Having trouble finding out what coordinate system a map is using, and how to convert it

Question

I'm using Small Area shapefiles in Ireland from the Central Statistics Office (CSO) website. The link to these is here http://census.cso.ie/censusasp/saps/boundaries/ED_SA%20Disclaimer1.htm (Look for the small area ones)

I can't seem to convert the coordinates to latitude longitude, can anyone see what I'm doing wrong?

I've loaded these in using R maptools package using the following code:

cte<- readShapePoly("Shapefile/Census2011_Small_Areas_generalised20m.shp") 

But when I check the cte@proj4string it reurns NA. Then I found this document which suggests the projection used is EPSG:3035 http://www.cso.ie/en/media/csoie/census/documents/census2011griddataset/1%20Km%C2%B2%20Grid%20dataset%20User%20Guide%202011.pdf

I then found a site which gives the proj4 string for EPSG:3035. I then tried to set the coorinates as this string.

cte@proj4string <- CRS("+proj=laea +lat_0=52 +lon_0=10 +x_0=4321000 +y_0=3210000 +ellps=GRS80 +units=m +no_defs" )

Finally I tried to transform the object to latlong coordinates using the following:

cte <- spTransform(cte, CRS("+proj=longlat +datum=WGS84"))

I also tried:

coordinates(cte) <- project(cte, "+proj=longlat +datum=WGS84")

The first transformed it to new coordinates, but they don't seem to be correct. The second one just put everything as infinite.

Can anybody let me know what I'm doing wrong?

Thanks.

Answer 1

If you read carefully the page you mentioned first, you see that EPSG:29902 is the correct CRS. The shapefiles do not have a .prj file. For reprojecting, you might create one on your own, or load the data in QGIS, set the CRS to EPSG:29902, and Save As...

QGIS creates the following prj file:

PROJCS["TM65_Irish_Grid",GEOGCS["GCS_TM65",DATUM["D_TM65",SPHEROID["Airy_Modified_1849",6377340.189,299.3249646]],PRIMEM["Greenwich",0],UNIT["Degree",0.017453292519943295]],PROJECTION["Transverse_Mercator"],PARAMETER["latitude_of_origin",53.5],PARAMETER["central_meridian",-8],PARAMETER["scale_factor",1.000035],PARAMETER["false_easting",200000],PARAMETER["false_northing",250000],UNIT["Meter",1]]

Off course, you can use QGIS also to reproject the data into any other CRS you want. Lat/lon WGS84 is EPSG:4326.