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[FEATURE][processing] New algorithm to extract specific nodes
This algorithm allows you to extract specific nodes from geometries. Eg you can extract the first or last node in the geometry. The algorithm accepts a comma separated list of node indices to extract, eg 0 = first node, 1 = second node, etc. Negative indices can be used to extract nodes from the end of the geometry. Eg -1 = last node, -2 = second last node.
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‎python/plugins/processing/algs/help/qgis.yaml‎

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@@ -177,6 +177,13 @@ qgis:extractnodes: >
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Additional fields are added to the nodes indicating the node index (beginning at 0), distance along original geometry and bisector angle of node for original geometry.
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qgis:extractspecificnodes: >
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This algorithm takes a line or polygon layer and generates a point layer with points representing specific nodes in the input lines or polygons. For instance, this algorithm can be used to extract the first or last nodes in the geometry. The attributes associated to each point are the same ones associated to the line or polygon that the point belongs to.
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The node indices parameter accepts a comma separated string specifying the indices of the nodes to extract. The first node corresponds to an index of 0, the second node has an index of 1, etc. Negative indices can be used to find nodes at the end of the geometry, eg an index of -1 corresponds to the last node, -2 corresponds to the second last node, etc.
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Additional fields are added to the nodes indicating the specific node position (eg 0, -1, etc), the original node index, distance along the original geometry and bisector angle of node for the original geometry.
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qgis:fieldcalculator: >
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This algorithm computes a new vector layer with the same features of the input layer, but with an additional attribute. The values of this new attribute are computed from each feature using a mathematical formula, based on te properties and attributes of the feature.
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‎python/plugins/processing/algs/qgis/ExtractSpecificNodes.py‎

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# -*- coding: utf-8 -*-
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"""
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***************************************************************************
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ExtractSpecificNodes.py
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--------------------
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Date : October 2016
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Copyright : (C) 2016 by Nyall Dawson
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Email : nyall dot dawson at gmail dot com
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***************************************************************************
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* *
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* This program is free software; you can redistribute it and/or modify *
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* it under the terms of the GNU General Public License as published by *
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* the Free Software Foundation; either version 2 of the License, or *
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* (at your option) any later version. *
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* *
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***************************************************************************
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"""
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__author__ = 'Nyall Dawson'
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__date__ = 'October 2016'
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__copyright__ = '(C) 2016, Nyall Dawson'
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# This will get replaced with a git SHA1 when you do a git archive323
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__revision__ = '$Format:%H$'
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import math
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from processing.core.GeoAlgorithm import GeoAlgorithm
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from processing.core.GeoAlgorithmExecutionException import GeoAlgorithmExecutionException
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from processing.core.parameters import ParameterVector, ParameterString
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from processing.core.outputs import OutputVector
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from processing.tools import dataobjects, vector
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from qgis.core import QgsWkbTypes, QgsFeature, QgsGeometry, QgsField
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from qgis.PyQt.QtCore import QVariant
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class ExtractSpecificNodes(GeoAlgorithm):
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INPUT_LAYER = 'INPUT_LAYER'
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OUTPUT_LAYER = 'OUTPUT_LAYER'
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NODES = 'NODES'
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def defineCharacteristics(self):
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self.name, self.i18n_name = self.trAlgorithm('Extract specific nodes')
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self.group, self.i18n_group = self.trAlgorithm('Vector geometry tools')
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self.addParameter(ParameterVector(self.INPUT_LAYER,
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self.tr('Input layer'), [dataobjects.TYPE_VECTOR_ANY]))
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self.addParameter(ParameterString(self.NODES,
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self.tr('Node indices'), default='0'))
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self.addOutput(OutputVector(self.OUTPUT_LAYER, self.tr('Nodes'), datatype=[dataobjects.TYPE_VECTOR_POINT]))
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def processAlgorithm(self, progress):
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layer = dataobjects.getObjectFromUri(
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self.getParameterValue(self.INPUT_LAYER))
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fields = layer.fields()
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fields.append(QgsField('node_pos', QVariant.Int))
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fields.append(QgsField('node_index', QVariant.Int))
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fields.append(QgsField('distance', QVariant.Double))
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fields.append(QgsField('angle', QVariant.Double))
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writer = self.getOutputFromName(
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self.OUTPUT_LAYER).getVectorWriter(
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fields,
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QgsWkbTypes.Point,
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layer.crs())
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node_indices_string = self.getParameterValue(self.NODES)
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indices = []
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for node in node_indices_string.split(','):
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try:
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indices.append(int(node))
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except:
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raise GeoAlgorithmExecutionException(
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self.tr('\'{}\' is not a valid node index').format(node))
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features = vector.features(layer)
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total = 100.0 / len(features)
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for current, f in enumerate(features):
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input_geometry = f.geometry()
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if not input_geometry:
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writer.addFeature(f)
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else:
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total_nodes = input_geometry.geometry().nCoordinates()
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for node in indices:
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if node < 0:
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node_index = total_nodes + node
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else:
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node_index = node
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if node_index < 0 or node_index >= total_nodes:
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continue
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distance = input_geometry.distanceToVertex(node_index)
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angle = math.degrees(input_geometry.angleAtVertex(node_index))
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output_feature = QgsFeature()
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attrs = f.attributes()
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attrs.append(node)
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attrs.append(node_index)
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attrs.append(distance)
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attrs.append(angle)
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output_feature.setAttributes(attrs)
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point = input_geometry.vertexAt(node_index)
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output_feature.setGeometry(QgsGeometry.fromPoint(point))
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writer.addFeature(output_feature)
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progress.setPercentage(int(current * total))
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del writer

