully saved to: {output_path}")
+
+# Exit QGIS to clean up resources and avoid memory leaks
+qgs.exitQgis()