I have a polygon, imported from a shapefile in QGIS. I would like transform it to a linestring instead so I can save it as a GPX file later.
How can I transform a polygon to a linestring in QGIS?
-
3simply cutting a polygonal object won't transform it to line. you would need to do a poly to line conversion. I can tell you how to do that in arcmap. I'm sure it is just as easy in qgis.– Brad NesomCommented Oct 17, 2011 at 14:45
Add a comment
|
4 Answers
Assuming you're using QGIS >= 1.6.0. There is a geoalgorithm called "Polygons to lines" accessible via Vector > Geometry Tools > Polygons to Lines (otherwise search in the Processing Toolbox Ctrl+Alt+T), which will create a new shapefile with all the attributes of the original.
For PyQGIS it is "native:polygonstolines"
>>> processing.algorithmHelp("native:polygonstolines")
Polygons to lines (native:polygonstolines)
Converts polygons to lines.
Converts polygons to lines
----------------
Input parameters
----------------
INPUT: Input layer
Parameter type: QgsProcessingParameterFeatureSource
Accepted data types:
- str: layer ID
- str: layer name
- str: layer source
- QgsProcessingFeatureSourceDefinition
- QgsProperty
- QgsVectorLayer
OUTPUT: Lines
Parameter type: QgsProcessingParameterFeatureSink
Accepted data types:
- str: destination vector file, e.g. 'd:/test.shp'
- str: 'memory:' to store result in temporary memory layer
- str: using vector provider ID prefix and destination URI, e.g. 'postgres:…' to store result in PostGIS table
- QgsProcessingOutputLayerDefinition
- QgsProperty
----------------
Outputs
----------------
OUTPUT: <QgsProcessingOutputVectorLayer>
Lines
answered Oct 17, 2011 at 16:02
Another option is the SAGA's "Convert Polygons to Lines" module (search in the Processing Toolbox Ctrl+Alt+T).
Input:
Working window:
Output:
For PyQGIS it is "saga:convertpolygonstolines"
>>> processing.algorithmHelp("saga:convertpolygonstolines")
Convert polygons to lines (saga:convertpolygonstolines)
----------------
Input parameters
----------------
POLYGONS: Polygons
Parameter type: QgsProcessingParameterFeatureSource
Accepted data types:
- str: layer ID
- str: layer name
- str: layer source
- QgsProcessingFeatureSourceDefinition
- QgsProperty
- QgsVectorLayer
LINES: Lines
Parameter type: QgsProcessingParameterVectorDestination
Accepted data types:
- str
- QgsProperty
- QgsProcessingOutputLayerDefinition
----------------
Outputs
----------------
LINES: <QgsProcessingOutputVectorLayer>
Lines
Another option is the GRASS's v.to.lines (search in the Processing Toolbox Ctrl+Alt+T).
Input:
Working window:
Output:
For PyQGIS it is "grass7:v.to.lines"
>>> processing.algorithmHelp("grass7:v.to.lines")
v.to.lines (grass7:v.to.lines)
Converts vector polygons or points to lines.
----------------
Input parameters
----------------
input: Name of input vector map
Parameter type: QgsProcessingParameterFeatureSource
Accepted data types:
- str: layer ID
- str: layer name
- str: layer source
- QgsProcessingFeatureSourceDefinition
- QgsProperty
- QgsVectorLayer
method: Method used for point interpolation
Parameter type: QgsProcessingParameterEnum
Available values:
- 0: delaunay
Accepted data types:
- int
- str: as string representation of int, e.g. '1'
- QgsProperty
output: Lines
Parameter type: QgsProcessingParameterVectorDestination
Accepted data types:
- str
- QgsProperty
- QgsProcessingOutputLayerDefinition
GRASS_REGION_PARAMETER: GRASS GIS 7 region extent
Parameter type: QgsProcessingParameterExtent
Accepted data types:
- str: as comma delimited list of x min, x max, y min, y max. E.g. '4,10,101,105'
- str: layer ID. Extent of layer is used.
