Reduce (geography Data Type)
Applies to: 
SQL Server
Azure SQL Database
Azure SQL Managed Instance
Returns an approximation of the given geography instance produced by running the Douglas-Peucker algorithm on the instance with the given tolerance.
This geography data type method supports FullGlobe instances or spatial instances that are larger than a hemisphere.
Syntax
.Reduce ( tolerance )
Arguments
| Term | Definition |
|---|---|
| tolerance | Is a value of type float. tolerance is the tolerance to input to the Douglas-Peucker algorithm. tolerance must be a positive number. |
Return Types
SQL Server return type: geography
CLR return type: SqlGeography
Remarks
For collection types, this algorithm operates independently on each geography contained in the instance. This algorithm does not modify Point instances.
This method will attempt to preserve the endpoints of LineString instances, but may fail to do so in order to preserve a valid result.
If Reduce() is called with a negative value, this method will produce an ArgumentException. Tolerances used in Reduce() must be positive numbers.
The Douglas-Peucker algorithm works on each curve or ring in the geography instance by removing all points except for the start point and end point. Each point removed is then added back, starting with the farthest outlying point, until no point is more than tolerance from the result. The result is then made valid if necessary, as a valid result is guaranteed.
In SQL Server 2012 (11.x), this method has been extended to FullGlobe instances.
This method is not precise.
Examples
The following example creates a LineString instance and uses Reduce() to simplify the instance.
DECLARE @g geography = 'LineString(120 45, 120.1 45.1, 199.9 45.2, 120 46)'
SELECT @g.Reduce(10000).ToString()