Compute partial polygon area

[Stephen Woodbridge]

Krung Saengpole wrote:
> Hi all,
>  
> I'd like to compute partial area of polygon that intersects with another 
> polygon. Is there a way to do with PHPMapscript? For example, I want to 
> know how many square kilometers of forests that reside in the area of 
> 50km-radius circled area. Or where can I get to the example?

If you use PostGIS this is a straight forward SQL query like:

select polyid, area2d(intersection(the_geom, circleAsPolygon(point(x, 
y), radius, npoints)) as area from mytable where the_geom && 
expand(point(x, y), radius);

where the_geom is the geometry column for the polygons
and x,y is the center of your circle radius = 50000
and npoints is the number of points to generate for the polygonal circle.

you would need to write a function circleAsPolygon() in pgsql, but that 
is pretty trivial and most of us that would with PostGIS could post that 
if you needed us to.

-Steve