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rrastershapefilergdal

R: save the shapefile IDs in a particular order as a vector

Sample data

      library(raster)
      dat <- getData('GADM', country='FRA', level=1)
      plot(dat)
      text(dat, labels=as.character(dat$ID_1), col="darkred", font=2, offset=0.5, adj=c(0,2))

enter image description here

To save the IDs of provinces, I can do this

      province.id <- dat$ID_1

However, I want to arrange these IDs according to some direction (i.e. south to north)

For example, my province.id id should start from 10 (since it is the southern most province) all the way till 17 since it is the northern most province One way I thought was I can generate centorid of each province and based on the centroid, I can determine which are the most south to most north location.

      library(rgeos)
      trueCentroids = gCentroid(dat,byid=TRUE)
      plot(dat)
      points(coordinates(dat),pch=1)

enter image description here

But I still cannot export the output or arrange the centroids in the south-to-north direction to save as a vector

Solution

  • An easy approach would be to take the minimum latitude of each polygon and sort your IDs based on that:

    # data 

library(raster)
dat <- getData('GADM', country='FRA', level=1)

# create south to north index
sn_index <- unlist(lapply(dat@polygons, function(x) min(x@Polygons[[1]]@coords[,2])))

#sort IDs

dat$ID_1[order(sn_index)]
# [1] 10 13 21 16 22  3  2 14 20  6 18  8 11  7  1  9 15  4  5 19 12 17