‎python/plugins/processing/algs/qgis/QGISAlgorithmProvider.py‎

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@@ -172,6 +172,7 @@
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from .ZonalStatisticsQgis import ZonalStatisticsQgis
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from .RemoveNullGeometry import RemoveNullGeometry
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from .ExtendLines import ExtendLines
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from .ExtractSpecificNodes import ExtractSpecificNodes
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pluginPath = os.path.normpath(os.path.join(
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os.path.split(os.path.dirname(__file__))[0], os.pardir))
@@ -233,7 +234,8 @@ def __init__(self):
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ReliefAuto(), ZonalStatisticsQgis(),
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IdwInterpolationZValue(), IdwInterpolationAttribute(),
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TinInterpolationZValue(), TinInterpolationAttribute(),
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RemoveNullGeometry(), ExtractByExpression(), ExtendLines()
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RemoveNullGeometry(), ExtractByExpression(), ExtendLines(),
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ExtractSpecificNodes()
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]
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if hasMatplotlib:

‎python/plugins/processing/tests/testdata/expected/extract_specific_nodes_lines.gfs‎

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<GMLFeatureClassList>
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<GMLFeatureClass>
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<Name>extract_specific_nodes_lines</Name>
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<ElementPath>extract_specific_nodes_lines</ElementPath>
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<GeometryType>1</GeometryType>
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<SRSName>EPSG:4326</SRSName>
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<DatasetSpecificInfo>
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<FeatureCount>21</FeatureCount>
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<ExtentXMin>-1.00000</ExtentXMin>
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<ExtentXMax>11.00000</ExtentXMax>
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<ExtentYMin>-3.00000</ExtentYMin>
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<ExtentYMax>5.00000</ExtentYMax>
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</DatasetSpecificInfo>
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<PropertyDefn>
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<Name>node_pos</Name>
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<ElementPath>node_pos</ElementPath>
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<Type>Integer</Type>
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</PropertyDefn>
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<PropertyDefn>
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<Name>node_index</Name>
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<ElementPath>node_index</ElementPath>
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<Type>Integer</Type>
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</PropertyDefn>
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<PropertyDefn>
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<Name>distance</Name>
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<ElementPath>distance</ElementPath>
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<Type>Real</Type>
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</PropertyDefn>
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<PropertyDefn>
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<Name>angle</Name>
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<ElementPath>angle</ElementPath>
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<Type>Real</Type>
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</PropertyDefn>
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</GMLFeatureClass>
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</GMLFeatureClassList>

