[Proj] [gdal-dev] convert shapefile from California State Plane to EPSG:4326

[Even Rouault]

Le lundi 08 février 2016 15:54:11, Daniel Fenton a écrit :
> Hi Maning,
> 
> For NAD83 you need to convert the Datum to WGS84. The WKT string you show
> does not include that conversion. Proj.4 is missing these in a lot of
> cases.
> 
> Try this string instead:
> 
> PROJCS["NAD83_California_zone_5_ftUS",GEOGCS["GCS_North_American_1983",DATU
> M["D_North_American_1983",SPHEROID["GRS_1980",6378137,298.257222101]],TOWGS
> 84[-0.9956,1.9013,0.5215,0.025915,0.009426,0.011599,-0.00062],PRIMEM["Green
> wich",0],UNIT["Degree",0.017453292519943295]],PROJECTION["Lambert_Conformal
> _Conic"],PARAMETER["standard_parallel_1",35.46666666666667],PARAMETER["stan
> dard_parallel_2",34.03333333333333],PARAMETER["latitude_of_origin",33.5],PA
> RAMETER["central_meridian",-118],PARAMETER["false_easting",6561666.667],PAR
> AMETER["false_northing",1640416.667],UNIT["Foot_US",0.30480060960121924]]
> 
> Hopefully that works for you. Let me know if it doesn't.

(CC'ing proj mailing list)

That's interesting, as indeed, proj assumes that WGS84 == NAD83, if no extra parameters
are specified. Comparing the TOWGS84 values you suggest with the EPSG database,
I see you suggest using EPSG:6866 coordinate transformation "ITRF2000 to NAD83(CORS96)"
(actually the opposite transformation, NAD83(CORS96) to ITRF2000).

Wondering if it might be appropriate to change proj.4 defaults to the above ?

Although it is not clear to me which realization of WGS84 is supposed to be the one used
by proj.4 as its pivot when datum transformations are involved. I guess this must depends
on the semantics of the grid or towgs84 parameters used...

There's also the EPSG:6864 (ITRF96 to NAD83(CORS96)) values that could be used if we rely rather
on the original WGS84 realization : -0.9910, 1.9072,0.5129,0.02579,0.00965,0.01166,0


Using the HTDP grids ( https://trac.osgeo.org/proj/wiki/HTDPGrids ) might also be an option.


Indeed Daniel's suggestion seems to do the trick, or at least better match
other data from the site http://geohub.lacity.org/datasets/813fcefde1f64b209103107b26a8909f_0 
. I see it has also a geojson output :
http://geohub.lacity.org/datasets/813fcefde1f64b209103107b26a8909f_0.geojson

And the first feature returned is OBJECT_ID=4001 whose first vertex is :
-118.276946349107,33.7920287419671

If I do, 
ogr2ogr -f kml /vsistdout/ /vsizip/Building_Footprints.zip  -where "OBJECTID=4001" 

I get :
-118.276934177326,33.7920239166625

which is off by a bit more than 1 meter from the position of the geojson output.

But If I do :

ogr2ogr -f kml /vsistdout/ /vsizip/Building_Footprints.zip  -where "OBJECTID=4001" \
 -s_srs "+proj=lcc +lat_1=35.46666666666667 +lat_2=34.03333333333333 \
+lat_0=33.5 +lon_0=-118 +x_0=2000000.000101601 +y_0=500000.0001016002 \
+ellps=GRS80 +towgs84=-0.9956,1.9013,0.5215,0.025915,0.009426,0.011599,-0.00062 +units=us-ft +no_defs" \
-t_srs EPSG:4326

I get:

-118.276946349109,33.7920287419681

which is identical to the geojson output (difference is in the 10 micrometer precision range)

> 
> Daniel Fenton
> Software Engineer
> Esri R&D
> 
> On Mon, Feb 8, 2016 at 6:56 AM maning sambale <emmanuel.sambale at gmail.com>
> 
> wrote:
> > Hi,
> > 
> > I'm trying to convert LA City building data from State Plane to
> > EPSG:4326. I converted the data using ogr2ogr.  Visually inspecting
> > using Bing imagery, I'm seeing an offset.
> > 
> > Details of SRS info and sample images here:
> > https://gist.github.com/maning/093a866fd465ea880032
> > 
> > It is possible that Bing has the offset, but the source of Bing comes
> > from LARIAC (Los Angeles Region Imagery Acquisition Consortium)
> > imagery, the same source as the building footprints were traced.
> > 
> > Any hints on the "proper" conversion from State Plane to EPSG:4326.
> > 
> > --
> > cheers,
> > maning
> > ------------------------------------------------------
> > "Freedom is still the most radical idea of all" -N.Branden
> > https://epsg4253.wordpress.com/
> > http://twitter.com/maningsambale
> > ------------------------------------------------------
> > _______________________________________________
> > gdal-dev mailing list
> > gdal-dev at lists.osgeo.org
> > http://lists.osgeo.org/mailman/listinfo/gdal-dev

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