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STBuffer (geometry Data Type)

  • Άρθρο

Applies to: SQL Server Azure SQL Database Azure SQL Managed Instance

Returns a geometric object that represents the union of all points whose distance from a geometry instance is less than or equal to a specified value.

Syntax

  
.STBuffer ( distance )

Arguments

distance
Is a value of type float (double in the .NET Framework) specifying the distance from the geometry instance around which to calculate the buffer.

Return Types

SQL Server return type: geometry

CLR return type: SqlGeometry

Remarks

STBuffer() calculates a buffer like BufferWithTolerance, specifying tolerance = distance * .001 and relative = false.

When distance > 0 then either a Polygon or MultiPolygon instance is returned.

Note

Since distance is a float, a very small value can equate to zero in the calculations. When this occurs, a copy of the calling geometry instance is returned. See float and real (Transact-SQL)

When distance = 0, then a copy of the calling geometry instance is returned.

When distance < 0, then

  • an empty GeometryCollection instance is returned when the dimensions of the instance are 0 or 1.

  • a negative buffer is returned when the dimensions of the instance are 2 or more.

    Note

    A negative buffer may also create an empty GeometryCollection instance.

A negative buffer removes all points enclosed in given distance of the boundary of the geometry.

The error between the theoretical and computed buffer is max(tolerance, extents * 1.E-7) where tolerance = distance * .001. For more information on extents, see geometry Data Type Method Reference.

Examples

A. Calling STBuffer() with parameter_value < 0 on one dimensional geometry instance

The following example returns an empty GeometryCollection instance:

 DECLARE @g geometry= 'LINESTRING(3 4, 8 11)'; 
 SELECT @g.STBuffer(-1).ToString();

B. Calling STBuffer() with parameter_value < 0 on a Polygon instance

The following example returns a Polygon instance with a negative buffer:

 DECLARE @g geometry = 'POLYGON((1 1, 1 5, 5 5, 5 1, 1 1))'; 
 SELECT @g.STBuffer(-1).ToString();

C. Calling STBuffer() with parameter_value < 0 on a CurvePolygon instance

The following example returns a Polygon instance with a negative buffer from a CurvePolygon instance:

 DECLARE @g geometry = 'CURVEPOLYGON(COMPOUNDCURVE(CIRCULARSTRING(0 4, 4 0, 8 4), (8 4, 0 4)))'; 
 SELECT @g.STBuffer(-1).ToString();

Note

A Polygon instance is returned instead of a CurvePolygon instance. To return a CurvePolygon instance, see BufferWithCurves (geometry Data Type)

D. Calling STBuffer() with a negative parameter value that returns an empty instance

The following example shows what occurs when the distance parameter equals -2 for the previous example.

 DECLARE @g geometry = 'CURVEPOLYGON(COMPOUNDCURVE(CIRCULARSTRING(0 4, 4 0, 8 4), (8 4, 0 4)))'; 
 SELECT @g.STBuffer(-2).ToString();

This SELECT statement returns a GEOMETRYCOLLECTION EMPTY.

E. Calling STBuffer() with parameter_value = 0

The following example returns a copy of the calling geometry instance:

 DECLARE @g geometry = 'LINESTRING(3 4, 8 11)'; 
 SELECT @g.STBuffer(0).ToString();

F. Calling STBuffer() with a non-zero parameter value that is extremely small

The following example also returns a copy of the calling geometry instance:

 DECLARE @g geometry = 'LINESTRING(3 4, 8 11)';  
 DECLARE @distance float = 1e-20;  
 SELECT @g.STBuffer(@distance).ToString();

G. Calling STBuffer() with parameter_value > 0

The following example returns a Polygon instance:

 DECLARE @g geometry= 'LINESTRING(3 4, 8 11)'; 
 SELECT @g.STBuffer(2).ToString();

H. Calling STBuffer() with a string parameter value

The following example returns the same Polygon instance as mentioned earlier, but a string parameter is passed to the method:

 DECLARE @g geometry= 'LINESTRING(3 4, 8 11)'; 
 SELECT @g.STBuffer('2').ToString();

The following example will throw an error:

 DECLARE @g geometry = 'LINESTRING(3 4, 8 11)'; 
 SELECT @g.STBuffer('a').ToString();

Note

The previous two examples passed a string literal to the STBuffer(). The first example works because the string literal can be converted to a numeric value. However, the second example throws an ArgumentException.

I. Calling STBuffer() on a MultiPoint instance

The following example returns two MultiPolygon instances and one Polygon instance:

 DECLARE @g geometry = 'MULTIPOINT((1 1),(1 4))'; 
 SELECT @g.STBuffer(1).ToString(); 
 SELECT @g.STBuffer(1.5).ToString(); 
 SELECT @g.STBuffer(1.6).ToString();

The first two SELECT statements return a MultiPolygon instance because the parameter distance is less than or equal to 1/2 the distance between the two points (1 1) and (1 4). The third SELECT statement returns a Polygon instance because the buffered instances of the two points (1 1) and (1 4) overlap.

See Also

BufferWithTolerance (geometry Data Type)
OGC Methods on Geometry Instances