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What I have:

  • Coordinates of Point A and Point C and therefore vector and azimuth from A to C

What I want:

  • Coordinates of Point B (between A and C) with a distance from A to B of 35.2 meters.
  • Suppose to work in every coordinate system

Some code:

import shapely.geometry as g
line = g.LineString([(0, 0), (0, 100)])  
print line.interpolate(35.2).wkt in UTM 32N but not in geographic coordinates?!

share|improve this question
One meter (measured east to west) at the equator is not the same as one meter at 40 degrees north latitude. What you're trying to do, I wouldn't expect it to work unless you're using a projected coordinate system. – Rayner Sep 27 '12 at 14:34
  • First transform from any CRS into geographic coordinates (lon/lat or WGS 84)
  • Now we can get identifier for UTM zone (called EPSG), for example 32632 (UTM zone 32N).
  • Transform again, this time into UTM
  • Interpolate in meters :-D
  • Transform back
  • Transform back

Some code:

import math
import shapely.geometry as sg

def get_utm_zone(longitude):
    return int((math.floor((longitude + 180.0) / 6.0) + 1) % 60)

def get_epsg(longitude, latitude):
    epsg = 32600
    if latitude < 0.0:
        epsg += 100
    epsg += get_utm_zone(longitude)
    return epsg

def get_point_b_between(point_a, point_c, distance_from_a_to_b):
    Input points as features
    point_a = point_a.geometry().asPoint()
    point_c = point_c.geometry().asPoint()

    crs_src = qgis.utils.iface.activeLayer().crs()
    crs_dest = QgsCoordinateReferenceSystem(4326) # WGS 84
    xform_wgs = QgsCoordinateTransform(crs_src, crs_dest)

    point_a_wgs = xform_wgs.transform(QgsPoint(point_a.x(), point_a.y()))
    point_c_wgs = xform_wgs.transform(QgsPoint(point_c.x(), point_c.y()))

    epsg = get_epsg(point_a_wgs.x(), point_a_wgs.y())

    crs_src = QgsCoordinateReferenceSystem(4326) # WGS 84
    crs_dest = QgsCoordinateReferenceSystem(epsg) # UTM
    xform_utm = QgsCoordinateTransform(crs_src, crs_dest)

    point_a_utm = xform_utm.transform(QgsPoint(point_a_wgs.x(), point_a_wgs.y()))
    point_c_utm = xform_utm.transform(QgsPoint(point_c_wgs.x(), point_c_wgs.y()))

    line = sg.LineString([(point_a_utm.x(),point_a_utm.y()),(point_c_utm.x(),point_c_utm.y())])

    point_b_utm = line.interpolate(distance_from_a_to_b)

    point_b_wgs = xform_utm.transform(QgsPoint(point_b_utm.x, point_b_utm.y), QgsCoordinateTransform.ReverseTransform)

    point_b = xform_wgs.transform(QgsPoint(point_b_wgs.x(), point_b_wgs.y()), QgsCoordinateTransform.ReverseTransform)

    f = QgsFeature()
    return f

Create a point layer with two selected features and enter in console:

a = qgis.utils.iface.activeLayer().selectedFeatures()[0]
c = qgis.utils.iface.activeLayer().selectedFeatures()[1]
b = get_point_b_between(a,c,135.0)

To be optimized…

share|improve this answer
Thank you, qräbnö for your good example. But a small correction is needed: get_utm_zone works fine when longitude <= 174. If we take longitude>174, the function returns 0, not 60. So we can use, for example def get_utm_zone(longitude): zone = int((math.floor((longitude + 180.0) / 6.0) + 1) % 60) if zone == 0: zone = 60 return zone – user24964 Dec 16 '13 at 16:04

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