2

On Qgis 2.18, I'm trying to write a python code that allows me to create a 1km, 2km, 3km, 4km, 5km buffer around a selected point feature. To avoid having to reproject things, I'm using the reverse haversine formula. My workflow is as follows:

  1. Get lat long of selected point feature.
  2. Use reverse haversine formula to get coordinates of new points 1km away from origin at specified angle intervals.
  3. Join new points to form buffer ring polygon feature.
  4. Repeat for 2km, 3km, 4km, 5km.

    from math import *
    from qgis.core import *
    from PyQt4.QtCore import *
    from PyQt4.QtGui import *        
    
    layer = iface.activeLayer()
    for feat in layer.selectedFeatures():
        geom = feat.geometry()
    
    start_x = geom.asPoint().x()
    start_y = geom.asPoint().y()
    start_lon = start_x * pi / 180
    start_lat = start_y * pi / 180
    sides = 64
    radius = 6378137.0  # meters
    points = []
    distance = [1000, 2000, 3000, 4000, 5000]
    
    for i in range(len(distance)):
        dist = distance[i]
        degrees = 0
        while degrees < 360:
            degrees = degrees + 360/sides
    
            bearing = degrees * pi / 180
    
            end_lat = asin((sin(start_lat) * cos(dist / radius))+ (cos(start_lat) * sin(dist / radius) * cos(bearing)))
            end_lon = start_lon + atan2(sin(bearing) * sin(dist / radius) * cos(start_lat), cos(dist / radius) - (sin(start_lat) * sin(end_lat)))
    
            points.append(QgsPoint(end_lon * 180 / pi, end_lat * 180 / pi))
    
            geometry = QgsGeometry.fromPolygon([points])
    

Two places where I'm stuck:

  1. When I add this script on the Qgis processing toolbox (Add script from file) and try to run it, I get an error which says "iface is not defined".
  2. How can I save each iteration of the distance buffer as a shp file?
4

To return the xy coordinates of a selected feature(s), you could use the following:

layer = iface.activeLayer()
for feat in layer.selectedFeatures():
    geom = feat.geometry()
    print geom.asPoint().x(), geom.asPoint().y()
  • Thank you I've incorporated your snippet into the code above. – Park Sang Jin Mar 14 '18 at 3:52
  • @ParkSangJin - Most welcome! For your first question, use from qgis.utils import iface. Secondly, have a look at this post to get an idea on how to create a memory layer and add features to it. Then use this post to save each memory layer as a shapefile :) – Joseph Mar 14 '18 at 10:03

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