保留UnionAggregate中的起始点

Question

DECLARE @Geom TABLE 
( 
   shape geometry, 
   shapeType nvarchar(50) 
); 

INSERT INTO @Geom(shape,shapeType) 
VALUES('LINESTRING(1 2, 3 4)', 'A'), 
('LINESTRING(3.2 4, 7 8)', 'B'); 

SELECT *
FROM @Geom

SELECT geometry::UnionAggregate(shape).ToString(), geometry::UnionAggregate(shape)
FROM @Geom;

输出的WKT是

MULTILINESTRING ((7 8, 3.2 4), (3 4, 1 2))

当我想要的时候

MULTILINESTRING ((1 2, 3 4), (3.2 4, 7 8))

其中"A"和"B"行的开始应该是(1 2) 和(3.2 4)尊敬.

这种行为UnionAggregate似乎并不关心几何的"方向",以便维持A联合B和B联合A是相同的结果.不过,我想保留的开始/终点,因为我unioning街道几何,我希望所有的线串在其原来的方向去了.

这里讨论这个问题:https://social.msdn.microsoft.com/Forums/sqlserver/en-US/89e95366-3649-4294-a0bc-f3921598157f/union-of-linestrings-and-reversing-direction?forum= sqlspatial

他们似乎建议在检查最终结果的可能解决方案,但我不清楚如何做到这一点.它是在一个链接的线程中暗示的

MultiLineString始终表示距离原点最远的点的图形.

我不清楚这究竟意味着什么,但我认为我不能假设UnionAggregate的结果总是与我想要的相反

如果很难知道方向意图,那么我可以添加M个度量,其中方向应该跟随增加的M值.

假设我有一种方法可以反转积分,我该如何解决这个问题呢？

我找到了一个模拟STUnionZ和M测量的附加支持的功能:http://www.spatialdbadvisor.com/files/SQLServer.html#robo48但是注意到"他们的方向可能会改变(例如,开始/起点关系) ).",这是我想要避免的.

Answer 1

几何类型不记录/编码方向性。您给它的线可能被认为是“无方向的”或“双向的”。返回1：

select geometry::STGeomFromText('LINESTRING(1 2, 3 4)',0).STEquals(
       geometry::STGeomFromText('LINESTRING(3 4, 1 2)',0))

因此，这些类型无法提供您要查找的内容。您认为“起点”很特殊。我建议您将那些单独记录为POINT。

不过，这确实使所有结果代码变得更丑陋-您必须将这些数据对一起处理：

DECLARE @Geom TABLE 
(
   start geometry, 
   shape geometry, 
   shapeType nvarchar(50) 
); 

INSERT INTO @Geom(start,shape,shapeType) 
VALUES('POINT(1 2)','LINESTRING(1 2, 3 4)', 'A'), 
('POINT(3.2 4)','LINESTRING(3.2 4, 7 8)', 'B'); 

SELECT *
FROM @Geom

SELECT
    geometry::UnionAggregate(start).ToString(), geometry::UnionAggregate(shape).ToString(),
    geometry::UnionAggregate(start), geometry::UnionAggregate(shape)
FROM @Geom;

此时，您可能决定直接停止使用地理类型-您可以创建引用SqlGeography（相同类型的CLR曲面）并在内部使用但也跟踪其“方向性” 的CLR UDT，并将其包装在一起开始使用它。

不过，您不太可能希望geography在该包装中显示所有方法-您必须选择并选择战斗。而且，当然，由于结果并不是真正出现在SQL Server geography中，因此您将无法从Management Studio中使用“空间结果”选项卡。

我只能想到的是，在这些类型中确实存在某些“方向性”的地方是消除geography形状歧义的左手规则。

Answer 2

最初，我建议...

DECLARE @Geom TABLE 
( 
   shape geometry, 
   shapeType nvarchar(50) 
); 

INSERT @Geom(shape,shapeType) VALUES
  ('LINESTRING(1 2, 3 4)', 'A'), 
  ('LINESTRING(3.2 4, 7 8)', 'B'); 

SELECT * FROM @Geom

SELECT 
  geometry::CollectionAggregate(shape).Reduce(0).ToString(), 
  geometry::CollectionAggregate(shape).Reduce(0)
FROM @Geom

你得到：

...但是，我很清楚我给出的答案还不够好。例如，很难Reduce()避免简化部分代码，

我仍然喜欢CollectionAggregate，将原始的线阵列整合到一件事中，但是后来我发现，必须有一种构建必要的几何结构的方法。

我玩了几次，根据输入中是否存在不相交的元素，该迭代将评估为a LineString或a ：MultiLineStringLineString

create function dbo.SimplifyToLine( @geo geometry ) returns geometry as
begin
  declare 
    @numSubGeos int = @geo.STNumGeometries(),
    @subGeoIdx int = 1,
    @sql nvarchar( max ) = N'',
    @subGeo geometry,
    @oldEndX float = -1.0e26,
    @oldEndY float = -1.0e26,
    @startX float,
    @startY float,
    @endX float,
    @endY float,
    @idx int,
    @numPoints int,
    @point geometry,
    @segment int = 1,
    @continue bit,
    @result geometry,
    @started bit = 0

  declare
    @geos table
    ( 
      Idx int primary key, 
      SubGeo geometry, 
      StartX decimal, 
      EndX decimal, 
      StartY decimal, 
      EndY decimal, 
      NumPoints int, 
      ContinueFromPrevious bit 
    ) 

  declare
    @multiLines table
    (
      Idx int primary key,
      Segment nvarchar(max)
    )

