Polygons from coordinates
I have data.frame
s lat
and lng
that define the bounds of rectangular rectangles like
geohash north_lat south_lat east_lng west_lng
1 gbsuv 48.69141 48.64746 -4.306641 -4.350586
2 gbsuy 48.69141 48.64746 -4.262695 -4.306641
What's the simplest way to convert this to an object sf
that contains column POLYGON
s?
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The key to creating polygons is that the coordinates must be sequential to form an enclosed area (i.e. the last point is the same as the first point).
This way your data will need a little manipulation to create coordinates and order. In my example, I did it withlapply
Then the rest can be taken from sf examples
lst <- lapply(1:nrow(df), function(x){
## create a matrix of coordinates that also 'close' the polygon
res <- matrix(c(df[x, 'north_lat'], df[x, 'west_lng'],
df[x, 'north_lat'], df[x, 'east_lng'],
df[x, 'south_lat'], df[x, 'east_lng'],
df[x, 'south_lat'], df[x, 'west_lng'],
df[x, 'north_lat'], df[x, 'west_lng']) ## need to close the polygon
, ncol =2, byrow = T
)
## create polygon objects
st_polygon(list(res))
})
## st_sfc : creates simple features collection
## st_sf : creates simple feature object
sfdf <- st_sf(geohash = df[, 'geohash'], st_sfc(lst))
sfdf
# Simple feature collection with 2 features and 1 field
# geometry type: POLYGON
# dimension: XY
# bbox: xmin: 48.64746 ymin: -4.350586 xmax: 48.69141 ymax: -4.262695
# epsg (SRID): NA
# proj4string: NA
# geohash st_sfc.lst.
# 1 gbsuv POLYGON((48.69141 -4.350586...
# 2 gbsuy POLYGON((48.69141 -4.306641...
plot(sfdf)
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