‎python/plugins/processing/tests/testdata/expected/extract_specific_nodes_lines.gml‎

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<?xml version="1.0" encoding="utf-8" ?>
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<ogr:FeatureCollection
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xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
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xsi:schemaLocation=""
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xmlns:ogr="http://ogr.maptools.org/"
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xmlns:gml="http://www.opengis.net/gml">
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<gml:boundedBy>
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<gml:Box>
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<gml:coord><gml:X>-1</gml:X><gml:Y>-3</gml:Y></gml:coord>
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<gml:coord><gml:X>11</gml:X><gml:Y>5</gml:Y></gml:coord>
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</gml:Box>
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</gml:boundedBy>
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<gml:featureMember>
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<ogr:extract_specific_nodes_lines fid="lines.1">
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<ogr:geometryProperty><gml:Point srsName="EPSG:4326"><gml:coordinates>6,2</gml:coordinates></gml:Point></ogr:geometryProperty>
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<ogr:node_pos>0</ogr:node_pos>
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<ogr:node_index>0</ogr:node_index>
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<ogr:distance>0</ogr:distance>
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<ogr:angle>90</ogr:angle>
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</ogr:extract_specific_nodes_lines>
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</gml:featureMember>
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<gml:featureMember>
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<ogr:extract_specific_nodes_lines fid="lines.2">
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<ogr:geometryProperty><gml:Point srsName="EPSG:4326"><gml:coordinates>11,5</gml:coordinates></gml:Point></ogr:geometryProperty>
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<ogr:node_pos>-1</ogr:node_pos>
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<ogr:node_index>3</ogr:node_index>
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<ogr:distance>6.82842712474619</ogr:distance>
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<ogr:angle>45</ogr:angle>
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</ogr:extract_specific_nodes_lines>
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</gml:featureMember>
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<gml:featureMember>
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<ogr:extract_specific_nodes_lines fid="lines.3">
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<ogr:geometryProperty><gml:Point srsName="EPSG:4326"><gml:coordinates>9,3</gml:coordinates></gml:Point></ogr:geometryProperty>
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<ogr:node_pos>2</ogr:node_pos>
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<ogr:node_index>2</ogr:node_index>
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<ogr:distance>4</ogr:distance>
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<ogr:angle>22.5</ogr:angle>
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</ogr:extract_specific_nodes_lines>
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</gml:featureMember>
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<gml:featureMember>
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<ogr:extract_specific_nodes_lines fid="lines.4">
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<ogr:geometryProperty><gml:Point srsName="EPSG:4326"><gml:coordinates>9,3</gml:coordinates></gml:Point></ogr:geometryProperty>
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<ogr:node_pos>-2</ogr:node_pos>
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<ogr:node_index>2</ogr:node_index>
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<ogr:distance>4</ogr:distance>
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<ogr:angle>22.5</ogr:angle>
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</ogr:extract_specific_nodes_lines>
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</gml:featureMember>
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<gml:featureMember>
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<ogr:extract_specific_nodes_lines fid="lines.5">
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<ogr:geometryProperty><gml:Point srsName="EPSG:4326"><gml:coordinates>-1,-1</gml:coordinates></gml:Point></ogr:geometryProperty>
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<ogr:node_pos>0</ogr:node_pos>
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<ogr:node_index>0</ogr:node_index>
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<ogr:distance>0</ogr:distance>
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<ogr:angle>90</ogr:angle>
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</ogr:extract_specific_nodes_lines>
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</gml:featureMember>
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<gml:featureMember>
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<ogr:extract_specific_nodes_lines fid="lines.6">
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<ogr:geometryProperty><gml:Point srsName="EPSG:4326"><gml:coordinates>1,-1</gml:coordinates></gml:Point></ogr:geometryProperty>
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<ogr:node_pos>-1</ogr:node_pos>
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<ogr:node_index>1</ogr:node_index>
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<ogr:distance>2</ogr:distance>
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<ogr:angle>90</ogr:angle>
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</ogr:extract_specific_nodes_lines>
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</gml:featureMember>
