fzenoni · November 2, 2023 11:06 · AWSisco · May 31, 2019 · csaybar · Aug 27, 2019
diff --git a/ortho_view.R b/ortho_view.R
 # Download earth data first
 # https://www.naturalearthdata.com/http//www.naturalearthdata.com/download/110m/physical/ne_110m_land.zip

 library(sf)
 library(lwgeom)
 library(dplyr)
 library(ggplot2)
 library(mapview)

 # Read the data
 mini_world <- read_sf('data/ne_110m_land/ne_110m_land.shp')

 # Define the orthographic projection
 # Choose lat_0 with -90 <= lat_0 <= 90 and lon_0 with -180 <= lon_0 <= 180
 lat <- 45
 lon <- 2
 ortho <- paste0('+proj=ortho +lat_0=', lat, ' +lon_0=', lon, ' +x_0=0 +y_0=0 +a=6371000 +b=6371000 +units=m +no_defs')

 # Define the polygon that will help you finding the "blade"
 # to split what lies within and without your projection
 circle <- st_point(x = c(0,0)) %>% st_buffer(dist = 6371000) %>% st_sfc(crs = ortho)
  
 # Project this polygon in lat-lon
 circle_longlat <- circle %>% st_transform(crs = 4326)

 # circle_longlat cannot be used as it is
 # You must decompose it into a string with ordered longitudes
 # Then complete the polygon definition to cover the hemisphere
 if(lat != 0) {
    circle_longlat <- st_boundary(circle_longlat)
    
    circle_coords <- st_coordinates(circle_longlat)[, c(1,2)]
    circle_coords <- circle_coords[order(circle_coords[, 1]),]
    circle_coords <- circle_coords[!duplicated(circle_coords),]
    
    # Rebuild line
    circle_longlat <- st_linestring(circle_coords) %>% st_sfc(crs = 4326)
    
    if(lat > 0) {
      rectangle <- list(rbind(circle_coords,
                              c(X = 180, circle_coords[nrow(circle_coords), 'Y']),
                              c(X = 180, Y = 90),
                              c(X = -180, Y = 90),
                              c(X = -180, circle_coords[1, 'Y']),
                              circle_coords[1, c('X','Y')])) %>% 
        st_polygon() %>% st_sfc(crs = 4326)
    } else {
      rectangle <- list(rbind(circle_coords,
                              c(X = 180, circle_coords[nrow(circle_coords), 'Y']),
                              c(X = 180, Y = -90),
                              c(X = -180, Y = -90),
                              c(X = -180, circle_coords[1, 'Y']),
                              circle_coords[1, c('X','Y')])) %>% 
        st_polygon() %>% st_sfc(crs = 4326)
    }
    
    circle_longlat <- st_union(st_make_valid(circle_longlat), st_make_valid(rectangle))
  }
  
 # This visualization shows the visible emisphere in red
 ggplot() +
    geom_sf(data = mini_world) +
    geom_sf(data = circle_longlat, color = 'red', fill = 'red', alpha = 0.3)
  
 # A small negative buffer is necessary to avoid polygons still disappearing in a few pathological cases
 # I should not change the shapes too much
  visible <- st_intersection(st_make_valid(mini_world), st_buffer(circle_longlat, -0.09)) %>%
    st_transform(crs = ortho)

 # DISCLAIMER: This section is the outcome of trial-and-error and I don't claim it is the best approach 
 # Resulting polygons are often broken and they need to be fixed
 # Get reason why they're broken
 broken_reason <- st_is_valid(visible, reason = TRUE)
  
 # First fix NA's by decomposing them
 # Remove them from visible for now
 na_visible <- visible[is.na(broken_reason),]
 visible <- visible[!is.na(broken_reason),]
  
 # Open and close polygons
 na_visible <- st_cast(na_visible, 'MULTILINESTRING') %>% 
    st_cast('LINESTRING', do_split=TRUE)
 na_visible <- na_visible %>% mutate(npts = npts(geometry, by_feature = TRUE))
 # Exclude polygons with less than 4 points
 na_visible <- na_visible %>%
    filter(npts >=4) %>%
    select(-npts) %>%
    st_cast('POLYGON')

 # Fix other broken polygons
 broken <- which(!st_is_valid(visible))
 for(land in broken) {
    result = tryCatch({
    # visible[land,] <- st_buffer(visible[land,], 0) # Sometimes useful sometimes not
    visible[land,] <- st_make_valid(visible[land,]) %>% 
        st_collection_extract()  
    }, error = function(e) {
      visible[land,] <<- st_buffer(visible[land,], 0)
    })
 }

