R help - Oct 2010 - calculate area between intersecting polygons

Dear list:



I am trying to calculate the intersection area between two irregular
polygons (see example data below).



set.seed(1234)

theta <- seq(0, 2 * pi, length=(100))

poly1<-  cbind(c(0 + 1 * cos(theta) + rnorm(100, sd=0.1)), c(0 + 2 *
sin(theta)))

poly2<-  cbind(c(0 + 2 * cos(theta) ), c(-1 + 1.5 * sin(theta)+ rnorm(100,
sd=0.1)))



plot(x, y, type = "n", , xlim=c(-5,5), ylim=c(-5,5))

polygon(poly1)

polygon(poly2)





My data correspond to geographical positions, therefore I cannot generate it
using equations as I did in the example.

Are there any straightforward algorithm to calculate the intersection area
between polygons poly1 and poly2?



Thank you so much in advance



Jonas

	[[alternative HTML version deleted]]

Dear Jonas,

if you can write the difference in y-values between your polygons as a
function, you can use
integrate() to get the area between the polygons.

It sounds like perhaps your x-values will not match between the polygons
because they come from different sources, so you probably have to do some
interpolating (with ?approx).


hope that helps,
Remko
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