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drawing a polygon from several lines

4 messages · Antonio Silva, Vijay Lulla

Antonio Silva
# 
Hello all,

I want to select the cells of a grid that in a range of latitudes and
depths.
Latitudes limits are indicated by two lines and the depths by two isobaths
from a shapefile.
The shapefile can be downloaded at
https://www.dropbox.com/sh/us859uyijns36r9/AAC9N4OSwweLuW-fdsBWUy-ra?dl=0

Below are the steps I've already taken. I could not find a way to create a
polygon from the lines of depth and latitude. With this polygon I could
easily select the overlapped cells.

I would really appreciate any help. Best regards

----
library(rgeos)
library(sp)
rm(list = ls())

# import shape
isobaths <- readOGR(".","isobath")
isobaths <- spTransform(bath, CRS("+proj=longlat +datum=WGS84"))
proj4string(isobaths)
summary(isobaths)

# create a spatial grid and polygons
grd <-
GridTopology(cellcentre.offset=c(-47.75,-25.416667),cellsize=c(10/60,10/60),cells.dim=c(23,12))
polys <- as.SpatialPolygons.GridTopology(grd)
proj4string(polys) <- CRS("+proj=longlat +datum=WGS84")
summary(polys)
IDs <- sapply(slot(polys, "polygons"), function(x) slot(x, "ID"))

# create spatial lines
# southern limit
x <- c(-47.5,-45.5)
y <- c(-24.8,-24.8)
ls <- SpatialLines(list(Lines(Line(cbind(x,y)),
ID="b")),proj4string=CRS("+proj=longlat +datum=WGS84"))

# northern limit
x <- c(-46.2,-44.5)
y <- c(-24.02,-24.02)
ln <- SpatialLines(list(Lines(Line(cbind(x,y)),
ID="c")),proj4string=CRS("+proj=longlat +datum=WGS84"))

# plot isolines polygons and lines
plot(polys)
axis(1);axis(4)
text(coordinates(polys), labels=IDs, cex=0.7)
plot(isobaths,add=T)
plot(ls,add=T,col="blue",lwd=2)
plot(ln,add=T,col="orange",lwd=2)

# select isobaths
summary(isobaths)
dmin <- subset(isobaths, ID %in% "-25")
proj4string(dmin) <- CRS("+proj=longlat +datum=WGS84")

dmax <- subset(isobaths, ID %in% "-60")
proj4string(dmax) <- CRS("+proj=longlat +datum=WGS84")

plot(dmin,add=T,col="red",lwd=2)
plot(dmax,add=T,col="red",lwd=2)

# from here I'd like to have a polygon to select the cells.
na.omit(over(polys,dmin))
1 day later
Antonio Silva
# 
Dear colleagues

I finally could select the squares (cells) under the polygon. I extract the
points from the selected isobaths that fell within lat / lon limits and
with them I created a spatial polygon.

Bellow I share the script I wrote. Considering that I don't know much about
mapping in R, I would appreciate to hear suggestions to improve the code.

Best regards.
Vijay Lulla
# 
Very nice, Antonio!!  Thanks for sharing your script.  The only minor
suggestion I have to offer is that you can make your "cut isobath" portion
easier to read (at least IMO) by using `with` and `between` (either
dplyr::between or data.table::between).  Below is how I would've done it.

limmin <- with(list(coords = coordinates(isomin)[[1]][[1]]),
               subset(coords, between(coords[, 1], lonmin, lonmax) &
                              between(coords[, 2], latmin, latmax)))

Again, thanks for sharing your problem and solution.  I found it
instructive!
Cordially,
Vijay.
On Fri, Aug 24, 2018 at 9:16 AM Antonio Silva <aolinto.lst at gmail.com> wrote:

            

      
      
      
    

        

      
      
    

        

  
    

      
      
    

  


  


      

    

    

      
      

        


  

        

      Antonio Silva
    

    

      
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It's my pleasure to share Lulla. Thanks for the suggestion, I will adopt it.

Regards,

Antonio

Em sex, 24 de ago de 2018 ?s 10:58, Vijay Lulla <vijaylulla at gmail.com>
escreveu: