CODEWITHSUNDEEP
rgeospatialshapefile
Perro
Perro

Reputation: 358

Create a grid inside a shapefile

I am trying to crate a grid inside a shapefile, something like this. However, I am unable to generate such grid. I was wondering if anyone has an idea on how to accomplish this.

Here is my code - 

WWWL.Shape<- readOGR("E:/Juan Arango", "WWL_Commerce_OK")
WWWL.Shape
plot(WWWL.Shape)
proj4string(WWWL.Shape)

bb <- bbox(WWWL.Shape)
cs <- c(3.28084, 3.28084)*6000  # cell size 
cc <- bb[, 1] + (cs/2)  # cell offset
cd <- ceiling(diff(t(bb))/cs)  # number of cells per direction
grd <- GridTopology(cellcentre.offset=cc, cellsize=cs, cells.dim=cd)
grd
sp_grd <- SpatialGridDataFrame(grd,
                               data=data.frame(id=1:prod(cd)),
                               proj4string=CRS(proj4string(WWWL.Shape)))
plot(sp_grd)

Output of WWL.Shape 

class       : SpatialPolygonsDataFrame 
features    : 1 
extent      : 334367, 334498.7, 4088915, 4089057  (xmin, xmax, ymin, ymax)
coord. ref. : +proj=utm +zone=15 +datum=WGS84 +units=m +no_defs +ellps=WGS84 +towgs84=0,0,0 
variables   : 1
names       : Id 
min values  :  0 
max values  :  0

Upvotes: 5

Views: 7772

Answers (1)

loki
loki

Reputation: 10340

sf version

see rgdal version below

First, we start of with a shapefile. You might probably load it from any geospatial file using st_read.

library(sf)
library(raster)
library(ggplot2)

# load some spatial data. Administrative Boundary
shp <- getData('GADM', country = 'aut', level = 0)
shp <- st_as_sf(shp)
# ggplot() + 
  # geom_sf(data = shp)

Now the only thing you need is a combination of st_make_grid and st_intersection:

grid <- shp %>% 
  st_make_grid(cellsize = 0.1, what = "centers") %>% # grid of points
  st_intersection(shp)                               # only within the polygon

# ggplot() + 
  # geom_sf(data = shp) + 
  # geom_sf(data = grid)

rgdal and version

To create a grid of pixels you can use the sp::makegrid function.
Let's start with a reproducible example: 

library(raster)
shp <- getData(country = "FRA", level = 0)

So now we have a (multi)polygon. Let's transform it to a metric coordinate system (since your data and cell sizes were also metric): 

shp <- spTransform(shp, CRSobj = "+proj=utm +zone=32 +datum=WGS84 +units=m +no_defs +ellps=WGS84 +towgs84=0,0,0")
plot(shp)

Afterwards, we create a grid within this polygon with your specified cell size. 

cs <- c(3.28084, 3.28084)*6000
grdpts <- makegrid(shp, cellsize = cs)

Then, we convert this grid (which basically is a matrix of center points) to a SpatialPoints object: 

spgrd <- SpatialPoints(grdpts, proj4string = CRS(proj4string(shp)))

This can then be converted into a SpatialPixels object. (Note: adding the subset [shp, ] only selects points within the original polygons)

spgrdWithin <- SpatialPixels(spgrd[shp,])
plot(spgrdWithin, add = T)

If you need the grid as Polygons or Grid you can use

spgrdWithin <- as(spgrdWithin, "SpatialPolygons")
# or
spgrdWithin <- as(spgrdWithin, "SpatialGrid")

Upvotes: 19

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