Element-by-element distance

[barryrowlingson]

st_distance(x,y) returns the full matrix of distances from all x to all y. Sometimes we might have two geometry vector - maybe source and destination - and want to compute the distance from each source to its corresponding destination.

This can be done with sapply(1:length(g1),function(i)st_distance(g1[[i]],g2[[i]])) if g1 and g2 are geometry vectors with the same length, and returns a vector of the same length. But maybe a "element_wise" parameter to st_distance that changes its behaviour might be a nice thing to have.

Would ideally use R's usual vector recycling rules if lengths don't match, with the usual warnings on vectors that don't repeat nicely.

I think this is even easier:

> mapply(st_distance, g1, g2)
[1] 152687.52 192552.37 297678.52 262699.58  76465.13
Warning message:
In mapply(st_distance, g1, g2) :
  longer argument not a multiple of length of shorter

Seems easy enough, maybe not worth adding the complexity of a new argument, but people will try things like this: https://gis.stackexchange.com/questions/249762/is-there-a-convenient-way-to-calculate-distances-between-two-geometry-columns if there's no way documented.

[edzer]

Good idea, but how to call it - would pairwise be a better description? mapply does the recycling already.

[barryrowlingson]

Maybe st_distance(g1, g2, by_element=TRUE), in a similar (but not identical) way that byid in gDistance works.

How would you do this with ST_Distance in PostGIS or SpatiaLITE? I don't have a spatial DB handy...

[edzer]

I have no clue!