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<gml:featureMember>
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<ogr:extract_specific_nodes_lines fid="lines.7">
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<ogr:geometryProperty><gml:Point srsName="EPSG:4326"><gml:coordinates>-1,-1</gml:coordinates></gml:Point></ogr:geometryProperty>
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<ogr:node_pos>-2</ogr:node_pos>
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<ogr:node_index>0</ogr:node_index>
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<ogr:distance>0</ogr:distance>
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<ogr:angle>90</ogr:angle>
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</ogr:extract_specific_nodes_lines>
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</gml:featureMember>
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<gml:featureMember>
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<ogr:extract_specific_nodes_lines fid="lines.8">
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<ogr:geometryProperty><gml:Point srsName="EPSG:4326"><gml:coordinates>2,0</gml:coordinates></gml:Point></ogr:geometryProperty>
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<ogr:node_pos>0</ogr:node_pos>
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<ogr:node_index>0</ogr:node_index>
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<ogr:distance>0</ogr:distance>
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<ogr:angle>0</ogr:angle>
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</ogr:extract_specific_nodes_lines>
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</gml:featureMember>
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<gml:featureMember>
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<ogr:extract_specific_nodes_lines fid="lines.9">
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<ogr:geometryProperty><gml:Point srsName="EPSG:4326"><gml:coordinates>3,3</gml:coordinates></gml:Point></ogr:geometryProperty>
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<ogr:node_pos>-1</ogr:node_pos>
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<ogr:node_index>3</ogr:node_index>
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<ogr:distance>4</ogr:distance>
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<ogr:angle>0</ogr:angle>
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</ogr:extract_specific_nodes_lines>
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</gml:featureMember>
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<gml:featureMember>
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<ogr:extract_specific_nodes_lines fid="lines.10">
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<ogr:geometryProperty><gml:Point srsName="EPSG:4326"><gml:coordinates>3,2</gml:coordinates></gml:Point></ogr:geometryProperty>
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<ogr:node_pos>2</ogr:node_pos>
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<ogr:node_index>2</ogr:node_index>
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<ogr:distance>3</ogr:distance>
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<ogr:angle>45</ogr:angle>
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</ogr:extract_specific_nodes_lines>
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</gml:featureMember>
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<gml:featureMember>
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<ogr:extract_specific_nodes_lines fid="lines.11">
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<ogr:geometryProperty><gml:Point srsName="EPSG:4326"><gml:coordinates>3,2</gml:coordinates></gml:Point></ogr:geometryProperty>
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<ogr:node_pos>-2</ogr:node_pos>
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<ogr:node_index>2</ogr:node_index>
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<ogr:distance>3</ogr:distance>
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<ogr:angle>45</ogr:angle>
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</ogr:extract_specific_nodes_lines>
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</gml:featureMember>
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<gml:featureMember>
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<ogr:extract_specific_nodes_lines fid="lines.12">
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<ogr:geometryProperty><gml:Point srsName="EPSG:4326"><gml:coordinates>3,1</gml:coordinates></gml:Point></ogr:geometryProperty>
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<ogr:node_pos>0</ogr:node_pos>
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<ogr:node_index>0</ogr:node_index>
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<ogr:distance>0</ogr:distance>
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<ogr:angle>90</ogr:angle>
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</ogr:extract_specific_nodes_lines>
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</gml:featureMember>
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<gml:featureMember>
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<ogr:extract_specific_nodes_lines fid="lines.13">
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<ogr:geometryProperty><gml:Point srsName="EPSG:4326"><gml:coordinates>5,1</gml:coordinates></gml:Point></ogr:geometryProperty>
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<ogr:node_pos>-1</ogr:node_pos>
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<ogr:node_index>1</ogr:node_index>
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<ogr:distance>2</ogr:distance>
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<ogr:angle>90</ogr:angle>
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</ogr:extract_specific_nodes_lines>
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</gml:featureMember>
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<gml:featureMember>
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<ogr:extract_specific_nodes_lines fid="lines.14">
