Changing CRS of a SF object

Question

I have some 'roads' as a sp object:

class       : SpatialLinesDataFrame 
features    : 17360 
extent      : 490176.4, 567680.9, 148639.1, 212821  (xmin, xmax, ymin, ymax)
coord. ref. : +init=epsg:27700 +proj=tmerc +lat_0=49 +lon_0=-2     +k=0.9996012717 +x_0=400000 +y_0=-100000 +datum=OSGB36 +units=m +no_defs     +ellps=airy +towgs84=446.448,-125.157,542.060,0.1502,0.2470,0.8421,-20.4894 
variables   : 4
names       :    osm_id, lanes,     type, width 
min values  :        73,     2, motorway,   6.3 
max values  : 587969162,     6,    trunk,  18.9

I convert them to a sf object like so:

roads                   <- st_as_sf(roads, crs = 27700)

After conversion, the SRID is empty, and the proj4string (possibly) looks incorrect. Why is this? Can I fix it?

Simple feature collection with 17360 features and 4 fields
geometry type:  LINESTRING
dimension:      XY
bbox:           xmin: 490176.4 ymin: 148639.1 xmax: 567680.9 ymax: 212821
epsg (SRID):    NA
proj4string:    +proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012717 +x_0=400000 +y_0=-100000 +ellps=airy +towgs84=446.448,-125.157,542.06,0.1502,0.247,0.8421,-20.4894 +units=m +no_defs
First 10 features:
     osm_id lanes    type width                       geometry
73       73     2 primary   6.3 LINESTRING (526256 191950, ...
74       74     2 primary   6.3 LINESTRING (525236.4 190755...
75       75     2 primary   6.3 LINESTRING (526324.4 192518...
79       79     2 primary   6.3 LINESTRING (524912.4 190235...
482     482     2 primary   6.3 LINESTRING (527283.9 198571...
546     546     2 primary   6.3 LINESTRING (506089.1 205091...
1210   1210     2 primary   6.3 LINESTRING (526023.9 195013...
1213   1213     2 primary   6.3 LINESTRING (524861.1 196243...
1214   1214     2 primary   6.3 LINESTRING (524482.3 196798...
1698   1698     2 primary   6.3 LINESTRING (527335.3 195524...

I think I need to fix this, because I then want to st_transform() my data to the same CRS as a raster I have to do some extraction of values.

crs(raster)
CRS arguments:
 +proj=laea +lat_0=52 +lon_0=10 +x_0=4321000 +y_0=3210000 +ellps=GRS80 +units=m +no_defs

I presume I can convert my 'roads' to the correct CRS by doing something like this? But it doesn't work. I get ....

roads <- st_transform(roads, crs(raster))
'Error: cannot create a crs from an object of class CRS

Help appreciated. Thank you.

Jonahjonas

raster uses S4 classes, hence you need to call the appropriate slot to get the prj4string which sf can understand. roads <- st_transform(roads, crs(raster)@projargs) should do the trick. – Bike

Answer 1

You have to adapt the st_transform arguments like that:

roads <- st_transform(roads, crs = 4326)

OR

roads <- st_transform(roads, crs = proj4string(raster))

OR as @TimSalabim said:

roads <- st_transform(roads, crs = crs(raster)@projargs)

OR as @R'n'E pointed out:

roads <- st_transform(roads, crs = st_crs(raster))

Hostess

Seems like the code needs a slight update. Using st_crs() from sf_0.6-3 : roads <- st_transform(roads, crs = st_crs(raster)) – Weiland