I am trying to create a grid over a polygon from which I can choose a random subset of cells for sampling. The problem occurs when I crop the grid, which I am converting to polygons, by the polygon shapefile. For some reason this results in a polygon the shape of the cropping polygon but with lines across it. After opening the resulting shapefile in QGIS I can see that the lines are "edges" of the polygon, because when I get rid of the outline of the polygon, the lines go away.

Can you explain what error, if any, I am making, or do you have an alternate means of generating a grid over a polygon from which a subset of grid cells can be selected (and ideally turned in a shapefile)?

Here is some sample code:

(California boundary found at https://data.ca.gov/dataset/ca-geographic-boundaries)


# Load shapefile
ca <- readOGR(dsn = "...", layer = "CA_State_TIGER2016")
ca.utm <- spTransform(ca, CRSobj = "+proj=utm +zone=11 ellps=WGS84")

# Calculate number of grid cells necessary
x.cells <- ceiling((ca.utm@bbox[1,2]-ca.utm@bbox[1,1])/10000)
y.cells <- ceiling((ca.utm@bbox[2,2]-ca.utm@bbox[2,1])/10000)

# Generate grid
ca.grid <- raster(extent(ca.utm), nrow=y.cells, 
                      ncol=x.cells, crs=ca.utm@proj4string)
ca.grid.poly <- as(as(ca.grid, "SpatialPixels"),"SpatialPolygons")
ca.grid.utm <- spTransform(ca.grid.poly, CRSobj = "+proj=utm +zone=11 ellps=WGS84")

# Crop grid to polygon
ca.grid.inter <- gIntersection(ca.grid.utm, ca.utm)
# or
ca.grid.crop <- crop(ca.grid.utm, ca.utm)

# both gIntersection and crop give same result as far as I can tell

Here is a screen shot:


enter image description here

  • Can you post a screenshot? Or a link to one, if you don't have the reputation?
    – srha
    Oct 5, 2017 at 19:14

1 Answer 1


The problem is in the cropping step. Change

ca.grid.inter <- gIntersection(ca.grid.utm, ca.utm)


ca.grid.inter <- gIntersection(ca.grid.utm, ca.utm, byid = TRUE)

I'm not sure why not including byid = True produces the odd result that it does, but adding definitely solves the problem.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.