Hi Ben,
I had a bit of fun with this [1]...the act of following a shortest path
and finding the endpoints are sort of related (in that if you follow the
shortest path setting a maximum distance threshold, eventually you will
fail to find any more points, and you've found (maybe) and endpoint!).
I hope this is helpful!
Cheers,
-dewey
[1] https://gist.github.com/paleolimbot/0be47836de5008f308959923dac02c5b
On 2024-06-05 13:20, Ben Tupper wrote:
Hello,
NOAA's Ocean Prediction Center (https://ocean.weather.gov/) serves the
US Navy's almost-daily estimate of the locations of the north and
south walls of the Gulf Stream in the North Atlantic
(https://ocean.weather.gov/gulf_stream_latest.txt). The also serve an
archive via a FTP server
(https://ftp.opc.ncep.noaa.gov/grids/experimental/GStream) for
downloads by year. The dataset provides manually estimated of
locations along the so-called north and south walls of the Gulf
Stream, although there is ongoing research and debate about the best
way to define the Gulf Stream. My colleagues and I have been slowly
pulling together a small package to make working with this data easier
(https://github.com/BigelowLab/gstream).
This dataset provides an interesting spatial puzzle because the points
for a specific wall on a given date are not ordered. Assuming that
the points could be ordered into LINESTRING, the problem is how to do
it.
We probably can use sfnetworks package
(https://luukvdmeer.github.io/sfnetworks/) extensive tools to follow a
shortest path... but first one of the two endpoints must be found.
Eeek! How does one do that programmatically? The plot attached shows
the points for the north wall on 2020-12-19. Also shown are the first
and last points in the record which are clearly not the endpoints of a
path we might assemble from the points.
#### START
suppressPackageStartupMessages({
library(sf)
library(readr)
library(dplyr)
})
file =
"https://raw.githubusercontent.com/BigelowLab/gstream/main/inst/examples/2020-12-19-north.csv";
x = readr::read_csv(file, col_types = "nn") |>
sf::st_as_sf(coords = c("X", "Y"), crs = 4326) |>
sf::st_as_sf()
p = sf::st_cast(dplyr::select(x, attr(x, "sf_column")), "POINT")
plot(sf::st_geometry(x), type = "b", axes = TRUE, reset = FALSE)
plot(dplyr::slice(p,1), col = "orange", add = TRUE, pch = 2, cex = 1.5)
plot(dplyr::slice(p,nrow(x)), col = "orange", add = TRUE, pch = 6, cex
= 1.5)
### END
--
Ben Tupper he/him
Research Associate
Bigelow Laboratory for Ocean Sciences
–––––––––––––––––––––––––––––––––––––––––––––––––––
E btup...@bigelow.org
O 207-315-2567
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Bold Science for Our Blue Planet | BIGELOW.ORG
60 BIGELOW DRIVE | EAST BOOTHBAY, MAINE 04544 USA
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