CurveToLineWithTolerance (geography Data Type)
Applies to: 
SQL Server
Azure SQL Database
Azure SQL Managed Instance
Returns a polygonal approximation of a geography instance that contains circular arc segments.
Syntax
.CurveToLineWithTolerance( tolerance, relative )
Arguments
tolerance
Is a double expression that defines the maximum error between the original circular arc segment and its linear approximation.
relative
Is a bool expression that indicates whether to use a relative maximum for the deviation. If relative is false (0), then an absolute maximum is set for the deviation that a linear approximate can have. When relative is true (1), then the tolerance is calculated as a product of the tolerance parameter and the bounding box's diameter for the spatial object.
Return Types
SQL Server return type: geography
CLR return type: SqlGeography
Exceptions
Setting tolerance <= 0 throws an ArgumentOutOfRange exception.
Remarks
This method allows for an error tolerance amount to be specified for the resultant LineString.
CurveToLineWithTolerance method will return a LineString instance for a CircularString or CompoundCurve instance and Polygon instance for a CurvePolygon instance.
Examples
A. Using different tolerance values on a CircularString instance
The following example shows how setting the tolerance affects the LineStringinstance returned from a CircularString instance:
DECLARE @g geography;
SET @g = geography::Parse('CIRCULARSTRING(-122.358 47.653, -122.348 47.649, -122.348 47.658, -122.358 47.658, -122.358 47.653)');
SELECT @g.CurveToLineWithTolerance(0.1,0).STNumPoints(), @g.CurveToLineWithTolerance(0.01, 0).STNumPoints();
B. Using the method on a MultiLineString instance containing one LineString
The following example shows what is returned from a MultiLineString instance that only contains one LineString instance:
DECLARE @g geography;
SET @g = geography::Parse('MULTILINESTRING((-122.358 47.653, -122.348 47.649))');
SELECT @g.CurveToLineWithTolerance(0.1,0).ToString();
C. Using the method on a MultiLineString instance containing multiple LineStrings
The following example shows what is returned from a MultiLineString instance that contains more than one LineString instance:
DECLARE @g geography;
SET @g = geography::Parse('MULTILINESTRING((-122.358 47.653, -122.348 47.649),(-123.358 47.653, -123.348 47.649))');
SELECT @g.CurveToLineWithTolerance(0.1,0).ToString();
D. Setting relative to true for an invoking CurvePolygon instance
The following example uses a CurvePolygon instance to call CurveToLineWithTolerance() with relative set to true:
DECLARE @g geography = 'CURVEPOLYGON(COMPOUNDCURVE(CIRCULARSTRING(-122.358 47.653, -122.348 47.649, -122.348 47.658), (-122.348 47.658, -122.358 47.658, -122.358 47.653)))';
SELECT @g.CurveToLineWithTolerance(.5,1).ToString();