Find the closest node to a given location

The function closestNode searches for the closest node in a gGraph or a gData object to a given location. It is possible to restrain the research to given values of a node attribute. For instance, one can search the closest node on land to a given location.

Usage

closestNode(x, ...)

# S4 method for class 'gGraph'
closestNode(
  x,
  loc,
  method = "knn",
  zoneSize = 5,
  attr.name = NULL,
  attr.values = NULL
)

# S4 method for class 'gData'
closestNode(
  x,
  method = "knn",
  zoneSize = 5,
  attr.name = NULL,
  attr.values = NULL
)

Arguments

x: a valid gGraph or gData object. In the latter case, the gGraph to which the gData is linked has to be in the current environment.
...: further arguments passed to specific methods.
loc: locations, specified as a list with two components indicating longitude and latitude of locations. Alternatively, this can be a data.frame or a matrix with longitude and latitude in columns, in this order. Note that locator() can be used to specify the locations interactively.
method: the method to use for finding the closest node. The default is "knn", which uses a fast FNN-based spatial index over unit-sphere Cartesian coordinates. The legacy "inArea" method is also available, but it is slower.
zoneSize: The initial size of the search area (in degrees). Only needs to be specified if method = "inArea". The search zone will be expanded until at least 3 candidate nodes are found.
attr.name: the optional name of a node attribute. See details.
attr.values: an optional vector giving values for attr.name. See details.

Value

If x is a gGraph object: a vector of node names.

If x is a gData object: a gData object with matching nodes stored in the @nodes.id slot. Note that previous content of @nodes.id will be erased.

Details

This function is also used to match locations of a gData object with nodes of the gGraph object to which it is linked.

When creating a gData object, if the gGraph.name argument is provided, then locations are matched with the gGraph object automatically, by an internal call to closestNode. Note, however, that it is not possible to specify node attributes (attr.name and attr.values) this way.

Functions

closestNode(gGraph): Method for gGraph
closestNode(gData): Method for gData

Examples

if (FALSE) { # \dontrun{
## interactive example ##
plot(worldgraph.10k, reset = TRUE)

## zooming in
geo.zoomin(list(x = c(-6, 38), y = c(35, 73)))
title("Europe")

## click some locations
myNodes <- closestNode(worldgraph.10k, locator(), attr.name = "habitat", attr.value = "land")
myNodes

## here are the closestNodes
points(getCoords(worldgraph.10k)[myNodes, , drop = FALSE], col = "red")
} # }

## example with a gData object ##
myLoc <- list(x = c(3, -8, 11, 28), y = c(50, 57, 71, 67)) # some locations
obj <- new("gData", coords = myLoc) # new gData object
obj
#> 
#> === gData object ===
#> 
#> @coords: spatial coordinates of 4 nodes
#>   lon lat
#> 1   3  50
#> 2  -8  57
#> 3  11  71
#> ...
#> 
#> @nodes.id: 0 nodes identifiers
#> character(0)
#> 
#> @data: data
#> NULL
#> ...
#> 
#> Associated gGraph:  

obj@gGraph.name <- "worldgraph.10k" # this could be done when creating obj
obj <- closestNode(obj, attr.name = "habitat", attr.value = "land")

## plot the result (original location -> assigned node)
plot(obj, type = "both", reset = TRUE)
#> Spherical geometry (s2) switched off
#> Spherical geometry (s2) switched on
title("'x'=location, 'o'=assigned node")