R: Retrieve coordinate reference system from object

st_crs {sf} | R Documentation

## Retrieve coordinate reference system from object

### Description

Retrieve coordinate reference system from sf or sfc object

Set or replace retrieve coordinate reference system from object

### Usage

    st_crs(x, ...)

    ## S3 method for class 'sf'
    st_crs(x, ...)

    ## S3 method for class 'numeric'
    st_crs(x, ...)

    ## S3 method for class 'character'
    st_crs(x, ...)

    ## S3 method for class 'sfc'
    st_crs(x, ..., parameters = FALSE)

    ## S3 method for class 'bbox'
    st_crs(x, ...)

    ## S3 method for class 'CRS'
    st_crs(x, ...)

    ## S3 method for class 'crs'
    st_crs(x, ...)

    st_crs(x) <- value

    ## S3 replacement method for class 'sf'
    st_crs(x) <- value

    ## S3 replacement method for class 'sfc'
    st_crs(x) <- value

    st_set_crs(x, value)

    NA_crs_

    ## S3 method for class 'crs'
    is.na(x)

    ## S3 method for class 'crs'
    x$name

    ## S3 method for class 'crs'
    format(x, ...)

    st_axis_order(authority_compliant = logical(0))

### Arguments

`x` | numeric, character, or object of class sf or sfc
`...` | ignored
`parameters` | logical; `FALSE` by default; if `TRUE` return a list of coordinate reference system parameters, with named elements `SemiMajor`, `InvFlattening`, `units_gdal`, `IsVertical`, `WktPretty`, and `Wkt`
`value` | one of (i) character: a string accepted by GDAL, (ii) integer, a valid EPSG value (numeric), or (iii) an object of class `crs`.
`name` | element name
`authority_compliant` | logical; specify whether axis order should be handled compliant to the authority; if omitted, the current value is printed.

### Format

An object of class `crs` of length 2.

### Details

The *crs functions create, get, set or replace the `crs` attribute of a simple feature geometry list-column. This attribute is of class `crs`, and is a list consisting of `input` (user input, e.g. "EPSG:4326" or "WGS84" or a proj4string), and `wkt`, an automatically generated wkt2 representation of the crs. If `x` is identical to the wkt2 representation, and the CRS has a name, this name is used for the `input` field.

Comparison of two objects of class `crs` uses the GDAL function `OGRSpatialReference::IsSame`.

In case a coordinate reference system is replaced, no transformation takes place and a warning is raised to stress this.

`NA_crs_` is the `crs` object with missing values for `input` and `wkt`.

the `$` method for `crs` objects retrieves named elements using the GDAL interface; named elements include `"SemiMajor"`, `"SemiMinor"`, `"InvFlattening"`, `"IsGeographic"`, `"units_gdal"`, `"IsVertical"`, `"WktPretty"`, `"Wkt"`, `"Name"`, `"proj4string"`, `"epsg"`, `"yx"` and `"ud_unit"` (this may be subject to changes in future GDAL versions).

format.crs returns NA if the crs is missing valued, or else the name of a crs if it is different from "unknown", or else the user input if it was set, or else its "proj4string" representation;

`st_axis_order` can be used to get and set the axis order: `TRUE` indicates axes order according to the authority (e.g. EPSG:4326 defining coordinates to be latitude,longitude pairs), `FALSE` indicates the usual GIS (display) order (longitude,latitude). This can be useful when data are read, or have to be written, with coordinates in authority compliant order. The return value is the current state of this (`FALSE`, by default).

### Value

If `x` is numeric, return `crs` object for EPSG:`x`; if `x` is character, return `crs` object for `x`; if `x` is of class `sf` or `sfc`, return its `crs` object.

Object of class `crs`, which is a list with elements `input` (length-1 character) and `wkt` (length-1 character). Elements may be `NA` valued; if all elements are `NA` the CRS is missing valued, and coordinates are assumed to relate to an arbitrary Cartesian coordinate system.

`st_axis_order` returns the (logical) current value if called without argument, or (invisibly) the previous value if it is being set.

### Examples

    sfc = st_sfc(st_point(c(0,0)), st_point(c(1,1)))
    sf = st_sf(a = 1:2, geom = sfc)
    st_crs(sf) = 4326
    st_geometry(sf)
    sfc = st_sfc(st_point(c(0,0)), st_point(c(1,1)))
    st_crs(sfc) = 4326
    sfc
    sfc = st_sfc(st_point(c(0,0)), st_point(c(1,1)))
    library(dplyr)
    x = sfc %>% st_set_crs(4326) %>% st_transform(3857)
    x
    st_crs("EPSG:3857")$input
    st_crs(3857)$proj4string
    st_crs(3857)$b     # numeric
    st_crs(3857)$units # character
    pt = st_sfc(st_point(c(0, 60)), crs = 4326)
    # st_axis_order() only has effect in GDAL >= 2.5.0:
    st_axis_order() # query default: FALSE means interpret pt as (longitude latitude)
    st_transform(pt, 3857)[[1]]
    old_value = FALSE
    if (sf_extSoftVersion()["GDAL"] >= "2.5.0")
       (old_value = st_axis_order(TRUE))
    # now interpret pt as (latitude longitude), as EPSG:4326 prescribes:
    st_axis_order() # query current value
    st_transform(pt, 3857)[[1]]
    st_axis_order(old_value) # set back to old value

[Package sf version 1.0-7 Index]