[R-sig-Geo] Re-scale units of coordinates in sf geometry?

[Andrea Gilardi]

Thank you very much for your example! I briefly checked the examples
reported in ?st_transform and the docs at the PROJ website around
"Unit conversion"
(https://proj.org/operations/conversions/unitconvert.html) and I found
that the following code also seems to work (although with a warning
message that I'm not sure I understand):

library(sp)
demo(meuse, echo = FALSE, ask = FALSE)
library(sf)
#> Linking to GEOS 3.10.2, GDAL 3.4.1, PROJ 7.2.1; sf_use_s2() is TRUE
sf_proj_network(TRUE)
#> [1] "https://cdn.proj.org"
st_as_sf(meuse) |> st_bbox()
#>   xmin   ymin   xmax   ymax
#> 178605 329714 181390 333611
meuse2 <- st_as_sf(meuse) |>
  st_transform(pipeline = "+proj=pipeline +step +proj=unitconvert
+xy_in=m +xy_out=km")
#> Warning in st_transform.sfc(st_geometry(x), crs, ...): pipeline not found in
#> PROJ-suggested candidate transformations
st_bbox(meuse2)
#>    xmin    ymin    xmax    ymax
#> 178.605 329.714 181.390 333.611

I think this might be easier than manually adjusting the WKT
representation of the CRS, but I'm not sure that this is really a
recommended way to transform units since, for some reason, it does not
modify the CRS of the objects which makes any analysis very difficult:

all.equal(st_crs(meuse), st_crs(meuse2))
#> [1] TRUE
library(mapview)
mapview(meuse) # seems right
mapview(meuse2) # completely off

Andrea

Il giorno mar 31 gen 2023 alle ore 12:33 Edzer Pebesma
<edzer.pebesma using uni-muenster.de> ha scritto:
>
>
>
> On 31/01/2023 12:12, Andrea Gilardi wrote:
> >>   st_transform can do transformations and conversions between different CRSs, unit conversions is a special case of that.
> >
> > Dear all, I'm sorry if this is a trivial or stupid question but I was
> > wondering if you could provide an example of using st_transform to
> > apply a unit conversion transformation. For example, if I define
> > something like
> >
> > library(sf)
> > pt <- st_sfc(st_point(c(1500000, 5000000)), crs = 3003) # some place
> > in North Italy
> > st_crs(3003) # units are expressed in metres
> >
> > can I use st_transform to convert the unit measurement of pt from
> > metres to km and preserve that information in the CRS?
>
> It seems so, using proj4strings (not recommended, but simple):
>
> library(sp)
> demo(meuse, echo = FALSE, ask = FALSE)
> library(sf)
> # Linking to GEOS 3.11.1, GDAL 3.6.2, PROJ 9.1.1; sf_use_s2() is TRUE
> st_as_sf(meuse) |> st_bbox()
> #   xmin   ymin   xmax   ymax
> # 178605 329714 181390 333611
> st_crs(meuse)$proj4string
> # [1] "+proj=sterea +lat_0=52.1561605555556 +lon_0=5.38763888888889
> +k=0.9999079 +x_0=155000 +y_0=463000 +ellps=bessel +units=m +no_defs"
> st_as_sf(meuse) |>
>    st_transform("+proj=sterea +lat_0=52.1561605555556
> +lon_0=5.38763888888889 +k=0.9999079 +x_0=155000 +y_0=463000
> +ellps=bessel +units=km +no_defs") |>
>    st_bbox()
> #    xmin    ymin    xmax    ymax
> # 178.605 329.714 181.390 333.611
>
> The better way would be to modify WKT representations of CRSs.
>
> >
> > Thanks
> > Andrea
> >
> >
> > Il giorno mer 25 gen 2023 alle ore 18:58 Edzer Pebesma
> > <edzer.pebesma using uni-muenster.de> ha scritto:
> >>
> >>
> >>
> >> On 25/01/2023 18:42, Josiah Parry wrote:
> >>> I wonder if `units::set_units()` is a better fit for the job
> >>> https://r-quantities.github.io/units/articles/measurement_units_in_R.html
> >>
> >> It could definitely tell you which number to choose when going from,
> >> say, US feet to km. Coordinates in sf geometries are not stored as units
> >> objects, unit info is encoded in the CRS. st_transform can do
> >> transformations and conversions between different CRSs, unit conversions
> >> is a special case of that.
> >>
> >>>
> >>> On Wed, Jan 25, 2023 at 12:37 PM Steve Gutreuter <sgutreuter using gmail.com>
> >>> wrote:
> >>>
> >>>> Is it safe to re-scale sf geometry coordinates from meters to
> >>>> kilometers using, for example:
> >>>>
> >>>> sfobj$geometry <- sfobj$geometry / 1000
> >>>>
> >>>> It seems to work, but I understand too little about spatial data to
> >>>> know whether that practice is actually safe.  I am working with spatial
> >>>> data in a continental-scale equidistant conic projection, and the units
> >>>> are meters.  It seems that kilometers would be better suited stochastic
> >>>> partial differential equation modeling on a finite-element mesh, but
> >>>> maybe that is a moot point.
> >>>>
> >>>> Any and all advice will be much appreciated.
> >>>>
> >>>> Thanks!
> >>>> --
> >>>> Steve Gutreuter
> >>>>
> >>>>           [[alternative HTML version deleted]]
> >>>>
> >>>> _______________________________________________
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> >>>> R-sig-Geo using r-project.org
> >>>> https://stat.ethz.ch/mailman/listinfo/r-sig-geo
> >>>>
> >>>
> >>>        [[alternative HTML version deleted]]
> >>>
> >>> _______________________________________________
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> >>
> >> --
> >> Edzer Pebesma
> >> Institute for Geoinformatics
> >> Heisenbergstrasse 2, 48151 Muenster, Germany
> >> Phone: +49 251 8333081
> >>
> >> _______________________________________________
> >> R-sig-Geo mailing list
> >> R-sig-Geo using r-project.org
> >> https://stat.ethz.ch/mailman/listinfo/r-sig-geo
>
> --
> Edzer Pebesma
> Institute for Geoinformatics
> Heisenbergstrasse 2, 48151 Muenster, Germany
> Phone: +49 251 8333081