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Richard QuadlingFlag for United Kingdom of Great Britain and Northern Ireland

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Calculate the radius of a n-sided polygon such that the area of the polygon matches the area of a circle with the same radius.

Hi.

I know I should be able to do this, but I'm old and I've forgotten.

The issue is that SQL Server 2008 doesn't support a circle in its geospatial data type (as far as I can tell - still waiting on my Dev Ed).

So, I need to create an n-sided polygon that covers the same area as a circle of the same radius.

I'm using Google Maps to show this to TPTB within the company (I'm on a learning exercise, so having a LOT of fun).

Currently, I have the following code:
		fn_makePolygon = function(map, polygon, i_Points, i_Radius)
			{
			// Using the geometry library, calculate an array of points for the polygon
			var a_Path = [];
			var ll_Centre = map.getCenter();
			var f_Angle = 360 / i_Points;
			var f_Distance = i_Radius * 1609.344; // Meters to miles conversion.
			
			for(var i_Point = 0 ; i_Point <= i_Points ; ++i_Point)
				{
				a_Path.push
					(
					google.maps.geometry.spherical.computeOffset
						(
						ll_Centre,
						f_Distance,
						i_Point * f_Angle
						)
					);
				}
			polygon.setPath(a_Path);
			}

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This creates an n-sided closed polygon whose points lie on a circle of the same radius - you can see this in action at n-sided polygon fits inside circle.

What I would like is actually 2 fold.

1 - To calculate the path of an n-sided polygon so that the circle fits inside the polygon (just like the existing polygon fits inside the circle)
2 - To calculate the path of an n-sided polygon so that the area of the circle and the polygon are the same.


Don't worry about the coding, if you have issues with that, just the maths required will be enough for me.

I'm guessing that all I really need is to adjust the meters/miles conversion as this is all that actually alters the side the polygon.


Regards,

Richard Quadling.

P.S. I'm using Google Chrome V10, so the sliders are not going to be visible in older browsers - simply enter the number and tab off the input seems to work OK for IE/FF. This is just a test script, so not an important issue.
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deighton
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TommySzalapski
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Here is where I got the other one (outer radius to area)
http://www.mathopenref.com/polygonregulararea.html
Avatar of Richard Quadling

ASKER

Excellent. I'll be incorporating the logic tomorrow and get back to you then.
Thanks for all of this.

I've got the following formula working ...


polygon_radius = sqrt( (pi * circle_radius^2 * 2) / (polygon_sides * SIN(360/polygon_sides) )

and in Javascript ...

var f_Distance = 1609.344 * Math.sqrt( (2 * Math.PI * i_Radius * i_Radius) / (i_Points * Math.sin(2 * Math.PI / i_Points) ) );

as Google Maps wants the value in meters and my users think in miles (as do the drivers).

Thank you all. Extremely useful comments.

Richard.