Distance to coastline

Description

This function is a wrapper for a few other functions and calculates the
distance of geo points to the nearest coastline. It is meant to be used
for marine data. In fact it calculates the distance to the nearest line.
When using the coastline = "mapdata" method, for points on land the
result may well be the distance to a national border that is not the
coastline! The method coastline = "ne" uses lines representing only
the coastline, but with much lower resolution.

The input (and the coastline) will be regarded as WGS84. This can be
reprojected to the planar UTM 32 references system by choosing
as_UTM32 = TRUE, which will make the processing faster, especially
when global coastline is not filtered via coastline_crop, but it is
only recommended for smaller spatial extend, or for a quick overview.

EDIT: After updates to the sf package, “the earth is no longer
flat”. This creates
problems with the use of coastline = "mapdata". As a workaround use
spherical = FALSE to turn off S2 processing.

The result is either a vector of distances (in m) corresponding to the
input coordinates (default) or a matrix of distances between each point
and each line of the coastline data (this may actually be quite useless
because the coastline data is not output). A figure of the results can
be plotted with the plot = TRUE argument.

NOTE: requires packages mapdata (for coastline = "mapdata") and
rnaturalearthdata (for coastline = "ne") to be installed.

Arguments

lons – vector of longitudes

lats – vector of latitudes

coastline_crop = NULL – numeric vector with xmin, ymin, xmax and ymax
for cropping the coastline to a bounding box, e.g. c(xmin = -1, ymin =
50, xmax = 11, ymax = 60), elements do not have to be named but must be
in the correct order

coastline = “ne” – coastline source; “ne” for Natural Earth
(www.naturalearthdata.com) (faster), “mapdata” for using map('world')
(more precise)

as_utm32 = FALSE – transform both points and coast to UTM32, i.e. a
planar projection, for more speed

spherical = FALSE – the default turns off S2 processing in the sf
package, the previous system state is saved and restored when the
function is run.

output = “mindist” – “mindist” for distance to nearest coastline,
“distmat” for matrix of distances between points (in rows) and every
line (in cols)

plot = FALSE – should the result be shown in a plot

Value

When output = "mindist" a vector of distances in m to the nearest
coastline corresponding to the input points.

When output = "distmat" a matrix of distances in m with the rows
corresponding to input points and columns corresponding to each line
element of the coastline data.

When plot = TRUE a plot will be output in the graphic device.

Examples

Dummy points:

lons <- seq(2, 9, length.out = 10)
lats <- seq(52, 58, length.out = 10)

Default result

dist <- dist2coast(lons, lats)
dist

##  [1]  32170.82  74544.29  81416.67 100955.32 143297.58 142759.84  89782.11
##  [8]  42071.88  31545.26  50412.19

Result with finer coastline, filtered coastline, and plot output

dist <- dist2coast(lons, lats, coastline_crop = c(-5, 50, 15, 60), coastline = "mapdata", plot = TRUE)

dist

##  [1]  31031.58  70734.84  83577.18  88236.61 138237.26 144945.10  91933.34
##  [8]  44650.78  34420.04  43762.62