[GIS] Extract center coordinate polygon

coordinatespolygonr

I am new to R and I am looking for a little help. I have a polygonized raster image and I want to get the middle coordinate of the image. I have tried gCentroid
but there I get a list of multiple coordinates… how do I get just the middle one?

Best Answer

You can convert your raster to polygon then use gCentroid to extract the centroid. Here is an example taken from this answer

library(raster)
library(rgeos)

# example data
x <- raster(system.file("external/test.grd", package="raster"))

### To get the rectangular extent
e <- extent(x)
# coerce to a SpatialPolygons object
p <- as(e, 'SpatialPolygons')  

# calculate the centroid 
c1 <- gCentroid(p)
plot(p)
plot(c1, col='blue', add=TRUE)

### To get a polygon that surrounds cells that are not NA
# make all values the same. Either do
r = x > -Inf
# or alternatively
# r = reclassify(x, cbind(-Inf, Inf, 1))

pp <- rasterToPolygons(r, dissolve=TRUE)

# calculate the centroid 
c2 <- gCentroid(pp)

# look at the results
plot(x)
plot(p,  lwd=5, border='blue',  add=TRUE)
plot(c1, col='blue', add=TRUE)
plot(pp, lwd=3, border='red', add=TRUE)
plot(c2, col='red',  add=TRUE, pch = 17)

Related Solutions

[GIS] Extract all the polygon coordinates from a SpatialPolygonsDataframe

There are several ways to get at all of the coordinates for a SpatialPolygonsDataFrame or a SpatialLinesDataFrame.

Each of these methods records the object and the part within the object from the source, albeit with subtle differences and different names.

Say p is a SpatialPolygonsDataFrame.

#1. coerce to points, then to data frame
# names are Lines.NR Lines.ID Line.NR coords.x1 coords.x2
## start here for polys
lin <- as(p, "SpatialLinesDataFrame")  
## start here for lines
pts <- as.data.frame(as(lin, "SpatialPointsDataFrame"))


#2. convert to matrix 
# names are      object part cump hole         x        y
mpts <- raster::geom(p)

#3. convert to data frame
# names are        long      lat order  hole piece  id group
dpts <- ggplot2::fortify(p)

#4. convert to data frame
## names are   object_ branch_ island_ order_        x_       y_
spts <- spbabel::sptable(p)

Each of these outputs can be used interchangeably as long as you deal with the different name conventions, and for raster::geom remember that it's a matrix. (Actually the coercion one doesn't record the hole/island status so that is less useful depending on what's needed).

You can round trip the last one by

p2 <- spbabel::sp(spts, attr_tab = as.data.frame(p))

(There's an added bonus with spbabel in that you get a 'raster-like' print for the object, but with the data in 'tibble' print form. )

[GIS] Extract cell values and coordinates from raster using a polygon in R

you can use the raster::xyFromCell function to get a matrix of coordinates from a vector of cell numbers, which you can cbind onto the extract (as the xyFromCell result is in exactly the same order as the vector provided)

I've used sf but of course this works with readOGR etc.;

require(raster)
require(sf)

# make 2 dummy rasters for a dummy stack
r <- raster(xmn=0,xmx=10,ymn=0,ymx=10,res=1)
r[] <- sample(1:4,100,replace=T)

r2 <- raster(xmn=0,xmx=10,ymn=0,ymx=10,res=1)
r2[] <- sample(8:10,100,replace=T)

s <- stack(r,r2)

# make a triangle simple feature
s2 <- rbind(c(2.2,2.2), c(3.3,8.5), c(5.3,0.8), c(2.2,2.2))
s2.sf <-st_sfc(st_polygon(list(s2)))
s2.pol = st_sf(ID = "tr", s2.sf)

# extract values into data frame
ex.df <- as.data.frame(extract(s,s2.pol,cellnumbers=T))

# create coordinate columns using xyFromCell
ex.df.coords <- cbind(ex.df, xyFromCell(s,ex.df[,1]))

Best Answer

Related Solutions

[GIS] Extract all the polygon coordinates from a SpatialPolygonsDataframe

[GIS] Extract cell values and coordinates from raster using a polygon in R

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