- str: layer name. Extent of layer is used.
- str: layer source. Extent of layer is used.
- QgsMapLayer: Extent of layer is used
- QgsProcessingFeatureSourceDefinition: Extent of source is used
- QgsProperty
- QgsRectangle
- QgsReferencedRectangle
- QgsGeometry: bounding box of geometry is used
GRASS_SNAP_TOLERANCE_PARAMETER: v.in.ogr snap tolerance (-1 = no snap)
Parameter type: QgsProcessingParameterNumber
Accepted data types:
- int
- float
- QgsProperty
GRASS_MIN_AREA_PARAMETER: v.in.ogr min area
Parameter type: QgsProcessingParameterNumber
Accepted data types:
- int
- float
- QgsProperty
GRASS_OUTPUT_TYPE_PARAMETER: v.out.ogr output type
Parameter type: QgsProcessingParameterEnum
Available values:
- 0: auto
- 1: point
- 2: line
- 3: area
Accepted data types:
- int
- str: as string representation of int, e.g. '1'
- QgsProperty
GRASS_VECTOR_DSCO: v.out.ogr output data source options (dsco)
Parameter type: QgsProcessingParameterString
Accepted data types:
- str
- QgsProperty
GRASS_VECTOR_LCO: v.out.ogr output layer options (lco)
Parameter type: QgsProcessingParameterString
Accepted data types:
- str
- QgsProperty
GRASS_VECTOR_EXPORT_NOCAT: Also export features without category (not labeled). Otherwise only features with category are exported
Parameter type: QgsProcessingParameterBoolean
Accepted data types:
- bool
- int
- str
- QgsProperty
----------------
Outputs
----------------
output: <QgsProcessingOutputVectorLayer>
Lines
Another option is using the boundary($geometry) expression in the "Geometry by expression" tool (search in the Processing Toolbox Ctrl+Alt+T).
Input:
Working window:
Output:
For PyQGIS it is "native:geometrybyexpression"
>>> processing.algorithmHelp("native:geometrybyexpression")
Geometry by expression (native:geometrybyexpression)
This algorithm updates existing geometries (or creates new geometries) for input features by use of a QGIS expression. This allows complex geometry modifications which can utilize all the flexibility of the QGIS expression engine to manipulate and create geometries for output features.
For help with QGIS expression functions, see the inbuilt help for specific functions which is available in the expression builder.
----------------
Input parameters
----------------
INPUT: Input layer
Parameter type: QgsProcessingParameterFeatureSource
Accepted data types:
- str: layer ID
- str: layer name
- str: layer source
- QgsProcessingFeatureSourceDefinition
- QgsProperty
- QgsVectorLayer
OUTPUT_GEOMETRY: Output geometry type
Parameter type: QgsProcessingParameterEnum
Available values:
- 0: Polygon
- 1: Line
- 2: Point
Accepted data types:
- int
- str: as string representation of int, e.g. '1'
- QgsProperty
WITH_Z: Output geometry has z dimension
Parameter type: QgsProcessingParameterBoolean
Accepted data types:
- bool
- int
- str
- QgsProperty
WITH_M: Output geometry has m values
Parameter type: QgsProcessingParameterBoolean
Accepted data types:
- bool
- int
- str
- QgsProperty
EXPRESSION: Geometry expression
Parameter type: QgsProcessingParameterExpression
Accepted data types:
- str
- QgsProperty
OUTPUT: Modified geometry
Parameter type: QgsProcessingParameterFeatureSink
Accepted data types:
- str: destination vector file, e.g. 'd:/test.shp'
- str: 'memory:' to store result in temporary memory layer
- str: using vector provider ID prefix and destination URI, e.g. 'postgres:…' to store result in PostGIS table
- QgsProcessingOutputLayerDefinition
- QgsProperty
----------------
Outputs
----------------
OUTPUT: <QgsProcessingOutputVectorLayer>
Modified geometry