VS: [mapserver-users] Converting datum using ogr2ogr

[Rahkonen Jukka]

Hi Isao,

I am sorry, I must admit that I haven't done my homework with datum shifting.
However, I believe that reason for not getting any change in your reprojection
is because the definition in the FWTools epsg ligrary for epsg:4301 is this:
# Tokyo
<4301> +proj=longlat +ellps=bessel +no_defs  <>

So it gives no towgs parameters which would tell what kind of shifting should 
be done.  For comparison, this projection using bessel does have towgs84
parameters and a trial with that should lead to different result:
# Bukit Rimpah
<4219> +proj=longlat +ellps=bessel +towgs84=-384,664,-48,0,0,0,0 +no_defs  <>

If you can find somewhere the towgs84 parameters and insert them to your source 
projection you may get what you want.  It is not necessary to edit the epsg text
file but even better to give the projection directly for ogr2ogr this way:

ogr2ogr -s_srs "+proj=longlat +ellps=bessel +towgs84=-384,664,-48,0,0,0,0 +no_defs" -t_srs epsg:4326

Of course with your correct towgs84 parameters.  This way it is simple to play with 
adjusting the parameters, and most important, you know for sure what parameters
ogr2ogr is going to use. It is possible that right parameters for you are
TOWGS84=-148,507,685,0,0,0,0 but I can't be sure. It was just the first hit from
altavista search.

-Jukka Rahkonen-
 

> -----Alkuperäinen viesti-----
> Lähettäjä: mapserver-users-bounces at lists.osgeo.org 
> [mailto:mapserver-users-bounces at lists.osgeo.org] Puolesta NASUNO Isao
> Lähetetty: 15. toukokuuta 2008 7:59
> Vastaanottaja: Rahkonen Jukka
> Kopio: mapserver-users at lists.osgeo.org
> Aihe: Re: [mapserver-users] Converting datum using ogr2ogr
> 
> Hi Jukka,
> 
> Thank you for your response.
> 
> yes, I tried with the options (s/t_srs), however it makes 
> conversion for projection but not for datum.
> 
> In my case, I used following options but the input and output 
> are exactly same.
> 
>   -s_srs "EPSG:4301"   -t_srs "EPSG:4326"
>   (GCS with bessel)    (GCS with WGS84)
> 
> I checked GDAL FAQ and found following.
> 
> http://trac.osgeo.org/gdal/wiki/FAQCoordinateSystemsAndProject
> ions#WhydoesntGDALautomaticallychoosethedatumtransformation
> ------------------------------------
> Why doesn't GDAL automatically choose the datum transformation?
>   There is no such thing as an accurate "default" set of 
> datum transformation
>   parameters for a datum. OGR uses (NADCON) by default, which 
> is the most
>   accurate available for North America, but in the general 
> case (worldwide)
>   this is very hard to determine and there normally is no 
> such thing as a default.
>   The transformation that should be used depends on the exact 
> area covered,
>   accuracy required etc. In other words, user beware and do 
> your homework.
> ------------------------------------
> 
> Is this the reason ?
> I am not sure if ogr2ogr can accept the parameters in manual.
> Please help if any idea.
> 
> Thanks,
> 
> Isao
> 
> 
> Rahkonen Jukka wrote:
> > Hi,
> > 
> > I don't think you need .prj file with your shapefiles and 
> you should be able even to overrun it.
> > You need to use parameters -s_srs for source projection and 
> -t_srs for target projection.
> > 
> > -Jukka Rahkonen-
> > 
> > 
> > -----Alkuperäinen viesti-----
> > Lähettäjä: mapserver-users-bounces at lists.osgeo.org puolesta: NASUNO 
> > Isao
> > Lähetetty: to 15.5.2008 4:59
> > Vastaanottaja: mapserver-users at lists.osgeo.org
> > Aihe: [mapserver-users] Converting datum using ogr2ogr
> >  
> > Hello,
> > 
> > I am new in the mailing list.
> > 
> > I want to use ogr2ogr to convert projection/datum of shape 
> files to a 
> > same SRS before insert them into postgres for serving by MapServer.
> > 
> > However ogr2ogr seems convert only projection even the 
> shape file has .prj file.
> > I checked out that GDAL has no default parameters to convert datum.
> > 
> > I need the conversion before inserting to postgres with 
> some reasons, 
> > I mean, with translation from shape to shape.
> > 
> > Any suggestion to convert datum using ogr2ogr or other 
> stand-alone program ?
> > - The ogr2ogr is included in FWTools 2.1.0.
> > - OS is windows xp.
> > 
> > Thanks,
> > 
> > Isao NASUNO
> > 
> 
> 
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