Problem with gIntersections {rgeos}

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This issue is occurring because your poly2 object contains a canopy polygon that is also in the poly object, as such when calculating the intersection the resulting geometry contains that canopy polygon as well as the intersection polygon which is not a valid geometry hence the error. You can either remove the offending canopy from poly, or more likely if you are attempting to work with a single set of canopy polygons you can do something like the following to obtain all the canopy intersections:

inter = gIntersects(poly,byid=TRUE)
inter[!lower.tri(inter)] = FALSE

ids = which(inter,arr.ind=TRUE)

results = list()
for(i in 1:nrow(ids))
{
    results[[i]] = gIntersection(poly[ids[i,1],], poly[ids[i,2],])
}

plot(poly)
for(i in 1:length(results))
{
    plot(results[[i]],col='red', add=TRUE)
}

-Colin

Dear R users,

I am having hard time while computing the intersections (overlaps) among
tree crowns in a forest stand. I am using gIntersection() from 'rgeos'
package, but I get the following error message:
"Error in RGEOSBinTopoFunc(spgeom1, spgeom2, byid, id,
"rgeos_intersection") :
 TopologyException: side location conflict at 55.270200771297759
44.641891473965337"

It seems that the function does not allow 2 polygons (within the same
layer) to be connected (touching each other). Is it a known problem? Any
way to overcome this issue?
Below is an example. Many thanks for your suggestion and help.
Best regards,
Ervan

_____________________________________

library(spatstat)
library(rgeos)
library(sp)
x<- c(37.5, 42, 51, 63, 89.5, 110, 123, 142.5, 143.5, 141.5)  # x
coordinates
y <- c(202, 27, 53, 39.5, 59, 25.5, 73.5, 98, 38, 5)  # y coordinates
crown <- c(11.8, 9.4, 9.4, 9.3, 9.1, 11.2, 9.2, 9.9, 11.1, 10.1)  # the
crown radius of the trees
test <- data.frame(x=x,y=y,crown=crown)
A <- list()
for (i in 1: nrow(test)) {
DISC  <-
disc(test$crown[i],centre=c(test$x[i],test$y[i]),npoly=50)[[4]][[1]][1:2]
# compute the crown area based on crown radius
DISC$x[50]<-DISC$x[1]  # Polygons need to be closed -> closed the circle by
replacing last values by initial one
DISC$y[50]<-DISC$y[1]
A[[i]] <- Polygons(list(Polygon(DISC)),i)
}
dat <- data.frame(ID=1:nrow(test),crown.radius=test$crown)
poly <- SpatialPolygons(A,1:nrow(test))
Poly <- SpatialPolygonsDataFrame(poly,dat)
plot(poly)

poly2 <- SpatialPolygons(A[2:3],1:2)
plot(poly2,border=2,add=T)
inter <- gIntersection(poly,poly2)

points(55.27, 44.64,bg=3,pch=21)

-- 
_____________________________

Ervan Rutishauser, PhD
carboforexpert.ch <http://carboforexpert.ch/index.html>
LinkedIn <http://www.linkedin.com/pub/ervan-rutishauser/36/463/a>
Skype: ervan-CH

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