R: [postgis-users] precision of GeomFromTex, Transform and text functions

[Ludovico Bianchini]

Thanks for the suggestion, actually there were not the
toWGS parameter in spatial_ref_sys table.

The situation seems to be a little better (here a
sequence of insert-select:
Insert:37.13953463591601 45.66405322544534
Select:37.1321637582786 45.6647731135686

Insert:37.1321637582786 45.6647731135686
Select:37.1248123607135 45.6654908577754

Insert:37.1248123607135 45.6654908577754
Select:37.1174803443424 45.6662064695791), but the
error is still too big.

I have this question: is there a parameter simmetric
to TOWGS? If so the project library shall use FROMWGS
to convert from the gps system and TOWGS to convert to
the gps system.
Sorry if the question is stupid, but I don't
understand how to solve this problem (to convert gps
(SRID=4326) to West Gauss Boaga Italy Rome Monte Mario
(SRID=26591 or 3003).

Any help will be very appreciated!!!

Ludovico


--- P.Rizzi Ag.Mobilità Ambiente
<paolo.rizzi at ama-mi.it> ha scritto:

> I forget to mention the fact that inside the PostGIS
> spatial_ref_sys
> table, TOWGS parameters are missing for EPSG:26591
> and for 3003 too.
> So any transform will not be precise in any case.
> To add parameters for EPSG:3003, you can use the
> following SQL
> in any DB that you need to.
> 
> Bye
> Paolo Rizzi
> 
> 
> ---
> --- To add TOWGS parameters, if needed, to:
> ---
> --- EPSG 3003 : Monte Mario / Italy zone 1
> ---
> --- If parameters are already present, no change
> will take place.
> ---
> UPDATE "spatial_ref_sys" SET
> "srtext"='PROJCS["Monte Mario / Italy zone
> 1",GEOGCS["Monte
> Mario",DATUM["Monte_Mario",SPHEROID["International
>
1924",6378388,297,AUTHORITY["EPSG","7022"]],TOWGS84[-104.1,-49.1,-9.9,0.971,-2.917,0.714,-11.68],AUTHORITY["EPSG","6265"]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],UNIT["degree",0.01745329251994328,AUTHORITY["EPSG","9122"]],AUTHORITY["EPSG","4265"]],PROJECTION["Transverse_Mercator"],PARAMETER["latitude_of_origin",0],PARAMETER["central_meridian",9],PARAMETER["scale_factor",0.9996],PARAMETER["false_easting",1500000],PARAMETER["false_northing",0],UNIT["metre",1,AUTHORITY["EPSG","9001"]],AUTHORITY["EPSG","3003"]]'
> WHERE "srid"=3003 AND "auth_name"='EPSG' AND
> "srtext" NOT LIKE '%TOWGS%';
> 
> UPDATE "spatial_ref_sys" SET
> "proj4text"='+proj=tmerc +lat_0=0 +lon_0=9
> +k=0.999600 +x_0=1500000 +y_0=0 +ellps=intl +units=m
> +no_defs
>
+towgs84=-104.1,-49.1,-9.9,0.971,-2.917,0.714,-11.68'
> WHERE "srid"=3003 AND "auth_name"='EPSG' AND
> "proj4text" NOT LIKE '%towgs%';
> 
> 
> 
> 
> > -----Messaggio originale-----
> > Da: postgis-users-bounces at postgis.refractions.net
> >
>
[mailto:postgis-users-bounces at postgis.refractions.net]Per
> conto di
> > P.Rizzi Ag.Mobilità Ambiente
> > Inviato: martedì 4 settembre 2007 11.25
> > A: PostGIS Users Discussion
> > Oggetto: R: [postgis-users] precision of 
> > GeomFromTex,Transform and text
> > functions
> > 
> > 
> > Probably it does not matter to you in this case,
> > but EPSG:26591 has been deprecated in favour of
> EPSG:3003.
> > There is an error (can't remember what) in 26591.
> > 
> > Bye
> > Paolo Rizzi
> > 
> > 
> > > -----Messaggio originale-----
> > > Da:
> postgis-users-bounces at postgis.refractions.net
> > >
>
[mailto:postgis-users-bounces at postgis.refractions.net]Per
> conto di
> > > Ludovico Bianchini
> > > Inviato: lunedì 3 settembre 2007 21.39
> > > A: postgis-users at postgis.refractions.net
> > > Oggetto: [postgis-users] precision of
> GeomFromTex,Transform and text
> > > functions
> > > 
> > > 
> > > Hi all,
> > > 
> > > I'm using this postgis function:
> > > - GeomFromText(text, int4) to create a geometry
> value
> > > in 4326 spatial_ref_sys
> > > - Transform(geometry, int4) to convert a
> geometry from
> > > 4326 to 26591 when I'm inserting a record and to
> > > convert 26591 to 4326 when I'm selecting records
> > > - text(geometry) to convert the geometry column
> in a
> > > human-readable string
> > > 
> > > The problem is that the operations are not
> precise:
> > > the value I obtain via
> text(transorm(geom_column,
> > > 4326)) is different from what I inserted via 
> > > Transform(GeomFromText('POINT(' || $10 || ')',
> 4326),
> > > 26591).
> > > 
> > > Here an example (lon-lat values separated by a
> space):
> > > Insert:37.13953463591601 45.66405322544534
> > > Select:37.1321614399253 45.6647733181425
> > > 
> > > Insert:37.1321614399253 45.6647733181425
> > > Read:37.1248077366115 45.6654912656918
> > > 
> > > Insert:37.1248077366115 45.6654912656918
> > > Read:37.1174734269995 45.6662070796171
> > > 
> > > Is this a 'normal' behaviour?
> > > Am I omitting anything?
> > > 
> > > Thanks in advance for any help!!
> > > 
> > > Bye,
> > > Ludovico 
> > > 
> > > 
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