Building a proj4 string for readShapePoly in R

Question

I have read in a shapefile of lodgepole pine seed planning units in R with readShapePoly, like so

spu <- readShapePoly("spus.shp")

When I map the spus alone, they show up.

plot(spu, border=TRUE)

When I try to add the shapefile to a map of Canada, where I know the spus should go, they do not show up.

map("worldHires","Canada", xlim=c(-141,-110), ylim=c(45,65), col="gray90", fill=TRUE)
plot(spu, add=TRUE, border=TRUE)

I think this is because the shapefile is missing the projection information contained in the .prj file:

PROJCS["NAD_1983_Albers",GEOGCS["GCS_North_American_1983",DATUM["D_North_American_1983",SPHEROID["GRS_1980",6378137.0,298.257222101]],
PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]],PROJECTION["Albers"],PARAMETER["False_Easting",1000000.0],
PARAMETER["False_Northing",0.0],PARAMETER["Central_Meridian",-126.0],PARAMETER["Standard_Parallel_1",50.0],
PARAMETER["Standard_Parallel_2",58.5],PARAMETER["Latitude_Of_Origin",45.0],UNIT["Meter",1.0]]

The partial output of

summary(spu)

Object of class SpatialPolygonsDataFrame
Coordinates:
     min     max
x 642162.8 1870556
y 457057.2 1421478
Is projected: NA 
proj4string : [NA]
Data attributes:

shows that no projection information is associated with the spu object in R. I think I can fix this by using the correct proj4string with readShapePoly,but could not find how to translate the information in the .prj file into a proj4 string. How can I do this/is this the correct approach?

Answer 1

To convert a .prj to proj4, you can use the command line tool gdalsrsinfo, as described here.

To run it from R, you could do:

system('gdalsrsinfo "spus.prj"')

where spus.prj is the full (or relative to wd) path to the .prj file.

Included in the returned info is the proj4 string.

Note that for this to work as provided here, the path containing gdalsrsinfo.exe should be included in the system PATH environment variable. Otherwise, you could provide the full path to gdalsrsinfo in the system call.