Construction
From a nodes and edges table
The most basic way to construct a sfnetwork with spatially explicit
edges is by providing the sfnetwork construction function
one sf object containing the nodes, and another sf object containing the
edges. This edges table should include a from and to
column referring to the node indices of the edge endpoints. With a node
index we mean the position of a node in the nodes table (i.e. its
rownumber). A small toy example:
p1 = st_point(c(7, 51))
p2 = st_point(c(7, 52))
p3 = st_point(c(8, 52))
p4 = st_point(c(8, 51.5))
l1 = st_sfc(st_linestring(c(p1, p2)))
l2 = st_sfc(st_linestring(c(p1, p4, p3)))
l3 = st_sfc(st_linestring(c(p3, p2)))
edges = st_as_sf(c(l1, l2, l3), crs = 4326)
nodes = st_as_sf(c(st_sfc(p1), st_sfc(p2), st_sfc(p3)), crs = 4326)
edges$from = c(1, 1, 3)
edges$to = c(2, 3, 2)
net = sfnetwork(nodes, edges)
#> Checking if spatial network structure is valid...
#> Spatial network structure is valid
net#> # A sfnetwork with 3 nodes and 3 edges
#> #
#> # CRS: EPSG:4326
#> #
#> # A directed acyclic simple graph with 1 component with spatially explicit edges
#> #
#> # Node Data: 3 × 1 (active)
#> # Geometry type: POINT
#> # Dimension: XY
#> # Bounding box: xmin: 7 ymin: 51 xmax: 8 ymax: 52
#> x
#> <POINT [°]>
#> 1 (7 51)
#> 2 (7 52)
#> 3 (8 52)
#> #
#> # Edge Data: 3 × 3
#> # Geometry type: LINESTRING
#> # Dimension: XY
#> # Bounding box: xmin: 7 ymin: 51 xmax: 8 ymax: 52
#> from to x
#> <int> <int> <LINESTRING [°]>
#> 1 1 2 (7 51, 7 52)
#> 2 1 3 (7 51, 8 51.5, 8 52)
#> 3 3 2 (8 52, 7 52)
class(net)
#> [1] "sfnetwork" "tbl_graph" "igraph"By default, the created network is a directed network. If you want to
create an undirected network, set directed = FALSE. Note
that for undirected networks, the indices in the from and
to columns are re-arranged such that the from index is
always smaller than (or equal to, for loop edges) the to index.
However, the linestring geometries remain unchanged. That means that in
undirected networks it can happen that for some edges the from
index refers to the last point of the edge linestring, and the
to index to the first point. The behavior of ordering the
indices comes from igraph and might be confusing, but
remember that in undirected networks the terms from and
to do not have a meaning and can thus be used
interchangeably.
net = sfnetwork(nodes, edges, directed = FALSE)
#> Checking if spatial network structure is valid...
#> Spatial network structure is valid
net#> # A sfnetwork with 3 nodes and 3 edges
#> #
#> # CRS: EPSG:4326
#> #
#> # An undirected simple graph with 1 component with spatially explicit edges
#> #
#> # Node Data: 3 × 1 (active)
#> # Geometry type: POINT
#> # Dimension: XY
#> # Bounding box: xmin: 7 ymin: 51 xmax: 8 ymax: 52
#> x
#> <POINT [°]>
#> 1 (7 51)
#> 2 (7 52)
#> 3 (8 52)
#> #
#> # Edge Data: 3 × 3
#> # Geometry type: LINESTRING
#> # Dimension: XY
#> # Bounding box: xmin: 7 ymin: 51 xmax: 8 ymax: 52
#> from to x
#> <int> <int> <LINESTRING [°]>
#> 1 1 2 (7 51, 7 52)
#> 2 1 3 (7 51, 8 51.5, 8 52)
#> 3 2 3 (8 52, 7 52)
Instead of from and to columns containing integers that refer to node indices, the provided edges table can also have from and to columns containing characters that refer to node keys. In that case, you should tell the construction function which column in the nodes table contains these keys. Internally, they will then be converted to integer indices.
nodes$name = c("city", "village", "farm")
edges$from = c("city", "city", "farm")
edges$to = c("village", "farm", "village")
edges
#> Simple feature collection with 3 features and 2 fields
#> Geometry type: LINESTRING
#> Dimension: XY
#> Bounding box: xmin: 7 ymin: 51 xmax: 8 ymax: 52
#> Geodetic CRS: WGS 84
#> x from to
#> 1 LINESTRING (7 51, 7 52) city village
#> 2 LINESTRING (7 51, 8 51.5, 8... city farm
#> 3 LINESTRING (8 52, 7 52) farm village
net = sfnetwork(nodes, edges, node_key = "name")
#> Checking if spatial network structure is valid...