  --> collect geometries and extents...
  while ( @subGeoIdx <= @numSubGeos )  
  begin

    select @subGeo = @geo.STGeometryN( @subGeoIdx )

    select 
      @startX = @subGeo.STPointN( 1 ).STX,
      @startY = @subGeo.STPointN( 1 ).STY,
      @endX = @subGeo.STPointN( @subGeo.STNumPoints( ) ).STX,
      @endY = @subGeo.STPointN( @subGeo.STNumPoints( ) ).STY

    insert @geos values
    ( 
      @subGeoIdx,
      @subGeo, 
      @startX, 
      @endX, 
      @startY, 
      @endY, 
      @subGeo.STNumPoints() ,
      case when @subGeoIdx = 1 then 1 when @oldEndX = @startX and @oldEndY = @startY then 1 else 0 end
    )   

    select 
      @oldEndX = @endX, 
      @oldEndY = @endY, 
      @subGeoIdx = @subGeoIdx + 1
  end


  if not exists ( select * from @geos where ContinueFromPrevious = 0 ) --> then all LineStrings are connected 
  begin
    --> build a single LINESTRING( )...
    select @sql = ''
    declare c cursor for select SubGeo, StartX, EndX, StartY, EndY, NumPoints, ContinueFromPrevious from @geos order by Idx  
    open c
    while ( 1 = 1 )
    begin
      fetch next from c into @subGeo, @startX, @endX, @startY, @endY, @numPoints, @continue
      if @@fetch_status != 0 break;
      select @idx = case when @started = 0 then 1 else 2 end, @started = 1  --> accrue all points, de-duplicating line ends...
      while ( @idx <= @numPoints )
      begin
        select @point = @subGeo.STPointN( @idx )
        select @sql += convert( nvarchar, @point.STX ) + N' ' + convert( nvarchar, @point.STY ) + N','
        select @idx = @idx + 1
      end
    end
    close c
    deallocate c
    select @sql = substring( @sql, 1, len( @sql ) -1 )
    select @result =  geometry::STGeomFromText(N'LINESTRING(' + @sql + N')', 0 ) 
  end
  else  --> we have disjoint lines in the inputs...
  begin
    select @sql = N'', @started = 0
    --> build a MULTILINESTRING((),()...) with line segements terminated at disjoint points..
    declare c cursor for select SubGeo, StartX, EndX, StartY, EndY, NumPoints, ContinueFromPrevious from @geos  order by Idx
    open c
      while ( 1=1 )
      begin
        fetch next from c into @subGeo, @startX, @endX, @startY, @endY, @numPoints, @continue
        if @@fetch_status != 0 break;
        if @continue = 1
        begin
          select @idx = case when @started = 0 then 1 else 2 end, @started = 1
          while ( @idx <= @numPoints )
          begin
            select @point = @subGeo.STPointN( @idx )
            select @sql += convert( nvarchar, @point.STX ) + N' ' + convert( nvarchar, @point.STY ) + N','
            select @idx = @idx + 1
          end
        end
        else
        begin
          insert @multiLines values ( @segment, substring( @sql, 1, len( @sql ) -1 ) ) --> collect the segment
          select @idx = 1, @sql = N'', @segment = @segment + 1
          while ( @idx <= @numPoints )
          begin
            select @point = @subGeo.STPointN( @idx )
            select @sql += convert( nvarchar, @point.STX ) + N' ' + convert( nvarchar, @point.STY ) + N','
            select @idx = @idx + 1
          end
        end
      end
    close c
    deallocate c
    insert @multiLines values ( @segment, substring( @sql, 1, len( @sql ) -1 ) )
    select @sql = N''
    select @sql += N'(' + Segment + N'),' from @multiLines order by Idx --> appends all segments
    select @sql = substring( @sql, 1, len( @sql ) -1 )
    select @result = geometry::STGeomFromText( 'MULTILINESTRING('+ @sql + N')', 1 )
  end

...最后，鉴于：

DECLARE @Geom TABLE 
( 
   shape geometry, 
   shapeType nvarchar(50) 
); 

INSERT @Geom(shape,shapeType) VALUES
  ('LINESTRING(1 2, 3 4)', 'A'), 
  ('LINESTRING(3 4, 9 9)', 'B'),  --> disjoint from here to the next LINESTRING
  ('LINESTRING(9 8, 3 4)', 'C'),
  ('LINESTRING(3 4, 1 2)', 'D'); 

select 
  dbo.SimplifyToLine(geometry::CollectionAggregate(shape)).ToString(),
  dbo.SimplifyToLine(geometry::CollectionAggregate(shape))
from 
  @Geom

delete @Geom

INSERT @Geom(shape,shapeType) VALUES
('LINESTRING(1 2, 3 4)', 'A'), 
('LINESTRING(3 4, 9 8)', 'B'),
('LINESTRING(9 8, 3 4)', 'C'),
('LINESTRING(3 4, 1 2)', 'D'); 

select 
  dbo.SimplifyToLine(geometry::CollectionAggregate(shape)).ToString(),
  dbo.SimplifyToLine(geometry::CollectionAggregate(shape))
from
  @Geom

...你得到：