 # Bind together the two tables
 visible <- rbind(visible, na_visible)

 # Final plot
 ggplot() +
    geom_sf(data = circle,
        #fill = 'aliceblue') + # if you like the color
        fill = NA) +
    geom_sf(data=st_collection_extract(visible)) +
    coord_sf(crs = ortho)
	# Download earth data first
	# https://www.naturalearthdata.com/http//www.naturalearthdata.com/download/110m/physical/ne_110m_land.zip

	library(sf)
	library(lwgeom)
	library(dplyr)
	library(ggplot2)
	library(mapview)

	# Read the data
	mini_world <- read_sf('data/ne_110m_land/ne_110m_land.shp')

	# Define the orthographic projection
	# Choose lat_0 with -90 <= lat_0 <= 90 and lon_0 with -180 <= lon_0 <= 180
	lat <- 45
	lon <- 2
	ortho <- paste0('+proj=ortho +lat_0=', lat, ' +lon_0=', lon, ' +x_0=0 +y_0=0 +a=6371000 +b=6371000 +units=m +no_defs')

	# Define the polygon that will help you finding the "blade"
	# to split what lies within and without your projection
	circle <- st_point(x = c(0,0)) %>% st_buffer(dist = 6371000) %>% st_sfc(crs = ortho)

	# Project this polygon in lat-lon
	circle_longlat <- circle %>% st_transform(crs = 4326)

	# circle_longlat cannot be used as it is
	# You must decompose it into a string with ordered longitudes
	# Then complete the polygon definition to cover the hemisphere
	if(lat != 0) {
	circle_longlat <- st_boundary(circle_longlat)

	circle_coords <- st_coordinates(circle_longlat)[, c(1,2)]
	circle_coords <- circle_coords[order(circle_coords[, 1]),]
	circle_coords <- circle_coords[!duplicated(circle_coords),]

	# Rebuild line
	circle_longlat <- st_linestring(circle_coords) %>% st_sfc(crs = 4326)

	if(lat > 0) {
	rectangle <- list(rbind(circle_coords,
	c(X = 180, circle_coords[nrow(circle_coords), 'Y']),
	c(X = 180, Y = 90),
	c(X = -180, Y = 90),
	c(X = -180, circle_coords[1, 'Y']),
	circle_coords[1, c('X','Y')])) %>%
	st_polygon() %>% st_sfc(crs = 4326)
	} else {
	rectangle <- list(rbind(circle_coords,
	c(X = 180, circle_coords[nrow(circle_coords), 'Y']),
	c(X = 180, Y = -90),
	c(X = -180, Y = -90),
	c(X = -180, circle_coords[1, 'Y']),
	circle_coords[1, c('X','Y')])) %>%
	st_polygon() %>% st_sfc(crs = 4326)
	}

	circle_longlat <- st_union(st_make_valid(circle_longlat), st_make_valid(rectangle))
	}

	# This visualization shows the visible emisphere in red
	ggplot() +
	geom_sf(data = mini_world) +
	geom_sf(data = circle_longlat, color = 'red', fill = 'red', alpha = 0.3)

	# A small negative buffer is necessary to avoid polygons still disappearing in a few pathological cases
	# I should not change the shapes too much
	visible <- st_intersection(st_make_valid(mini_world), st_buffer(circle_longlat, -0.09)) %>%
	st_transform(crs = ortho)

	# DISCLAIMER: This section is the outcome of trial-and-error and I don't claim it is the best approach
	# Resulting polygons are often broken and they need to be fixed
	# Get reason why they're broken
	broken_reason <- st_is_valid(visible, reason = TRUE)

	# First fix NA's by decomposing them
	# Remove them from visible for now
	na_visible <- visible[is.na(broken_reason),]
	visible <- visible[!is.na(broken_reason),]

	# Open and close polygons
	na_visible <- st_cast(na_visible, 'MULTILINESTRING') %>%
	st_cast('LINESTRING', do_split=TRUE)
	na_visible <- na_visible %>% mutate(npts = npts(geometry, by_feature = TRUE))
	# Exclude polygons with less than 4 points
	na_visible <- na_visible %>%
	filter(npts >=4) %>%
	select(-npts) %>%
	st_cast('POLYGON')

	# Fix other broken polygons
	broken <- which(!st_is_valid(visible))
	for(land in broken) {
	result = tryCatch({
	# visible[land,] <- st_buffer(visible[land,], 0) # Sometimes useful sometimes not
	visible[land,] <- st_make_valid(visible[land,]) %>%
	st_collection_extract()
	}, error = function(e) {
	visible[land,] <<- st_buffer(visible[land,], 0)
	})
	}

	# Bind together the two tables
	visible <- rbind(visible, na_visible)

	# Final plot
	ggplot() +
	geom_sf(data = circle,
	#fill = 'aliceblue') + # if you like the color
	fill = NA) +
	geom_sf(data=st_collection_extract(visible)) +
	coord_sf(crs = ortho)