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

[Edzer Pebesma]

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