[MetaCRS] Coordinate offset after transformation from 4267 to 4326

[Jody Garnett]

G'Day Martin:

The GeoTools has been configured with GridShft files for NAD27 - this is
not something I have personally tried before so I am not quite sure how it
works.

I had a look with Betsey, the data being projected is in Texas (so slightly
different from Cuba so the TOWGS84 parameters sound legit).


--
Jody Garnett

On 24 May 2016 at 17:51, Martin Desruisseaux <
martin.desruisseaux at geomatys.com> wrote:

> Hello Betsy
>
> The difference that you get may depend on whether a datum shift has been
> applied or not. If both Proj.4 and GeoTools have applied a datum shift,
> then it may depend on the values in the TOWGS84 expression. There is
> many different possible TOWGS84 values for a transformations between
> NAD27 and WGS84, depending on the geographic area. If you are
> transforming coordinates in USA, you should have TOWGS84[-8, 160, 176]
> (for an error below 10 metres). If you are transforming coordinates in
> Canada, you should have TOWGS84[-10, 158, 187] (for an error below 20
> metres). The TOWGS84 parameters in the GeoTools CRS provided with your
> email are for Cuba (operation code EPSG:15978) except the sign of 0.526
> which should be -0.526.
>
> In order to compare with Proj.4, we would need to know which parameters
> Proj.4 has selected. It may also use the NADCON grids in USA, or NTv2
> grids in Canada. I do not know how to get this information from Proj.4.
>
> Alternatively you could also try Apache SIS 0.7 for coordinate
> transformations. It takes in account the geographic area of the points
> to transform, support GML and WKT version 1 and 2, tell you which
> parameters it selected, in which geographic area they are valid and what
> accuracy to expect, etc. Version 0.7 should be released this week.
>
>     Martin
>
>
> Le 24/05/16 à 18:11, Betsy Emmons a écrit :
> > Hello,
> >
> > I'm using Proj.4 v4.9.1 and GeoTools v13.x. When a WMS layer is
> > brought into QGIS as EPSG:4326 there is a noticeable offset compared
> > to the same local layer. The WMS layer source is EPSG:4267.
> >
> > A WMS 4326 sample point has a coordinate of -97.2341220, 28.0611696
> > and the same local 4326 sample point has a coordinate of -97.2339531,
> > 28.0608752. This is roughly a 40m difference. Some local calculations
> > have shown the error margin to be <=111m.
> >
> > Here is the source 4267 projection
> > definition:
> GEOGCS["GCS_North_American_1927",DATUM["D_North_American_1927",SPHEROID["Clarke_1866",6378206.4,294.9786982]],PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]].
> >
> > Here's what geotools is using:
> > GEOGCS["NAD27",
> >   DATUM["North American Datum 1927",
> >     SPHEROID["Clarke 1866", 6378206.4, 294.9786982138982,
> > AUTHORITY["EPSG","7008"]],
> >     TOWGS84[2.478, 149.752, 197.726, 0.526, -0.498, 0.501, 0.685],
> >     AUTHORITY["EPSG","6267"]],
> >   PRIMEM["Greenwich", 0.0, AUTHORITY["EPSG","8901"]],
> >   UNIT["degree", 0.017453292519943295],
> >   AXIS["Geodetic longitude", EAST],
> >   AXIS["Geodetic latitude", NORTH],
> >   AUTHORITY["EPSG","4267"]]
> >
> > Proj.4 is really a great library! Thank you for maintaining and
> > contributing to it!
> >
> > Thank you,
> > Betsy Emmons
>
> _______________________________________________
> MetaCRS mailing list
> MetaCRS at lists.osgeo.org
> http://lists.osgeo.org/mailman/listinfo/metacrs
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osgeo.org/pipermail/metacrs/attachments/20160526/fbe43518/attachment.html>