#> Spatial network structure is valid
net#> # A sfnetwork with 3 nodes and 3 edges
#> #
#> # CRS: EPSG:4326
#> #
#> # A directed acyclic simple graph with 1 component with spatially explicit edges
#> #
#> # Node Data: 3 × 2 (active)
#> # Geometry type: POINT
#> # Dimension: XY
#> # Bounding box: xmin: 7 ymin: 51 xmax: 8 ymax: 52
#> x name
#> <POINT [°]> <chr>
#> 1 (7 51) city
#> 2 (7 52) village
#> 3 (8 52) farm
#> #
#> # Edge Data: 3 × 3
#> # Geometry type: LINESTRING
#> # Dimension: XY
#> # Bounding box: xmin: 7 ymin: 51 xmax: 8 ymax: 52
#> from to x
#> <int> <int> <LINESTRING [°]>
#> 1 1 2 (7 51, 7 52)
#> 2 1 3 (7 51, 8 51.5, 8 52)
#> 3 3 2 (8 52, 7 52)
If your edges table does not have linestring geometries, but only references to node indices or keys, you can tell the construction function to create the linestring geometries during construction. This will draw a straight line between the endpoints of each edge.
st_geometry(edges) = NULL
other_net = sfnetwork(nodes, edges, edges_as_lines = TRUE)
#> Checking if spatial network structure is valid...
#> Spatial network structure is valid
plot(net, cex = 2, lwd = 2, main = "Original geometries")
plot(other_net, cex = 2, lwd = 2, main = "Straight lines")
A sfnetwork should have a valid spatial network structure. For the nodes, this currently means that their geometries should all be of type POINT. In the case of spatially explicit edges, edge geometries should all be of type LINESTRING, nodes and edges should have the same CRS and endpoints of edges should match their corresponding node coordinates.
If your provided data do not meet these requirements, the construction function will throw an error.
st_geometry(edges) = st_sfc(c(l2, l3, l1), crs = 4326)
net = sfnetwork(nodes, edges)
#> Checking if spatial network structure is valid...
#> Error: Edge boundaries do not match their corresponding nodesYou can skip the validity checks if you are already sure your input
data meet the requirements, or if you don’t care that they don’t. To do
so, set force = TRUE. However, remember that all functions
in sfnetworks are designed with the assumption that the
network has a valid structure.
From an sf object with linestring geometries
Instead of already providing a nodes and edges table with a valid network structure, it is also possible to create a network by only providing an sf object with geometries of type LINESTRING. Probably, this way of construction is most convenient and will be most often used.
It works as follows: the provided lines form the edges of the network, and nodes are created at their endpoints. Endpoints that are shared between multiple lines become one single node.
See below an example using the Roxel dataset that comes with the package. This dataset is an sf object with LINESTRING geometries that form the road network of Roxel, a neighborhood in the German city of Münster.
roxel#> Simple feature collection with 851 features and 2 fields
#> Geometry type: LINESTRING
#> Dimension: XY
#> Bounding box: xmin: 7.522594 ymin: 51.94151 xmax: 7.546705 ymax: 51.9612
#> Geodetic CRS: WGS 84
#> # A tibble: 851 × 3
#> name type geometry
#> * <chr> <fct> <LINESTRING [°]>
#> 1 Havixbecker Strasse residential (7.533722 51.95556, 7.533461 51.95576)
#> 2 Pienersallee secondary (7.532442 51.95422, 7.53236 51.95377, 7.53…
#> 3 Schulte-Bernd-Strasse residential (7.532709 51.95209, 7.532823 51.95239, 7.5…
#> 4 NA path (7.540063 51.94468, 7.539696 51.94479, 7.5…
#> 5 Welsingheide residential (7.537673 51.9475, 7.537614 51.94562)
#> 6 NA footway (7.543791 51.94733, 7.54369 51.94686, 7.54…
#> 7 NA footway (7.54012 51.94478, 7.539931 51.94514)
#> 8 NA path (7.53822 51.94546, 7.538131 51.94549, 7.53…
#> 9 NA track (7.540063 51.94468, 7.540338 51.94468, 7.5…
#> 10 NA track (7.5424 51.94599, 7.54205 51.94629, 7.5419…
#> # … with 841 more rows
net = as_sfnetwork(roxel)
plot(net)
Other methods to convert ‘foreign’ objects into a sfnetwork exists as
well, e.g. for SpatialLinesNetwork objects from stplanr and
linnet objects from spatstat. See here
for an overview.
