can somebody help me understand (probably giving visual examples) what does this sentence mean which is quoted from the wiki page of nine intersection model:
Those points that are in the boundaries of an odd number of its elements (curves).
I run the following example in R:
elem1 <- matrix(c(0,0,0,1),2) elem2 <- matrix(c(0,0,0,-1),2) elem3 <- matrix(c(0,-1,0,0),2) elem4 <- matrix(c(0,0,-1,-2),2) mls <- sf::st_multilinestring(list(elem1,elem2,elem3,elem4)) plot(mls,type='o') sf::st_boundary(mls)
The output is all 4 points on the boundaries of each individual element.
MULTIPOINT(-1 0, 0 -2, 0 0, 0 1)