[R-sig-Geo] proj4string read by readOGR doesn't seem to precisely specify source shapefile projection

[Edzer Pebesma]

Also relevant to this question is this discussion/issue:

https://github.com/edzer/sfr/issues/380

On 14/06/17 17:14, Shaun Walbridge wrote:
> Glen,
> 
> Round-tripping projection information can be tricky, in part because the
> normalization routines vary between different stacks, and what counts
> as the same form is also implementation dependent. In this case
> (round-tripping with ArcGIS), you may be better off using the
> arcgisbinding package [1], which can do the conversion to and from
> ArcGIS native data types (including some that aren't available via
> OGR/GDAL). It's easy enough to install [2], and also lets you build
> Geoprocessing tools which speak directly to R. I imagine there's also a
> pure R solution to the normalization, but it would likely have side
> effects for existing packages, and may not be easily solved.
> 
> If you know that your data is fine and want a simple solution, you could
> also just overwrite the new shapefile’s .prj file with a copy of the
> original.
> 
> Cheers,
> Shaun
> 
> 1. https://urldefense.proofpoint.com/v2/url?u=https-3A__r-2Darcgis.github.io_assets_arcgisbinding.pdf&d=DwIGaQ&c=n6-cguzQvX_tUIrZOS_4Og&r=fCPRb7QX-vd5bnO9gIJHCiX852SVUtyYX--xtCKtpfk&m=9su2o7yjMBqHmqZT3kZAlTRZwPC_W0Tm65yvo7NmjLw&s=pIBxR3icnDLcn639TzBc0mkXy6Tv7G9Ip7xcWSAMweU&e= 
> 2. https://urldefense.proofpoint.com/v2/url?u=https-3A__github.com_R-2DArcGIS_r-2Dbridge-2Dinstall&d=DwIGaQ&c=n6-cguzQvX_tUIrZOS_4Og&r=fCPRb7QX-vd5bnO9gIJHCiX852SVUtyYX--xtCKtpfk&m=9su2o7yjMBqHmqZT3kZAlTRZwPC_W0Tm65yvo7NmjLw&s=JADt055JJUuCG1eltd7N9o5f_fu_geqGlnzO62SGtGM&e= 
> 
> On 6/14/17, 9:35 AM, "R-sig-Geo on behalf of Glenn Stauffer" <r-sig-geo-bounces at r-project.org on behalf of gestauffer at gmail.com> wrote:
> 
>     I have a shapefile that I imported into R using readOGR from the rgdal
>     package. I do a little bit of work with it, like adding attribute
>     information, etc, then export it as an ESRI shapefile again, with a new
>     name. However, when I bring both the original and new shapefile into ArcGIS,
>     it tells me that the CRS do not match. 
>     
>     So, noting that all the projection parameters remain the same, but the
>     projection and coordinate system names are different, and the datum name is
>     dropped, my questions are:
>     
>     1.	Is the second CRS the same as the first?
>     2.	If so, why did the names change, and why does ArcGIS no longer
>     recognize it as the same?
>     3.	If not, how did it get changed?
>     4.	Can the proj4string be modified to be more specific, and if so, why
>     did readOGR not already do this to preserve all the information?
>     
>     I can use the new shapefiles just fine and readOGR interprets them as
>     identical, but it is a bit vexing that ArcGIS does not. And, I could of
>     course define it again in ArcGIS, but part of the motivation to work in R is
>     to obviate the need to point and click for many files.
>     
>     I appreciate any insights or enlightenment. 
>     
>     Thanks, 
>     
>     Glenn
>     
>     Here is the original projection information from ArcGIS:
>     
>     Projected Coordinate System:    NAD_1983_HARN_Transverse_Mercator
>     
>     Projection: Transverse_Mercator
>     
>     False_Easting:  520000.00000000
>     
>     False_Northing: -4480000.00000000
>     
>     Central_Meridian:   -90.00000000
>     
>     Scale_Factor:   0.99960000
>     
>     Latitude_Of_Origin: 0.00000000
>     
>     Linear Unit:    Meter
>     
>      
>     
>     Geographic Coordinate System:   GCS_North_American_1983_HARN
>     
>     Datum:  D_North_American_1983_HARN
>     
>     Prime Meridian:     Greenwich
>     
>     Angular Unit:   Degree
>     
>     Here is the proj4string from R, which also agrees with the proj4string given
>     for this projection at www.spatialreference.org for epsg:3071 and also for
>     SR-ORG:7396.
>     
>     +proj=tmerc +lat_0=0 +lon_0=-90 +k=0.9996 +x_0=520000 +y_0=-4480000
>     +ellps=GRS80 +units=m +no_defs
>     
>     When I use writeOGR to export the SpatialPolygonsDataFrame with the above
>     proj4string, then bring it back into ArcGIS, the projection information is
>     given as the following, and is no longer recognized as identical to the
>     original.
>     
>     Projected Coordinate System:    Transverse_Mercator
>     
>     Projection: Transverse_Mercator
>     
>     false_easting:  520000.00000000
>     
>     false_northing: -4480000.00000000
>     
>     central_meridian:   -90.00000000
>     
>     scale_factor:   0.99960000
>     
>     latitude_of_origin: 0.00000000
>     
>     Linear Unit:    Meter
>     
>      
>     
>     Geographic Coordinate System:   GCS_GRS 1980(IUGG, 1980)
>     
>     Datum:  D_unknown
>     
>     Prime Meridian:     Greenwich
>     
>     Angular Unit:   Degree
>     
>      
>     
>      
>     
>     
>     	[[alternative HTML version deleted]]
>     
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>     
> 
> 
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-- 
Edzer Pebesma
Institute for Geoinformatics  (ifgi),  University of Münster
Heisenbergstraße 2, 48149 Münster, Germany; +49 251 83 33081
Journal of Statistical Software:   http://www.jstatsoft.org/
Computers & Geosciences:   http://elsevier.com/locate/cageo/

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