[GIS] Getting polygon shapefile node coordinates and point order

coordinatesorderpointqgisqgis-plugins

I am trying to get the coordinates of each node of a polygon in QGIS. What I basically did is use the following tools:

Vector -> Geometry tools -> Extract nodes

Vector -> Geometry tools -> Export Geometry column

However only around the first 300 points had successfully obtained coordinate values in the new xcoord and ycoord fields. The other three thousand plus points had null values. What can I do to get the other points' coordinates? Also is there an easy way to get the point order of each node?

Best Answer

The mmqgis plugin also allows exporting geometry data to csv. The point order will be preserved.

On importing, the plugin does not like multipolygons.

Related Solutions

[GIS] find polygon corner coordinates

Not in one step:

Vector -> Geometry tools -> Extract nodes

will give you a point layer with all nodes of the polygons

Vector -> Geometry tools -> Extract Geometry column

on the point layer will create a new point layer with X and Y columns

python – Getting Lines and Nodes Around the Confluence Point in a Network System (Line Shapefile)

You work here with a Graph (Graph Theory) and your confluence point is simply a node with a degree (number of edges adjacent to that node) > 2.

As you mentioned a Python tag, a solution in Python with Fiona (for reading the shapefile) and NetworkX (Graph Theory)

The Polylines

Open the shapefile and convert all the segments of the Polylines in egdes of a Graph

import fiona
import networkx as nx
import itertools # for extracting all the line segments
# creation of an undirected Graph
G = nx.Graph()
for line in fiona.open("lines.shp"):
   # convert the line segments to Graph edges
   for seg_start, seg_end in itertools.izip(line['geometry']['coordinates'],line['geometry']['coordinates'][1:]):
       G.add_edge(seg_start, seg_end)

The Graph with Nodes(x) and edges

Now, in NetworkX, the degrees of a node are obtained by degree (I use Shapely here but you can save the resulting shapefiles with Fiona and even Networkx)

from shapely.geometry import Point, LineString
for node in G.nodes_iter():
    if G.degree(node) > 2:
        print Point(node) #the nodes
        for edge in G.edges(node):
            print LineString(edge) # the edges (>2)

Result

You can translate easily the Fiona solution in PyQGIS.

Best Answer

Related Solutions

[GIS] find polygon corner coordinates

python – Getting Lines and Nodes Around the Confluence Point in a Network System (Line Shapefile)

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