[mapserver-users] How to convert from 2393(Finish YKJ) to 4326 (WGS84)

[Rahkonen Jukka]

Hi,

If you do not need to reproject or you are reprojecting just between KKJ zones you can live without towgs84 parameters.  However, reprojecting to/from all the other systems will give an error of 150-200 meters. That is the thing that is not safe.  Updating spatial_ref_sys is safe.

The most official parameters are in the JHS document http://docs.jhs-suositukset.fi/jhs-suositukset/JHS153/JHS153.pdf, table in page 10.  Parameters seem to be the ones I have been using with Mapserver. I do not understand the mathematics myself but luckily it is not necessary. Just use correct parameters.

-Jukka Rahkonen-

 Pena Kupen wrote:

> Thank's Jukka,

> On my spatial_ref_sys-table, there is proj4text like:
+proj=tmerc +lat_0=0 +lon_0=27 +k=1.000000 +x_0=3500000 +y_0=0 +ellps=intl +units=m +no_defs
So towgs84-parameter is missing.

>> If it is not, update the record.
>>
> It this safe? Can it affect with unwanted result somewhere?

>>You can read also http://latuviitta.org/documents/YKJ-TM35FIN_muunnos_ogr2ogr_cs2cs.txt (in Finnish, I suppose it is not a problem for you).
>>
> Yeps, language not, but understanding is something else ;-)

> -Pena

Rahkonen Jukka [Jukka.Rahkonen at mmmtike.fi] kirjoitti:
> Hi,
>
> I suppose that towgs84 parameters are missing. You can check this with psql:
>
>  select proj4text from spatial_ref_sys where srid=2393;
>
> The result should be close to
>  +proj=tmerc +lat_0=0 +lon_0=27 +k=1.000000 +x_0=3500000 +y_0=0 +ellps=intl +towgs84=-90.7,-106.1,-119.2,4.09,0.218,-1.05,1.37 +units=m +no_defs
> If it is not, update the record.
>
> A related thing is that Mapserver epsg text file often comes without towgs84 parameters for the Finnish KKJ zones. This is what I seem to have on this computer:
>  KKJ / Finland Uniform Coordinate System
> <2393> +proj=tmerc +lat_0=0 +lon_0=27 +k=1 +x_0=3500000 +y_0=0 +ellps=intl +towgs84=-96.0617,-82.4278,-121.743,4.80107,0.34543,-1.37646,1.4964 +units=m +no_defs  <>
>
> You can see that the parameters are not the same.  There are more parameter sets around and that's actually the reason why Proj4 comes or at least used to come without any parameters because none of the sets was authorized. The results of these transformations are rather close and accurate to about one meter.  You can read also http://latuviitta.org/documents/YKJ-TM35FIN_muunnos_ogr2ogr_cs2cs.txt (in Finnish, I suppose it is not a problem for you).
>
> -Jukka Rahkonen-
>
> Pena Kupen wrote:
>
> > Hello all,
>
> > Last time I was successfully helped by Ralf - Thank's once more time!
> http://postgis.refractions.net/pipermail/postgis-users/2011-March/029055.html
>
> > Now I have same type of problem, but I cannot get correct result.
>
> Table is:
> CREATE TABLE location (
>     location_id integer NOT NULL,
>     coord geometry
> );
>
> Data is in coord column as 2393 format.
>
> My sql is like:
> select
> y(location.coord) as y_ykj,
> x(location.coord) as x_ykj,
> y(ST_Transform(location.coord,4326)) as y_wgs84,
> x(ST_Transform(location.coord,4326)) as x_wgs84
> from location where location_id = 'AAA123A'
>
> result for y_ykj and x_ykj are ok, but point of x_wgs84 and y_wgs84 are about 180meters too much in east.
>
> -Pena
>
>


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