layerslineqgisvector
Is it possible to show only every other contour line? And how do you do that?
(I'm new to QGIS Lisboa 1.8.0, but even newer to QGIS Dufour 2.0.1, but have used ArcGIS a bit a while ago, and also some ArcView 3.x earlier. But I'm now using QGIS Dufour 2.0.1. for this).
Beyond my comments above. A good practice for installing Python modules is to use pip. Though, for your initial use, I would forgo the setup of a virtualenv at this time as that may complicate your use of those modules from within QGIS (though certainly doable).
As you will be installing pip and Python modules into Mac system areas (/usr/local for pip, modules in /Library/Python/2.7/site-packages/), you will need to preface any install commands in Terminal with sudo:
sudo pip install Shapely
For matplotlib, consider using the Kynchaos.com installer for that. I'm running 10.7.4 and QGIS (1.8 and master branch build) has no problem finding matplotlib after using said installer.
from shapely.geometry import Point, LineString, mapping
import fiona
import math
def azimuth(point1, point2):
'''azimuth between 2 shapely points (interval 0 - 180°, for my work)'''
angle = math.atan2(point2.x - point1.x, point2.y - point1.y)
return math.degrees(angle)if angle>0 else math.degrees(angle) + 180
def pair(list):
'''Iterate over pairs in a list '''
for i in range(1, len(list)):
yield list[i-1], list[i]
with fiona.collection('testline.shp', 'r') as input:
# copy of the input schema'
schema = input.schema.copy()
# creation of a new field for storing azimuth
schema['properties']['azimuth'] = 'int'
# copy of the original shapefile with the field azimuth to a new shapefile
with fiona.collection('testline_azim.shp', 'w', 'ESRI Shapefile', schema) as output:
for line in input:
# use of pair function to extract the line segments
for seg_start, seg_end in pair(line['geometry']['coordinates']):
line_start = Point(seg_start)
line_end = Point(seg_end)
segment = LineString([line_start.coords[0],line_end.coords[0]])
elem = {}
elem['properties'] = line['properties']
elem['properties']['azimuth'] = azimuth(line_start, line_end)
elem['geometry'] = mapping(segment)
output.write(elem)
Result
image from PortailSIG
With GRASS:
see Angle between segments of a polyline or azimuth of lines with v.to.db?
Update
in the Python console of QGIS with only PyQGIS
from PyQt4.QtCore import *
import math
def select_all(layer):
layer.select([])
layer.setSelectedFeatures([obj.id() for obj in layer])
# theoretical azimuth function but you can use `point1.azimuth(point2)`, look below
def azimut(point1, point2):
'''azimuth between 2 QGIS points ->must be adapted to 0-360°'''
angle = math.atan2(point2.x() - point1.x(), point2.y() - point1.y())
return math.degrees(angle)
def pair(list):
'''Iterate over pairs in a list '''
for i in range(1, len(list)):
yield list[i-1], list[i]
mylayer = qgis.utils.iface.activeLayer()
# select all the elements of the layer
select_all(mylayer)
# Memory layer
v_layer = QgsVectorLayer("LineString", "azimuth_lines", "memory")
pr = v_layer.dataProvider()
pr.addAttributes( [ QgsField("azimuth", QVariant.Int), QgsField("az_pt1pt2",QVariant.Int),QgsField("az_pt2-1",QVariant.Int)])
for elem in mylayer.selectedFeatures():
line = elem.geometry().asPolyline()
for seg_start, seg_end in pair(line):
line_start = QgsPoint(seg_start)
line_end = QgsPoint(seg_end)
seg = QgsFeature()
seg.addAttribute(0, QVariant(int(azimut(line_start,line_end))))
# with the functions of PyQGIS
seg.addAttribute(1, QVariant(int(line_start.azimuth(line_end))))
seg.addAttribute(2, QVariant(int(line_end.azimuth(line_start))))
seg.setGeometry(QgsGeometry.fromPolyline([line_start, line_end]))
pr.addFeatures( [ seg ] )
v_layer.updateExtents()
v_layer.updateFieldMap()
QgsMapLayerRegistry.instance().addMapLayers([v_layer])
Result
and 360 -159 = 201 -180 = 21
Best Answer
If you have a contour lines layer with associated elevation attribute, you can query the contour lines in reference to the elevation values. More in detail, you could define a rule-based style in
Vector properties --> Stylein order to query the elevation values you are interested in.For instance, suppose you have contour lines with equidistance
5 mand you want to show only contour lines with equidistance10 m. A possible query in a rule-based style could be:Only the contour lines which match this expression (
elevvalue which ends with0) will be filtered and consequently shown. Instead, contour lines withelevvalue which ends with5will be hidden (but they are always there).