根据一条线和一个值创建一条线弧

我正在尝试重新创建一个Origin-Destination图，如下所示：

我已经设法将数据整理到MSOA到LAD表中，并且可以为原始MSOA之一绘制这样的地图。

一旦允许峰顶区的人们上下班（现在很荒谬）上下班，那距离就很近了。

但是我非常喜欢作者通过“张开”线条来达到的效果。显然，在流量为522和371的情况下，每个通勤者我都不能走一条线路，但是最好能产生成比例的弧线来显示出行的人数。

我以为我可以使用Geometry Generator，但是如果没有循环结构，我似乎就无法取得进展。

巨大的挑战！

该答案主要使用Geometry生成器，并用QGIS 3.2编写。在我第一次建立线条之后，QGIS崩溃了（没有保存！），我几乎放弃了，但是最近使用的表达式列表节省了这一天-使用Geometry生成器的另一个好处

我从两个点集开始，一个来源和三个目的地。目的地标有计数：

然后，我使用以下代码使用虚拟层生成了将源点连接到所有目标的线：

SELECT d.Count_MF, Makeline( s.geometry, d.geometry) 'geometry' 
  FROM Source AS s JOIN Destinations AS d

然后，我使用以下几何生成器表达式来设置线条样式：

 intersection(
   geom_from_wkt( 
     'MULTILINESTRING ((' ||  $x_at( 0)  || ' ' || $y_at( 0)  || ', ' || 
     array_to_string(
       array_remove_at( string_to_array( regexp_replace(
             geom_to_wkt(nodes_to_points( tapered_buffer(  $geometry ,0, "Count_MF" * 200, floor("Count_MF" / 10)),true)),
             '[\\(\\)]','')),0)
     , ') , ('  ||  $x_at( 0)  || ' ' || $y_at( 0)  || ', ' )
    || '))')
    ,buffer( point_n(  $geometry ,1), $length))

这将占用每一行并应用以下步骤：

1. 生成一个锥形缓冲区，该缓冲区从源的零宽度到目标端的目标计数缩放的宽度。缓冲点密度也由目标计数属性缩放。
2. 将缓冲区多边形的顶点转换为点（这可能是多余的），然后导出到WKT，并在转换为数组之前使用正则表达式删除括号。
3. 然后将数组扩展回多线字符串的WKT字符串，插入源点的坐标以及相关格式-这为连接到源点的每个提取顶点创建一条单独的线
4. WKT转换回几何对象，并最终与源点的缓冲区相交，以将其裁剪回目标点所在的圆（请参阅的输出tapered_buffer以了解为什么这样做是必要的）

在编写步骤时，我意识到到数组的转换是不必要的，并且所有WKT操作都可以使用正则表达式来完成。该表达式在下面，如果该tapered_array函数可以替换为其他函数，则也可以在QGIS 2.18中使用。

intersection(
   geom_from_wkt(
    'MULTILINESTRING ((' ||  $x_at( 0)  || ' ' || $y_at( 0)  || ', ' ||
  replace(
    regexp_replace(
      regexp_replace(
        geom_to_wkt(tapered_buffer(  $geometry ,0, "Count_MF" * 200, floor("Count_MF" / 10))),
      '^[^,]*,',''),
    ',[^,]*$',''),
  ',',') , ('  ||  $x_at( 0)  || ' ' || $y_at( 0)  || ', ')
  || '))')
,buffer( point_n(  $geometry ,1), $length))

您的问题使我感到好奇。

该解决方案仅适用于Python控制台中的QGIS 2.x

就像我在评论中提到的那样，我的想法是用Python创建线条弧。

我有两层：

一世。一个持有资本（id，大写）

ii。一个持有城镇（id，城镇，通勤者）

通勤者的数量被“分成钞票”，这些将构成弧线。因此371个通勤者是3x100、1x50、2x10和1x1的组合，总共7张钞票。然后，通过基于规则的样式对线条进行样式设置。

这是代码：

from qgis.gui import *
from qgis.utils import *
from qgis.core import *
from PyQt4 import QtGui, uic
from PyQt4.QtGui import *
from PyQt4.QtCore import *

for lyr in QgsMapLayerRegistry.instance().mapLayers().values():
    if lyr.name() == "capital":
        capital_layer = lyr

for lyr in QgsMapLayerRegistry.instance().mapLayers().values():
    if lyr.name() == "town":
        town_layer = lyr

