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All Blog Articles, Data Models and Free Source Code by Simon Greener, The SpatialDB Advisor is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License.

COGO: Compute arc length subtended by angle centre of circle

Thursday May 05 2011 at 11:27

In working with circles (and circular arcs) in spatial data one must be able to compute the length of an arc defined by an angle subtended at the centre of a circle.

In particular this is important for converting circles to vertex-stroked polygons and vice versa.

Here is an encoding of the calculation of the length of a arc defined by an angle subtended at the centre of a circle for use in SQL Server 2008 spatial.

Note that I have a schema call cogo in which I create functions like this. You can use anything you like.

/* ----------------------------------------------------------------------------------------
* @function   : ComputeArcLength
* @precis     : Computes the length of the arc on the circumference of a circle subtended 
*               at its centre by the the supplied angle
* @version    : 1.0
* @usage      : FUNCTION ComputeArcLength( @p_dRadius Float,
*                                               @p_dAngle Float)
*                        RETURNS INTEGER 
*               eg select cogo.ComputeArcLength(10,90.0) as arcLength;
* @param      : @p_dRadius  : Radius of Circle
* @paramtype  : @p_dRadius  : Float
* @param      : @p_dAngle   : Angle subtended at centre of circle expressed as 
*                             degree value > 0 and <= 360
* @paramtype  : @p_dAngle   : Float
* @return     : ArcLength   : the length of the arc on the circumference subtended by the supplied angle
* @rtnType    : ArcLength   : INTEGER
* @note       : Does not throw exceptions
* @note       : Assumes planar projection eg UTM.
* @history    : Simon Greener - Feb 2005 - Original coding.
* @history    : Simon Greener - May 2011 - Converted for use in TSQL
  * @copyright  : Licensed under a Creative Commons Attribution-Share Alike 2.5 Australia License. (http://creativecommons.org/licenses/by-sa/2.5/au/)
Create Function cogo.ComputeArcLength(@p_dRadius Float,
                                      @p_dAngle  Float)
Returns Float
      @dArc      Float,
      @dAngleRad Float;
      SET @dAngleRad = RADIANS(@p_dAngle);
      SET @dArc      = @p_dRadius * @dAngleRad;
      Return @dArc;

An example of how to execute this function is as follows:

select ( 2 * PI() * 10 ) as circumference,
       cogo.ComputeArcLength(10,90.0) as arcLength,
       cogo.ComputeArcLength(10,90.0) / ( 2 * PI() * 10 ) as ratio;

circumference arcLength ratio
62.8318530717959 15.707963267949 0.25

I hope this helps someone.

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