Functions to snap points to nodes or edges

[luukvdmeer]

Is your feature request related to a problem? Please describe.
It would be nice to have a function that takes a (set of) spatial point(s), either as sf, sfc or sfg, and snaps them to the nearest node or nearest edge. In that way you can provide any spatial point to a shortest path query, no matter if it lies exactly on the network or not.

Describe the solution you'd like
A function st_snap_point_to_node that replaces the geometry of the given point by the geometry of the nearest node, and a function st_snap_point_to_edge that replaces the geometry of the given point by the nearest point on the nearest edge. The first can simply use sf's st_nearest_feature (or st_nearest_node if we implement that), the latter can use sf's st_nearest_points to find the nearest point on the nearest edge.

Probably these functions will need a tolerance argument, like st_snap in sf.

[luukvdmeer]

See also how the shp2graph package handles this: https://www.mdpi.com/2220-9964/7/8/293/pdf

[Robinlovelace]

shp2graph is slow. The fastest approach I know of is with the nabor package, which I use in stplanr: https://github.com/ropensci/stplanr/blob/129b679c89a4ff9ba7f73dd5d59beb9bb734ace4/R/node-funs.R#L122

[luukvdmeer]

Ah, yes, I remember now we also used nabor in our blogpost. Regarding shp2graph I was mainly pointing to the figure with the explanation of the four strategies, which I like!

[Robinlovelace]

Yes it's a great figure, we need more good visuals like that. Good point.

[rafapereirabr]

Hi all. In the gtfs2gps package we use a quite fast function based on C++ to snap points to the closest points of another set using. Perhaps it might be useful here.

[luukvdmeer]

This topic was getting a little bit scattered.

There are now separate issues:

• Snapping points to the nearest node Snap spatial points to the nearest node #57
• Snapping points to the nearest point on the nearest edge Snap spatial points to the nearest point on the nearest edge #54
• Snapping points to the network with pseudo-edges Blend points into the network using pseudo-edges #58