    # creating the memory layer
d_lyr = QgsVectorLayer('LineString', 'distance', 'memory')
QgsMapLayerRegistry.instance().addMapLayer(d_lyr)
prov = d_lyr.dataProvider()
prov.addAttributes( [ QgsField("id", QVariant.Int), QgsField("banknote",QVariant.Int)])

    # function to create the banknotes
def banknoteOutput(number):
    number_list = []
    number_list.append(number)
    banknote_count = []
    temp_list = []
    banknote_list = []
    for n in number_list:
        total_sum = 0
        total = int(n/100)
        total_sum = total_sum + total
        if total > 0:
            banknote_count.append([total, 100])
        n = n-(total*100)
        total = int(n/50)
        total_sum = total_sum + total
        if total > 0:
            banknote_count.append([total, 50])
        n = n-(total*50)
        total = int(n/10)
        total_sum = total_sum + total
        if total > 0:
            banknote_count.append([total, 10])
        n = n-(total*10)
        total = int(n/5)
        total_sum = total_sum + total
        if total > 0:
            banknote_count.append([total, 5])
        n = n-(total*5)
        total = int(n/1)
        total_sum = total_sum + total
        if total > 0:
            banknote_count.append([total, 1])
        for i in banknote_count:
            temp_list.append(i*i[0])
        banknote_list = [item for sublist in temp_list for item in sublist][1::2]
        return banknote_list

        # creating lines with the amount of banknotes
for capital in capital_layer.getFeatures():
    for town in town_layer.getFeatures():
        commuter_splitting = banknoteOutput(town['commuters'])
        for i,banknote in enumerate(commuter_splitting):
            angle = 2
            distance = QgsDistanceArea()
            distance.measureLine(capital.geometry().asPoint(), town.geometry().asPoint())
            vect = QgsFeature()
            vect.setGeometry(QgsGeometry.fromPolyline([capital.geometry().asPoint(), town.geometry().asPoint()]))
            vect.geometry().rotate(0+(i*angle), capital.geometry().asPoint())
            vect.setAttributes([int(town["id"]), int(banknote)])
            prov.addFeatures([vect])

d_lyr.updateExtents()
d_lyr.triggerRepaint()
d_lyr.updateFields()

结果可能如下所示：

更新：区分男性/女性

产生4个存储层。

from qgis.gui import *
from qgis.utils import *
from qgis.core import *
from PyQt4 import QtGui, uic
from PyQt4.QtGui import *
from PyQt4.QtCore import *

for lyr in QgsMapLayerRegistry.instance().mapLayers().values():
    if lyr.name() == "capital":
        capital_layer = lyr

for lyr in QgsMapLayerRegistry.instance().mapLayers().values():
    if lyr.name() == "town":
        town_layer = lyr

    # function to create the banknotes
def banknoteOutput(number):
    number_list = []
    number_list.append(number)
    banknote_count = []
    temp_list = []
    banknote_list = []
    for n in number_list:
        total_sum = 0
        total = int(n/100)
        total_sum = total_sum + total
        if total > 0:
            banknote_count.append([total, 100])
        n = n-(total*100)
        total = int(n/50)
        total_sum = total_sum + total
        if total > 0:
            banknote_count.append([total, 50])
        n = n-(total*50)
        total = int(n/10)
        total_sum = total_sum + total
        if total > 0:
            banknote_count.append([total, 10])
        n = n-(total*10)
        total = int(n/5)
        total_sum = total_sum + total
        if total > 0:
            banknote_count.append([total, 5])
        n = n-(total*5)
        total = int(n/1)
        total_sum = total_sum + total
        if total > 0:
            banknote_count.append([total, 1])
        for i in banknote_count:
            temp_list.append(i*i[0])
        banknote_list = [item for sublist in temp_list for item in sublist][1::2]
        return banknote_list

    # creating the male memory layer
cmt_male = QgsVectorLayer('LineString', 'Commuters_Male', 'memory')
QgsMapLayerRegistry.instance().addMapLayer(cmt_male)
prov_male = cmt_male.dataProvider()
prov_male.addAttributes( [ QgsField("id", QVariant.Int), QgsField("banknote",QVariant.Int)])

