[R-sig-Geo] Polygons changed when the geom column is converted to text and then back to sfc

[Loïc Valéry]

Dear Michael,

Thank you very much for your prompt and efficient reply : it solved the problem (the default value of the number of digits was set to 7).

To tell you the truth, I don't think I'd have found it on my own !
Cheers,
Loïc

________________________________________
De : Michael Sumner <mdsumner using gmail.com>
Envoyé : vendredi 27 septembre 2024 12:08
À : Loïc Valéry <lvalery using outlook.fr>
Cc : R-sig-geo Mailing List <R-sig-Geo using r-project.org>
Objet : Re: [R-sig-Geo] Polygons changed when the geom column is converted to text and then back to sfc
 
try 'digits = 8' or higher in st_as_text e.g. 

test_chr <- test |>
  dplyr::mutate(geom = sf::st_as_text(geom, EWKT = TRUE, digits = 8)) |>
  as.data.frame()

(or set with options("digits" = <8 or higher>))

Default value doesn't seem to be documented in sf, but the source shows it calls out to lwgeom::st_astext which defaults to whatever options("digits") value is. 

HTH





On Fri, Sep 27, 2024 at 6:23 PM Loïc Valéry <lvalery using outlook.fr> wrote:
Dear list members,

I hope this message finds you well.
I submit to you a problem with which I have been struggling for several days without finding the beginning of a solution.

I need (i) to convert the geom column of a sf object into text and transform the sf object into a dataframe,
           (ii) then convert this dataframe back into an sf object. 
My problem is that this only operation changes the shape of the original polygons.
I guess the problem lies with the layer itself, but so far I haven't been able to find the cause (the crs seems O.K., the geometry seems valid).

Please find attached a test layer containing only three contiguous polygons and, below, the code needed to reproduce the problem :


test <- sf::st_read("Please add your path/polygons.gpkg")

# Converting the sf object into a dataframe with geom as text
test_chr <- test |>
  dplyr::mutate(geom = sf::st_as_text(geom, EWKT = TRUE)) |>
  as.data.frame()

# Converting back to a sf object
test_new <- test_chr |>
  dplyr::mutate(geom = sf::st_as_sfc(geom)) |>
  sf::st_sf()

# Comparing the original ('test') and the new ('test_new') sf objects
plot(test$geom, lwd = 1, border = "blue")
plot(test_new$geom, lwd = 2, lty = 2, border = "red", add = TRUE)


Thank you in advance for your help.
Cheers,
Loïc
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--
Michael Sumner
Research Software Engineer
Australian Antarctic Division
Hobart, Australia
e-mail: mdsumner using gmail.com