how to get point set (x,y) in a desired area in r

Question

The figure is the plot of x,y set in a excel file, total 8760 pair of x and y. I want to remove the noise data pair in red circle area and output a new excel file with remain data pair. How could I do it in R?

Answer 1

Using @G5W's example:

Make up data:

set.seed(2017)
x = runif(8760, 0,16)
y = c(abs(rnorm(8000, 0, 1)), runif(760,0,8)) 
XY = data.frame(x,y)

Fit a quantile regression to the 90th percentile:

library(quantreg)
library(splines)
qq <- rq(y~ns(x,20),tau=0.9,data=XY)

Compute and draw the predicted curve:

xvec <- seq(0,16,length.out=101)
pp <- predict(qq,newdata=data.frame(x=xvec))
plot(y~x,data=XY)
lines(xvec,pp,col=2,lwd=2)

Keep only points below the predicted line:

XY2 <- subset(XY,y<predict(qq,newdata=data.frame(x)))

plot(y~x,data=XY2)
lines(xvec,pp,col=2,lwd=2)

You can make the line less wiggly by lowering the number of knots, e.g. y~ns(x,10)