rleafletmapstmapmapview

# How to make small polygons from shapefile and extract coordinates

I have a small shapefile here: https://login.filesanywhere.com/fs/v.aspx?v=8c6d62865c626fb4a2ab called bay.RDS

`````` library(tmap)
library(leaflet)
library(mapview)
mapview(bay)
``````

I am trying to draw 50 meter wide polygons from shore to shore right at the below point and extract coordinates from it. I need multiple polygons next to each other but independently drawn as I need to color code them based on a column of values

``````pt <- st_sfc(st_point(c(-122.91, 38.17)), crs=4326)
mapview(bay) + mapview(pt, color ='red')
``````

I have included a snapshot below of what I am trying to accomplish. I would appreciate it if someone can show me how I can make just a couple of polygons and extract the coordinates from it so I can merge them to my dataset. Let me know if is not clear

Solution

• One approach, using {sf} and {dplyr}:

TLDR: below solution creates a horizontal baseline, buffers this baseline to result in a horizontal strip of chosen width, creates adjacent clones of this strip, combines them into a multipolygon, and rotates and shifts the mulitpolygon to expand southwards from a chosen anchor.

1. helper function `get_strips(...)` to return a multipolygon of adjacent strips:
``````get_strips <- function(n = 10, ## number of adjacent strips
strip_width = 1e3, # unit: m
strip_length = 1e5,
crs = 3857 ## default: 'google' mercator
){
## make horizontal line of length 'strip_length':
baseline <- matrix(c(-strip_length/2, strip_length/2, 0, 0), ncol = 2) |>
st_linestring()
## make line a polygon of width 'strip_width'
basestrip  <- baseline |>
st_buffer(strip_width/2, endCapStyle = 'FLAT')
## return a spatial dataframe of n adjacent strips from north to south:
st_sf(strip_id = sprintf('strip_%02.f', 1:n),
geometry = Map(1:n,
f = \(index) basestrip - c(0, index * strip_width - strip_width/2)
)|>
st_sfc() |> st_cast('MULTIPOLYGON') |> st_set_crs(crs)
)
}
``````
1. helper function to rotate the geometry of a spatial dataframe
``````    rotate_feature <- function(feature,
angle = 0, ## rotation in degree
anchor = c(0, 0) ## top center point of grid
){
crs <- st_crs(feature)
a <- -pi * angle/180 ## convert to radiant ccw
## create rotation matrix:
rotmat <- matrix(c(cos(a), sin(a), -sin(a), cos(a)), 2, 2)
## rotate, recenter, restore CRS:
st_geometry(feature) <- st_geometry(feature) * rotmat + anchor
feature |> st_set_crs(crs)
}
``````
1. load libraries, create play data
``````    library(sf)
library(dplyr)

the_lake <- readRDS(file.choose()) ## spatial feature supplied by OP
the_anchor <- c(5.08e5, 4.224e6) ## anchor strip grid here
``````
1. generate desired spatial features
``````    the_strips <-
get_strips(crs = st_crs(the_lake)) |>
rotate_feature(angle = 30, anchor = the_anchor)

the_cropped_strips <- the_strips |>
st_intersection(the_lake)

## casting polygons to multilinestrings to only
## obtain the corner points (not areas) on intersection:
the_corners <-
st_intersection(
st_cast(the_strips, 'MULTILINESTRING'),
st_cast(the_lake, 'MULTILINESTRING')
)
``````
1. inspect
``````    library(ggplot2)

anchor_point <- the_anchor |> st_point() |>
st_sfc(crs = st_crs(the_lake))

ggplot() +
geom_sf(data = the_lake) +
geom_sf(data = the_strips, fill = NA) +
geom_sf(data = the_cropped_strips, aes(fill = strip_id)) +
geom_sf(data = the_corners) +
geom_sf(data = anchor_point, pch = '+', size = 5) +
geom_sf_label(data = anchor_point, nudge_y = 1e3, label = 'Anchor') +
coord_sf(xlim = 1e5 * c(5, 5.2), ylim = 1e6 * c(4.21, 4.235))

``````

1. extract corners per polygon:
``````    the_corners |>
rowwise() |>
reframe(strip_id,
coords = st_coordinates(geometry)
)
``````
``````    ## # A tibble: 50 x 2
##    strip_id coords[,1]     [,2]  [,3]
##    <chr>         <dbl>    <dbl> <dbl>
##  1 strip_01    508864. 4223922.     1
##  2 strip_01    507927. 4223381.     1
##  3 strip_01    508230. 4222400.     1
##  4 strip_01    509328. 4223035.     1
##  5 strip_02    508230. 4222400.     1
##  6 strip_02    508791. 4221570.     1
``````