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)$proj4string
# [1] "+proj=sterea +lat_0=52.1561605555556 +lon_0=5.38763888888889
+k=0.9999079 +x_0=155000 +y_0=463000 +ellps=bessel +units=m +no_defs"
st_as_sf(meuse) |>
st_transform("+proj=sterea +lat_0=52.1561605555556
+lon_0=5.38763888888889 +k=0.9999079 +x_0=155000 +y_0=463000
+ellps=bessel +units=km +no_defs") |>
st_bbox()
# xmin ymin xmax ymax
# 178.605 329.714 181.390 333.611
The better way would be to modify WKT representations of CRSs.
Thanks
Andrea
Il giorno mer 25 gen 2023 alle ore 18:58 Edzer Pebesma
<edzer.pebesma at uni-muenster.de> ha scritto:
On 25/01/2023 18:42, Josiah Parry wrote:
It could definitely tell you which number to choose when going from,
say, US feet to km. Coordinates in sf geometries are not stored as units
objects, unit info is encoded in the CRS. st_transform can do
transformations and conversions between different CRSs, unit conversions
is a special case of that.
On Wed, Jan 25, 2023 at 12:37 PM Steve Gutreuter <sgutreuter at gmail.com>
wrote:
Is it safe to re-scale sf geometry coordinates from meters to
kilometers using, for example:
sfobj$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
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