尽管有可见的相交，相交层仍会返回空集合答案

【问题标题】：Intersecting Layers Returns Empty Collection Despite Visible Intersection尽管有可见的相交，相交层仍会返回空集合
【发布时间】：2013-08-03 01:53:39
【问题描述】：

当我在 OpenLayers 中绘制的多边形与 postgis 数据库层相交时，似乎得到了不正确的结果。

Intersection 在某些图层上可以正常工作。例如，如果我将一个三角形与代表农田的多边形图层相交，我会得到以下结果：

我的应用生成上述结果的查询是：

SELECT ST_AsText(ST_Intersection(%(geometries_0)s::geometry, %(geometry)s::geometry))

geometries_0 是我的三角形：

POLYGON((-104.84928345939991 40.518951354186285,-104.82319093011056 40.51953858115158,-104.83700967095314 40.50707521626648,-104.84928345939991 40.518951354186285))

而geometry是我的农田层，众所周知的文字：

MULTIPOLYGON(((-104.841309611298 40.5075331998226,-104.84173356681 40.5069932245841,-104.842041204329 40.50640946683,-104.842224948796 40.5057962996657,-104.842280275816 40.5051688207073,-104.842205823049 40.5045424803865,-104.842003423773 40.5039327015263,-104.841678061729 40.5033544995574,-104.841237748411 40.502822112724,-104.840693325791 40.5023486513933,-104.840058199365 40.5019457751149,-104.839348008051 40.5016234053897,-104.838580239118 40.5013894812384,-104.837773797582 40.5012497635973,-104.836948540713 40.501207693373,-104.836124789073 40.5012643066572,-104.83532282616 40.5014182091969,-104.834562398965 40.5016656107496,-104.833862231727 40.5020004184754,-104.833239564888 40.5024143870601,-104.832709730574 40.5028973218633,-104.83228577506 40.5034373300773,-104.831978137541 40.5040211136997,-104.831794393074 40.5046342970926,-104.831739066055 40.5052617810515,-104.831813518821 40.5058881146554,-104.832015918097 40.5064978757385,-104.832341280141 40.5070760506105,-104.83278159346 40.5076084036796,-104.833326016079 40.5080818278834,-104.833961142505 40.5084846673069,-104.834671333819 40.5088070040565,-104.835439102753 40.5090409023397,-104.836245544289 40.5091806037522,-104.837070801158 40.5092226689759,-104.837894552799 40.5091660624086,-104.83869651571 40.509012177646,-104.839456942906 40.5087648031902,-104.840157110143 40.5084300292289,-104.840779776982 40.5080160977709,-104.841309611298 40.5075331998226)))

但是，如果我对不同的层（“土壤”）执行相同的查询，我会得到一个空结果：

查询是一样的：

SELECT ST_AsText(ST_Intersection(%(geometries_0)s::geometry, %(geometry)s::geometry))

带有应该重叠的多边形geometries_0：

POLYGON((-104.84627938530097 40.54511058649626,-104.83460641167578 40.545175808723876,-104.84070039055733 40.537283458057615,-104.84627938530097 40.54511058649626))

还有一个代表土壤的geometry 图层，类似于上述查询中的农田：

MULTIPOLYGON(((-104.939716 40.258166,-104.939775 40.258174,-104.939963 40.258159,-104.940159 40.258065,-104.940039 40.257671,-104.939917 40.25749,-104.939928 40.257419,-104.94003 40.257404,-104.940265 40.257641,-104.940632 40.257902,-104.940826 40.258061,-104.941051 40.258188,-104.941123 40.258235,-104.941205 40.258283,-104.941246 40.258275,-104.941287 40.258212,-104.941186 40.258094,-104.941186 40.258007,-104.941167 40.257921,-104.941105 40.257858,-104.941044 40.257786,-104.941045 40.257716,-104.941127 40.257676,-104.94122 40.257653,-104.94141 40.257731,-104.941559 40.257671,-104.941255 40.257181,-104.940857 40.256794,-104.940644 40.256478,-104.940319 40.255997,-104.940003 40.255728,-104.939676 40.255561,-104.939419 40.255544,-104.938895 40.255529,-104.938287 40.255512,-104.938046 40.255528,-104.937733 40.255549,-104.937322 40.255533,-104.937012 40.255577,-104.936947 40.255593,-104.936623 40.255774,-104.936581 40.255924,-104.93658 40.256042,-104.936661 40.256223,-104.93671 40.256436,-104.936842 40.256618,-104.937262 40.256753,-104.937662 40.256818,-104.937897 40.257,-104.938181 40.25745,-104.938374 40.257742,-104.938465 40.257931,-104.938782 40.258051,-104.939121 40.258092,-104.939439 40.258133,-104.939716 40.258166)))