    # creating the male polygon memory layer
cmt_male_polygon = QgsVectorLayer('Polygon', 'Commuters_Male_Poly', 'memory')
QgsMapLayerRegistry.instance().addMapLayer(cmt_male_polygon)
prov_cmt_male_polygon = cmt_male_polygon.dataProvider()
prov_cmt_male_polygon.addAttributes( [ QgsField("id", QVariant.Int), QgsField("banknote",QVariant.Int)])

    # creating lines with the amount of banknotes
for capital in capital_layer.getFeatures():
    for town in town_layer.getFeatures():
        commuter_splitting = banknoteOutput(town['cmt_male'])
        points = []
        for i,banknote in enumerate(reversed(commuter_splitting)):
            angle = 2
            distance = QgsDistanceArea()
            distance.measureLine(capital.geometry().asPoint(), town.geometry().asPoint())
            vect = QgsFeature()
            vect.setGeometry(QgsGeometry.fromPolyline([capital.geometry().asPoint(), town.geometry().asPoint()]))
            vect.geometry().rotate(0+(i*angle), capital.geometry().asPoint())
            vect.setAttributes([int(town["id"]), int(banknote)])
            points.append(vect.geometry().asPolyline()[1])
            prov_male.addFeatures([vect])
        polygon = QgsFeature()
        points.insert(0,capital.geometry().asPoint())
        points.insert(len(points),capital.geometry().asPoint())
        polygon.setGeometry(QgsGeometry.fromPolygon([points]))
        polygon.setAttributes([1, 2])
        prov_cmt_male_polygon.addFeatures([polygon])

cmt_male.updateExtents()
cmt_male.triggerRepaint()
cmt_male.updateFields()
cmt_male_polygon.updateExtents()
cmt_male_polygon.triggerRepaint()
cmt_male_polygon.updateFields()

    # creating the female memory layer
cmt_female = QgsVectorLayer('LineString', 'Commuters_Female', 'memory')
QgsMapLayerRegistry.instance().addMapLayer(cmt_female)
prov_female = cmt_female.dataProvider()
prov_female.addAttributes( [ QgsField("id", QVariant.Int), QgsField("banknote",QVariant.Int)])

    # creating the female polygon memory layer
cmt_female_polygon = QgsVectorLayer('Polygon', 'Commuters_Female_Poly', 'memory')
QgsMapLayerRegistry.instance().addMapLayer(cmt_female_polygon)
prov_cmt_female_polygon = cmt_female_polygon.dataProvider()
prov_cmt_female_polygon.addAttributes( [ QgsField("id", QVariant.Int), QgsField("banknote",QVariant.Int)])

    # creating lines with the amount of banknotes
for capital in capital_layer.getFeatures():
    for town in town_layer.getFeatures():
        commuter_splitting = banknoteOutput(town['cmt_female'])
        points = []
        for i,banknote in enumerate(commuter_splitting):
            angle = 2
            distance = QgsDistanceArea()
            distance.measureLine(capital.geometry().asPoint(), town.geometry().asPoint())
            vect = QgsFeature()
            vect.setGeometry(QgsGeometry.fromPolyline([capital.geometry().asPoint(), town.geometry().asPoint()]))
            vect.geometry().rotate(-angle-(i*angle), capital.geometry().asPoint())
            vect.setAttributes([int(town["id"]), int(banknote)])
            points.append(vect.geometry().asPolyline()[1])
            prov_female.addFeatures([vect])
        polygon = QgsFeature()
        points.insert(0,capital.geometry().asPoint())
        points.insert(len(points),capital.geometry().asPoint())
        polygon.setGeometry(QgsGeometry.fromPolygon([points]))
        polygon.setAttributes([1, 2])
        prov_cmt_female_polygon.addFeatures([polygon])

cmt_female.updateExtents()
cmt_female.triggerRepaint()
cmt_female.updateFields()
cmt_female_polygon.updateExtents()
cmt_female_polygon.triggerRepaint()
cmt_female_polygon.updateFields()

结果可能如下所示：

从制图角度看，一件不理想的事情：

乍一看，弧线的大小可能会令人不快，因为较大的弧线可能代表更多的通勤者。通勤者较少的弧（289名通勤者/ 11张钞票）比通勤者较多的弧（311名通勤者/ 5张钞票）更大。