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BufferWithTolerance (geography Data Type)

 

THIS TOPIC APPLIES TO:yesSQL Server (starting with 2008)yesAzure SQL DatabasenoAzure SQL Data Warehouse noParallel Data Warehouse

Returns a geometric object representing the union of all point values whose distance from a geography instance is less than or equal to a specified value, allowing for a specified tolerance.

This geography data type method supports FullGlobe instances or spatial instances that are larger than a hemisphere.

  
.BufferWithTolerance ( distance, tolerance, relative )  

distance
Is a float expression specifying the distance from the geography instance around which to calculate the buffer.

The maximum distance of the buffer cannot exceed 0.999 * π * minorAxis * minorAxis / majorAxis (~0.999 * 1/2 Earth’s circumference) or the full globe.

tolerance
Is a float expression specifying the tolerance of the buffer distance.

The tolerance value refers to the maximum variation in the ideal buffer distance for the returned linear approximation.

For example, the ideal buffer distance of a point is a circle, but this must be approximated by a polygon. The smaller the tolerance, the more points the polygon will have, which increases the complexity of the result, but decreases the error.

The minimum limit is 0.1 percent of the distance, and any tolerance less than that will be rounded up to the minimum limit.

relative
Is a bit specifying whether the tolerance value is relative or absolute. If 'TRUE' or 1, tolerance is relative and is calculated as the product of the tolerance parameter and the angular extent * equatorial radius of the ellipsoid. If 'FALSE' or 0, tolerance is absolute and the tolerance value is the absolute maximum variation in the ideal buffer distance for the returned linear approximation.

SQL Server return type: geography

CLR return type: SqlGeography

This method will throw an ArgumentException if the distance is not a number (NAN), or if distance is positive or negative infinity. This method will also throw an ArgumentException if tolerance is zero (0), not a number (NaN), negative, or positive or negative infinity.

STBuffer() will return a FullGlobe instance in certain cases; for example, STBuffer() returns a FullGlobe instance on two poles when the buffer distance is greater than the distance from the equator to the poles.

This method will throw an ArgumentException in FullGlobe instances where the distance of the buffer exceeds the following limitation:

0.999 * π * minorAxis * minorAxis / majorAxis (~0.999 * 1/2 Earth’s circumference)

The error between the theorectical and computed buffer is max(tolerance, extents * 1.E-7) where tolerance is the value of the tolerance parameter. For more information on extents, see geography Data Type Method Reference.

This method is not precise.

The following example creates a Point instance and uses BufferWithTolerance() to obtain a rough buffer around it.

DECLARE @g geography;  
SET @g = geography::STGeomFromText('POINT(-122.34900 47.65100)', 4326);  
SELECT @g.BufferWithTolerance(1, .5, 0).ToString();  

STBuffer (geography Data Type)
Extended Methods on Geography Instances

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