我使用 postgis ST_AsText 函数将数据库层转换为知名文本，并检查以确保所有层都具有 EPSG:4326 投影（使用 Find_SRID 函数）。

为什么一层（农田）正确相交，而另一层（土壤）却不正确？我尝试使用地理而不是几何进行相同的查询，结果相同。

【问题讨论】：

请勿交叉发帖：gis.stackexchange.com/questions/67679/…

标签： postgresql postgis intersection

【解决方案1】：

它返回一个空集合，因为它们不相交。事实上，它们相距 32 公里。

SELECT ST_Intersects(A, B), ST_Distance(A, B)/1000 AS dist_km
FROM (
 SELECT
   'POLYGON((-104.84627938530097 40.54511058649626,-104.83460641167578 40.545175808723876,-104.84070039055733 40.537283458057615,-104.84627938530097 40.54511058649626))'::geography AS A,
   'MULTIPOLYGON(((-104.939716 40.258166,-104.939775 40.258174,-104.939963 40.258159,-104.940159 40.258065,-104.940039 40.257671,-104.939917 40.25749,-104.939928 40.257419,-104.94003 40.257404,-104.940265 40.257641,-104.940632 40.257902,-104.940826 40.258061,-104.941051 40.258188,-104.941123 40.258235,-104.941205 40.258283,-104.941246 40.258275,-104.941287 40.258212,-104.941186 40.258094,-104.941186 40.258007,-104.941167 40.257921,-104.941105 40.257858,-104.941044 40.257786,-104.941045 40.257716,-104.941127 40.257676,-104.94122 40.257653,-104.94141 40.257731,-104.941559 40.257671,-104.941255 40.257181,-104.940857 40.256794,-104.940644 40.256478,-104.940319 40.255997,-104.940003 40.255728,-104.939676 40.255561,-104.939419 40.255544,-104.938895 40.255529,-104.938287 40.255512,-104.938046 40.255528,-104.937733 40.255549,-104.937322 40.255533,-104.937012 40.255577,-104.936947 40.255593,-104.936623 40.255774,-104.936581 40.255924,-104.93658 40.256042,-104.936661 40.256223,-104.93671 40.256436,-104.936842 40.256618,-104.937262 40.256753,-104.937662 40.256818,-104.937897 40.257,-104.938181 40.25745,-104.938374 40.257742,-104.938465 40.257931,-104.938782 40.258051,-104.939121 40.258092,-104.939439 40.258133,-104.939716 40.258166)))'::geography AS B
) AS data;

 st_intersects |     dist_km
---------------+------------------
 f             | 32.1052124928391

你的地图有些地方不正确。

【讨论】：

【解决方案2】：

经过一些调试（除了一些helpful troubleshooting steps），我意识到我使用ST_AsText 函数错误地构建了我的WKT blob，特别是针对我的土壤层。结果，我的交集没有应用于我的土壤层中包含的整个几何图形集。

目前我的土壤层包含SSURGO soil map units 的子集，其中一些实际上不包含几何。为了正确构建代表 all 非空几何的文本字符串，我需要在将结果转换为 WKT 之前显式加入几何：

SELECT ST_AsText(ST_Union(the_geom)) FROM schema.layer

成功了。

【讨论】：