[R-sig-Geo] Combined projections

[Alex Mandel]

On 05/08/2013 04:34 PM, Sarah Haas wrote:
> Previously using ESRI's ArcMap, I projected both shapefiles and rasters
> into NAD27.  Now I do not have access to ESRI and am using various R
> packages instead. When I read both types of files into R, the shapefile
> shows, "proj4string : [+proj=aea +lat_1=34 +lat_2=40.5 +lat_0=0 +lon_0=-120
> +x_0=0 +y_0=-4000000 +datum=NAD27 +units=m +no_defs +ellps=clrk66
> +nadgrids=@conus, at alaska, @ntv2_0.gsb, @ntv1_can.dat]".  Likewise, the
> raster files states, "coord. ref. : +proj=aea +lat_1=34 +lat_2=40.5
> +lat_0=0 +lon_0=-120 +x_0=0 +y_0=-4000000 +datum=NAD27 +units=m +no_defs
> +ellps=clrk66 +nadgrids=@conus, at alaska, at ntv2_0.gsb, at ntv1_can.dat".
> <http://webpages.uncc.edu/~shaas1/index.htm>
> 
> Do you know why it appears that I have "combined" projections?  Could this
> be due to the fact that I "projected" them in ArcMap at a previous time and
> the previous/new information is being carried over? Is this cause for
> concern?
> 
> Thank you for your time.
> 
> --Sarah <http://webpages.uncc.edu/%7Eshaas1/index.htm>

What part of that seems combined?
I see Albers Equal Area in NAD27.

Are you referring to:
+nadgrids=@conus, at alaska, @ntv2_0.gsb, @ntv1_can.dat


My understanding is that parameter defines the transformation used to
get from WGS84 or to be used to go back.

Here's the docs on what that means
http://trac.osgeo.org/proj/wiki/GenParms#nadgrids-GridBasedDatumAdjustments

Thanks,
Alex