13

我一直在尝试调试此问题,但无法这样做。我试图找到两个Polygon对象的交集。它大部分时间都有效,但对于以下情况,它会引发以下异常:

P1 area: 13.125721955
P2 area: 1.0
Traceback (most recent call last):
File "geom2d.py", line 235, in <module>
print p1.intersection(p2)
File "/usr/local/lib/python2.7/dist-packages/shapely/geometry/base.py", line 334, in     intersection
return geom_factory(self.impl['intersection'](self, other))
  File "/usr/local/lib/python2.7/dist-packages/shapely/topology.py", line 47, in __call__
    "The operation '%s' produced a null geometry. Likely cause is invalidity of the geometry %s" % (self.fn.__name__, repr(this)))
shapely.geos.TopologicalError: The operation 'GEOSIntersection_r' produced a null     geometry. Likely cause is invalidity of the geometry <shapely.geometry.polygon.Polygon      object at 0x8e5ad6c>

代码如下。

from shapely.geometry import Point,Polygon,MultiPolygon

poly1 = [(35.0041000000000011, -88.1954999999999956), (34.9917999999999978,         -85.6068000000000069), (32.8404000000000025, -85.1756000000000029), (32.2593000000000032, -84.8927000000000049), (32.1535000000000011, -85.0341999999999985), (31.7946999999999989, -85.1358000000000033), (31.5199999999999996, -85.0438000000000045), (31.3384000000000000, -85.0836000000000041), (31.2092999999999989, -85.1069999999999993), (31.0023000000000017, -84.9943999999999988), (30.9953000000000003, -87.6008999999999958), (30.9422999999999995, -87.5926000000000045), (30.8538999999999994, -87.6256000000000057), (30.6744999999999983, -87.4072000000000031), (30.4404000000000003, -87.3687999999999931), (30.1463000000000001, -87.5240000000000009), (30.1545999999999985, -88.3863999999999947), (31.8938999999999986, -88.4742999999999995), (34.8937999999999988, -88.1020999999999930), (34.9478999999999971, -88.1721000000000004), (34.9106999999999985, -88.1461000000000041)]
poly2 = [(34.7998910000000024, -88.2202139999999986), (34.7998910000000024,  -87.2202139999999986), (35.7998910000000024, -87.2202139999999986), (35.7998910000000024, -88.2202139999999986)]

p1 = Polygon(poly1)
p2 = Polygon(poly2)
print 'P1 area:',p1.area
print 'P2 area:',p2.area
print p1.intersection(p2)

由于它打印了两个多边形的面积,我假设多边形的形成是正确的。我还(不知何故)打印了第一个多边形以确保它确实是一个简单的多边形。

谁能解释一下为什么我会得到这个例外?

编辑:我打印了 p1.is_valid ,结果是假的。这里有一些解释。搜索字符串is_valid。它说

一个有效的多边形可能不具有任何重叠的外环或内环。

有人可以解释一下这意味着什么,是否有可能的解决方法?顺便说一句,我还注意到,如果我从 中删除最后一个坐标poly1,则整个过程都有效。也许整个坐标列表使多边形变得复杂

4

2 回答 2

26

如前所述,p1无效。在绘制它时,我注意到右下方有一个小“领结”。我假设您的多边形中不需要它;如果没有,你可以尝试 Shapely 的buffer(0)技巧(记录在 Shapely 手册中)来解决这个问题:

In [382]: p1.is_valid
Out[382]: False

In [383]: p1 = p1.buffer(0)

In [384]: p1.is_valid
Out[384]: True

buffer(0)有以下效果:

前:

在此处输入图像描述

后:

在此处输入图像描述

你现在可以这样做:

print p1.intersection(p2)
POLYGON ((34.9396324323625151 -88.1614025927056559, 34.8937999999999988 -88.1020999999999930, 34.7998910000000024 -88.1137513649788247, 34.7998910000000024 -87.2202139999999986, 34.9994660069532983 -87.2202139999999986, 35.0041000000000011 -88.1954999999999956, 34.9396324323625151 -88.1614025927056559))

请注意,我有一些例子(看起来更像“鸟巢”而不是简单的领结的区域)这不起作用;检查以确保您返回一个Polygon对象而不是一个对象MultiPolygon

于 2012-12-30T19:46:42.157 回答
16

您收到此异常是因为p1不是有效的多边形。

>>> p1.is_valid
False
>>> p2.is_valid
True

文档说:

一个有效的多边形可能不具有任何重叠的外环或内环。

请记住,由于多边形的第一个点和最后一个点形状不同,因此会将第一个点附加到列表的末尾。

>>> list(p1.exterior.coords)
[(35.004100000000001, -88.195499999999996), (34.991799999999998, -85.606800000000007), (32.840400000000002, -85.175600000000003), (32.259300000000003, -84.892700000000005), (32.153500000000001, -85.034199999999998), (31.794699999999999, -85.135800000000003), (31.52, -85.043800000000005), (31.3384, -85.083600000000004), (31.209299999999999, -85.106999999999999), (31.002300000000002, -84.994399999999999), (30.9953, -87.600899999999996), (30.942299999999999, -87.592600000000004), (30.853899999999999, -87.625600000000006), (30.674499999999998, -87.407200000000003), (30.4404, -87.368799999999993), (30.1463, -87.524000000000001), (30.154599999999999, -88.386399999999995), (31.893899999999999, -88.474299999999999), (34.893799999999999, -88.102099999999993), (34.947899999999997, -88.1721), (34.910699999999999, -88.146100000000004), (35.004100000000001, -88.195499999999996)]

你的多边形的外部是一个线性环,它也似乎是无效的:

>>> p1.exterior.type
'LinearRing'
>>> p1.exterior.is_valid
False

您还可以看到,如果要将多边形的外部变成线串,那将很复杂:

>>> l1 = LineString(p1.exterior.coords)
>>> l1.is_simple
False

不知何故,多边形的外部交叉或接触自身。

深入挖掘数据,我们可以在地图上将其可视化:

>>> import cgpolyencode
>>> encoder = cgpolyencode.GPolyEncoder()
>>> encoder.encode((y, x) for x, y in p1.exterior.coords)
{'points': 'svstEzthyOzkAkrxNfecL_fsAznpBcgv@ftSjsZnaeA~yRzst@_~P~mb@vwFzeXfqCvlg@w~Tvj@ra|NfjI{r@ngPfmEf`b@_ti@bvl@_oFbmx@~h]{r@~lgDsurIjdPk|hQgugAaqIntLlgFoaDwfQvsH', 'numLevels': 18, 'zoomFactor': 2, 'levels': 'PPLMKMKGKPNIKLMNPLLKJP'}

如果你把它插入谷歌的折线编码器,你可以看到它是阿拉巴马州。如果您放大阿拉巴马州的左上角,您可以看到其中两条线相互交叉。要解决此问题,您需要删除最后一点或将最后一点与第二点交换到最后一点。

于 2012-10-25T06:05:49.820 回答