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<ogr:geometryProperty><gml:Point srsName="EPSG:4326"><gml:coordinates>3,1</gml:coordinates></gml:Point></ogr:geometryProperty>
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<ogr:node_pos>-2</ogr:node_pos>
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<ogr:node_index>0</ogr:node_index>
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<ogr:distance>0</ogr:distance>
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<ogr:angle>90</ogr:angle>
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</ogr:extract_specific_nodes_lines>
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</gml:featureMember>
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<gml:featureMember>
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<ogr:extract_specific_nodes_lines fid="lines.15">
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<ogr:geometryProperty><gml:Point srsName="EPSG:4326"><gml:coordinates>7,-3</gml:coordinates></gml:Point></ogr:geometryProperty>
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<ogr:node_pos>0</ogr:node_pos>
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<ogr:node_index>0</ogr:node_index>
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<ogr:distance>0</ogr:distance>
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<ogr:angle>90</ogr:angle>
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</ogr:extract_specific_nodes_lines>
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</gml:featureMember>
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<gml:featureMember>
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<ogr:extract_specific_nodes_lines fid="lines.16">
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<ogr:geometryProperty><gml:Point srsName="EPSG:4326"><gml:coordinates>10,-3</gml:coordinates></gml:Point></ogr:geometryProperty>
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<ogr:node_pos>-1</ogr:node_pos>
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<ogr:node_index>1</ogr:node_index>
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<ogr:distance>3</ogr:distance>
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<ogr:angle>90</ogr:angle>
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</ogr:extract_specific_nodes_lines>
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</gml:featureMember>
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<gml:featureMember>
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<ogr:extract_specific_nodes_lines fid="lines.17">
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<ogr:geometryProperty><gml:Point srsName="EPSG:4326"><gml:coordinates>7,-3</gml:coordinates></gml:Point></ogr:geometryProperty>
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<ogr:node_pos>-2</ogr:node_pos>
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<ogr:node_index>0</ogr:node_index>
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<ogr:distance>0</ogr:distance>
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<ogr:angle>90</ogr:angle>
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</ogr:extract_specific_nodes_lines>
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</gml:featureMember>
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<gml:featureMember>
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<ogr:extract_specific_nodes_lines fid="lines.18">
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<ogr:geometryProperty><gml:Point srsName="EPSG:4326"><gml:coordinates>6,-3</gml:coordinates></gml:Point></ogr:geometryProperty>
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<ogr:node_pos>0</ogr:node_pos>
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<ogr:node_index>0</ogr:node_index>
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<ogr:distance>0</ogr:distance>
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<ogr:angle>45</ogr:angle>
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</ogr:extract_specific_nodes_lines>
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</gml:featureMember>
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<gml:featureMember>
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<ogr:extract_specific_nodes_lines fid="lines.19">
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<ogr:geometryProperty><gml:Point srsName="EPSG:4326"><gml:coordinates>10,1</gml:coordinates></gml:Point></ogr:geometryProperty>
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<ogr:node_pos>-1</ogr:node_pos>
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<ogr:node_index>1</ogr:node_index>
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<ogr:distance>5.65685424949238</ogr:distance>
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<ogr:angle>45</ogr:angle>
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</ogr:extract_specific_nodes_lines>
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</gml:featureMember>
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<gml:featureMember>
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<ogr:extract_specific_nodes_lines fid="lines.20">
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<ogr:geometryProperty><gml:Point srsName="EPSG:4326"><gml:coordinates>6,-3</gml:coordinates></gml:Point></ogr:geometryProperty>
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<ogr:node_pos>-2</ogr:node_pos>
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<ogr:node_index>0</ogr:node_index>
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<ogr:distance>0</ogr:distance>
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<ogr:angle>45</ogr:angle>
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</ogr:extract_specific_nodes_lines>
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</gml:featureMember>
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<gml:featureMember>
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<ogr:extract_specific_nodes_lines fid="lines.21">
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</ogr:extract_specific_nodes_lines>
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</gml:featureMember>
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</ogr:FeatureCollection>

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