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Converts sf::sfc_LINSTRING objects to polygons by connecting end points around the given bounding box. This is particularly useful for plotting water and land delineated by coastlines. Coastlines in OpenStreetMap are lines, not polygons, and so there is no directly way to plot ocean water distinct from land. This function enables that by connecting the end points of coastline LINESTRING objects to form closed polygons.

Usage

osm_line2poly(obj, bbox)

Arguments

obj

A Simple Features (sf) data frame of lines, typically as returned by extract_osm_objects, or by osmdata::osmdata_sf.

bbox

bounding box (Latitude-longitude range) to be plotted. A 2-by-2 matrix of 4 elements with columns of min and max values, and rows of x and y values. Can also be an object of class sf, for example as returned from extract_osm_objects or the osmdata package, in which case the bounding box will be extracted from the object coordinates.

Value

A list of three Simple Features (sf) data frames, labelled sea islands and land.

Details

This is a tricky problem for a number of reasons, and the current implementation may not be correct, although it does successfully deal with a few tough situations. Some of the issues are: an osm coastline query returns a mixture of "ways" and polygons.

Polygons correspond to islands, but not all islands are polygons. A "way" is a connected set of points with the land on the left. A piece of coastline in a bounding box may consist of multiple ways, which need to be connected together to create a polygon. Also, ways extend outside the query bounding box, and may join other ways that enter the bounding box (e.g ends of a peninsula). The degree to which this happens depends on the scale of the bounding box. Coastlines may enter at any bounding box edge and exit at any other, including the one they entered from.

Examples

# This example uses the \code{osmdata} package to extract data from
# a named bounding box
if (FALSE) {
library (magrittr)
library (osmdata)
bb <- osmdata::getbb ("melbourne, australia")
coast <- extract_osm_objects (bbox = bb,
                              key = "natural",
                              value = "coastline",
                              return_type = "line")
coast <- osm_line2poly (coast, bbox = bb)
# The following map then colours in just the ocean:
map <- osm_basemap (bbox = bb) %>%
    add_osm_objects (coast$sea, col = "lightsteelblue") %>%
    print_osm_map ()
}