[R-sig-Geo] proj4string read by readOGR doesn't seem to precisely specify source shapefile projection
Glenn Stauffer
gestauffer at gmail.com
Wed Jun 14 19:48:33 CEST 2017
Thanks for the link. That does shed some light, but I'm not sure I entirely understand. The .prj file does not store the EPSG ID, but it looks like the shapefile properties does include that information. Furthermore, when I overwrite the .prj file as Shaun suggested, the properties of the associated shapefile now does show " WKID: 3071 Authority: EPSG " whereas it did not previously. But aside from that, I'm not sure why readOGR didn't identify the datum information in the first place. When I add "+datum=NAD83" to the proj4string, ArcGIS seems to accept it as the same CRS, even though the original datum was NAD83 HARN (which I don't know how to specify in R). But I am pretty new to this.... For now, overwriting the .prj with the desired one seems to work for me (and it is easy to do in a script).
Glenn
-----Original Message-----
From: R-sig-Geo [mailto:r-sig-geo-bounces at r-project.org] On Behalf Of Edzer Pebesma
Sent: Wednesday, June 14, 2017 11:12 AM
To: r-sig-geo at r-project.org
Subject: Re: [R-sig-Geo] proj4string read by readOGR doesn't seem to precisely specify source shapefile projection
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=fCPRb
> 7QX-vd5bnO9gIJHCiX852SVUtyYX--xtCKtpfk&m=9su2o7yjMBqHmqZT3kZAlTRZwPC_W
> 0Tm65yvo7NmjLw&s=pIBxR3icnDLcn639TzBc0mkXy6Tv7G9Ip7xcWSAMweU&e=
> 2.
> https://urldefense.proofpoint.com/v2/url?u=https-3A__github.com_R-2DAr
> cGIS_r-2Dbridge-2Dinstall&d=DwIGaQ&c=n6-cguzQvX_tUIrZOS_4Og&r=fCPRb7QX
> -vd5bnO9gIJHCiX852SVUtyYX--xtCKtpfk&m=9su2o7yjMBqHmqZT3kZAlTRZwPC_W0Tm
> 65yvo7NmjLw&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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--
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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