R: calculate the area within no closed contour lines - r

Question

I want to calculate the area within a 0.975 contour lines, which some of them aren't closed. This is the plot:
contour(zpropMAteo, levels = c(0.975),lty = 1,drawlabels = F, col=2)
plot(contorno, add=T )
where contorno is a window: polygonal boundary, with the continuous border of the plot:
str(contorno)
#List of 5
# $ type : chr "polygonal"
# $ xrange: num [1:2] 704787 727062
# $ yrange: num [1:2] 4239419 4261570
# $ bdry :List of 1
# ..$ :List of 4
# .. ..$ x : num [1:9188] 704787 705760 705892 706135 706311 ...
# .. ..$ y : num [1:9188] 4251037 4248333 4247517 4247191 4246915 ...
# .. ..$ area: num 1.76e+08
# .. ..$ hole: logi FALSE
# $ units :List of 3
# ..$ singular : chr "unit"
# ..$ plural : chr "units"
# ..$ multiplier: num 1
# ..- attr(*, "class")= chr "units"
# - attr(*, "class")= chr "owin"
and zpropMAteo is a pixel image:
str(zpropMAteo)
#List of 10
# $ v : num [1:99, 1:100] NA NA NA NA NA NA NA NA NA NA ...
# ..- attr(*, "dimnames")=List of 2
# .. ..$ : chr [1:99] "4239530.43796768" "4239753.18885493" "4239975.93974217" "4240198.69062942" ...
# .. ..$ : chr [1:100] "704898.406701755" "705121.157589002" "705343.908476249" "705566.659363497" ...
# $ dim : int [1:2] 99 100
# $ xrange: num [1:2] 704787 727062
# $ yrange: num [1:2] 4239419 4261570
# $ xstep : num 223
# $ ystep : num 224
# $ xcol : num [1:100] 704898 705121 705344 705567 705789 ...
# $ yrow : num [1:99] 4239531 4239755 4239978 4240202 4240426 ...
# $ type : chr "real"
# $ units :List of 3
# ..$ singular : chr "unit"
# ..$ plural : chr "units"
# ..$ multiplier: num 1
# ..- attr(*, "class")= chr "units"
# - attr(*, "class")= chr "im"
The problem, as you can see in the plot(right), is that there are open contour lines. Maybe a solution can be calculate the intersection with contours lines and the border and then the area inside, or maybe first calculate a contour line which it be exactly the border, I don't know, contourline(100%) or something like that... and then to obtain the interseccion and the area inside.
I tried to do
clinessMAteo<-contourLines(zpropMAteo$xcol,zpropMAteo$yrow,zpropMAteo$v,levels = c(0.975))
with this result:
str(clinessMAteo)
#List of 7
# $ :List of 3
# ..$ level: num 0.975
# ..$ x : num [1:5] 710690 710584 710690 710716 710690
# ..$ y : num [1:5] 4246190 4246243 4246296 4246243 4246190
# $ :List of 3
# ..$ level: num 0.975
# ..$ x : num [1:19] 714031 713978 713978 714031 714066 ...
# ..$ y : num [1:19] 4245519 4245572 4245796 4245814 4245796 ...
# $ :List of 3
# ..$ level: num 0.975
# ..$ x : num [1:6] 715258 715258 715145 715136 715145 ...
# ..$ y : num [1:6] 4260226 4260339 4260510 4260563 4260581 ...
# $ :List of 3
# ..$ level: num 0.975
# ..$ x : num [1:38] 718932 719154 719377 719574 719600 ...
# ..$ y : num [1:38] 4256978 4256942 4256891 4256759 4256742 ...
# $ :List of 3
# ..$ level: num 0.975
# ..$ x : num [1:45] 719377 719600 719823 720045 720268 ...
# ..$ y : num [1:45] 4255696 4255691 4255710 4255710 4255687 ...
# $ :List of 3
# ..$ level: num 0.975
# ..$ x : num [1:42] 724959 724946 724723 724562 724500 ...
# ..$ y : num [1:42] 4253166 4253162 4253138 4252956 4252900 ...
# $ :List of 3
# ..$ level: num 0.975
# ..$ x : num [1:15] 722273 722238 722273 722496 722718 ...
# ..$ y : num [1:15] 4251802 4251837 4251858 4251920 4251848 ...
and the area:
areaMASteo <- sum(sapply(clinessMAteo,function(ring){areapl(cbind(ring$x,ring$y))
but I know it isn't correct because I think I should obtain contours lines closed first.
Any idea?? :-)