WKT coordinate system string (PRJ) to PROJ.4

It's too early I guess as it didn't occur to me that if writing it out
locally was naff using an external service is more so. The
SpatialReference "importFromESRI()" function doesn't seem to be mapped
in rgdal. That would be the function needed to avoid a
write-out/external call.

On Tue, Jan 5, 2016 at 5:57 AM, boB Rudis <bob at rudis.net> wrote:

This may not be optimal as it uses an external service:

  prj_to_epsg <- function(prj) {

    require(sp)
    require(httr)
    require(jsonlite)

    res <- GET("http://prj2epsg.org/search.json",
               query=list(exact=TRUE,
                          error=TRUE,
                          mode="wkt",
                          terms=prj))

    # one shld prbly do more error checking than this
    stop_for_status(res)

    dat <- fromJSON(content(res, as="text", flatten=TRUE))

    # NOTE: there could be more in dat$codes if prj was ambiguous
    CRS(paste0("+init=epsg:", dat$codes[1, "code"]))

  }

  prj <- paste0(readLines("110m_admin_1_states_provinces_shp.prj"))
  prj_to_epsg(prj)
  ## CRS arguments:
  ##  +init=epsg:4326 +proj=longlat +datum=WGS84 +no_defs +ellps=WGS84
  ## +towgs84=0,0,0

  prj_1 <- 'PROJCS["Transverse_Mercator",
    GEOGCS["GCS_OSGB 1936",
    DATUM["D_OSGB_1936",
    SPHEROID["Airy_1830",6377563.396,299.3249646]],
    PRIMEM["Greenwich",0],UNIT["Degree",0.017453292519943295]],
    PROJECTION["Transverse_Mercator"],
    PARAMETER["latitude_of_origin",49],
    PARAMETER["central_meridian",-2],
    PARAMETER["scale_factor",0.9996012717],
    PARAMETER["false_easting",400000],
    PARAMETER["false_northing",-100000],
    UNIT["Meter",1]]'
  prj_to_epsg(prj_1)
  ## CRS arguments:
  ##  +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

On Tue, Jan 5, 2016 at 5:09 AM, Michael Sumner <mdsumner at gmail.com> wrote:

Hello, is there any R-level way to convert a WKT/PRJ *coordinate system*
string to PROJ.4 used by sp/rgdal?

(Previously I've written out to SHP/PRJ, and read back in with rgdal, but
that's obviously a bit naff.)

(I know you can rgeos::readWKT and wkb::readWKB for *geometry*).

Cheers, Mike.




--
Dr. Michael Sumner
Software and Database Engineer
Australian Antarctic Division
203 Channel Highway
Kingston Tasmania 7050 Australia

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