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

[Andrea Gilardi]

>  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?

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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> --
> Edzer Pebesma
> Institute for Geoinformatics
> Heisenbergstrasse 2, 48151 Muenster, Germany
> Phone: +49 251 8333081
>
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