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I want to sample points (latitude-longitude pairs) on borders of the US states, or points close to borders. With the following code, I am able to sample points in an example multipolygon. How can I do this on the borders instead?

library(sf)
nc = st_read(system.file("shape/nc.shp", package="sf"))
plot(nc$geometry[1])
plot(st_sample(nc$geometry[1], 1000))
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1 Answer 1

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Cast the polygon into a multi-linestring and sample from that:

> plot(st_sample(st_cast(nc$geometry[1],"MULTILINESTRING"), 1000))
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  • Cool approach! I have not seen this before.
    – Jeffrey Evans
    Commented Mar 4, 2020 at 0:02
  • Thanks! Does it oversamples from the line segments with longer lengths? In other words, is the expected number of points from a line segment is proportional to its length?
    – Ragıp Gürlek
    Commented Mar 4, 2020 at 14:28
  • It distributes points uniformly along the perimeter of the feature. So if you have a triangle with sides 3,4,5 then the number of points on each side would have expected ratio of 3:4:5.
    – Spacedman
    Commented Mar 4, 2020 at 17:29

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