materialize sf example

[mdsumner]

  library(L3bin)
  nr <- 360  ## NUMROWS, i.e. how many steps in latitude seq(-90, 90, length.out = nr) is the basis of the binning

  ## this just stores the information about the scheme
  bins <- L3bin(nr)
  
  ## we can now use the L3bin:: functions to lookup bin by longlat, or generate longlat by bin, etc. 
  ## but here we materialize a relatively small set of global bins and use sf
  
  
  ## matrix of cbind(xmin, xmax, ymin, ymax ) for every bin (don't do this for really large NUMROWS, even 180 is pretty dense at global scale)
  ex <- extent_from_bin(1:bins$totbins, nr)
  #plot(range(ex[,1:2]), range(ex[,3:4]))
  #rect(ex[,1], ex[,3], ex[,2], ex[,4])
  #lbins <- crop_bins(bins, extent = c(100, 180, -90, -20))
  #ex <- extent_from_bin(lbins, nr)

#' Create sf from matrix of xmin, xmax, ymin, ymax
#' @param ex matrix 4-columns
#' @param crs projection string of the coordinates
to_sf <- function(ex, crs = "OGC:CRS84") {
  idx <- c(1, 2, 2, 1, 1, 
           3, 3, 4, 4, 3)
  proto <- sf::st_polygon(list(cbind(0, c(1:4, 1))))
  sf::st_sfc(lapply(split(t(ex), rep(1:nrow(ex), each = 4L)), 
                    function(.x) {
                      proto[[1L]][] <- .x[idx]
                      proto
                    }), crs = crs)
}
sf::sf_use_s2(FALSE)
#> Spherical geometry (s2) switched off

#' Densify segments of sf safely
#' ‘dx’ is in the units of the geometry coordinates, so 1 (111111m) or 0.1 (111111m) maybe lower near the poles
#' @param sf lines or polygons
#' @param dx delta for shortest length of new segments from old (straight-line native to the geometry coordinates)
segit <- function(x, dx) {
  crs <- sf::st_crs(x)
  sf::st_set_crs(sf::st_segmentize(sf::st_set_crs(x, NA), dx), crs)
} 

## 70Mb at 360 rows
sfx <- to_sf(ex)

## we can crop the bins with crop_bins(bins) to get which bins fall in a longlat extent
## but, we've already materialized as polygons so we can use the GIS stuff
## everything within 1000km (beware of unit shenanigans though)
keep <- unclass(sf::st_distance(sfx, sf::st_sfc(sf::st_point(cbind(147, -42)), crs = sf::st_crs(sfx)))) <= 1e6
plot(sfx[keep])
maps::map(add = TRUE)
abline(v = 100:180,h = -50:-10, lty = 2, col = rgb(0, 0, 0, .4))
axis(1); axis(2)

## dx that's sensible, portion of  a degree - will depend on where (and it's only a constant which is not good)
utm <- "+proj=utm +zone=55 +south"
plot(st_transform(segit(sfx[keep], .1), utm))
plot(st_transform(rnaturalearth::ne_countries(country = "Australia", returnclass = "sf"), utm)[1], add = T, colour = NA)

^{Created on 2022-07-21 by the reprex package (v2.0.1)}

[mdsumner]

ping @ozjimbob