[Proj] Define custom proj4 CRS

[Hermann Peifer]

Dear both,

I just came to the same conclusion that EPSG:22523 would be the most 
appropriate CRS. It's proj4 representation is:

+proj=utm +zone=23 +south +ellps=intl +towgs84=-206,172,-6,0,0,0,0 
+units=m +no_defs

However, as Stephane mentioned: the units of IBGE's DGN files are in km, 
so one has to define a custom CRS with these parameters (same as above 
with the exception of +units=km)

+proj=utm +zone=23 +south +ellps=intl +towgs84=-206,172,-6,0,0,0,0 +units=km

Hope this helps, Hermann


On 2013-01-31 16:03, Jose Gonçalves wrote:
> Dear Stephane
>
> The projection is standard UTM, south hemisphere, zone 23 (central
> meridian -45 deg).
> Datum Corrego Alegre has two versions: "Corrego Alegre 1961" and
> "Corrego Alegre 1970-72". They are similiar, with differences of only a
> few meters. (The lat/long coordinates of the datum origin are not needed
> for the CRS definition).
>
> The EPSG file included in Quantum GIS has both datums but for the older
> one no parameters are considered for the transformation to WGS84. So you
> would better consider "Corrego Alegre 1970-72".
> So, the EPSG code for your CRS is 22523. You can chose that in Quantum GIS.
>
> If you need to transform your data to WGS84 and need better accuracy
> transformations, there are datum shift grids for datum Corrego Alegre.
> Have a look at:
>
> http://www.ibge.gov.br/home/geociencias/geodesia/param_transf/default_param_transf.shtm
>
>
> Regards
>
> José Gonçalves
>
>
> PS: The IBGE ftp site seems to be off.
>
>
>
>
> 2013/1/31 Stephane Goldstein <s.n.g at gmx.com <mailto:s.n.g at gmx.com>>
>
>     Hello.
>
>     I have a vectorized version of a map from the Ilhabela
>     <http://www.openstreetmap.org/?lat=-23.845&lon=-45.343&zoom=11&layers=M>
>     island in Brazil, dating from 1975.
>
>     I could not find in QGIS the corresponding CRS, so I want to
>     establish it based on the information that is in the PDF version of
>     the map:
>
>     Universal Transverse Mercator Projection
>
>     Units are in kilometers, starting 10.000 km south of the equator
>     (around de south pole), counting towards the north.
>
>     and 500 km west from the 45º W meridian, couting towards the east.
>
>     Horizontal Datum is Corrego Alegre with the Hayford 1924 ellipsoid
>     and origins:
>
>     Latitude19° 50' 14.91" S
>     Longitude 48° 57' 41.98" W
>
>     Vertical Datum is from Marégrafo de Imbituba, SC, but I don't really
>     understand what are the parameters of the vertical datum that need
>     to be taken in account.
>
>     I would be really thankful if someone can help me to set a proj4 CRS
>     to this map.
>
>     PDF map can be downloaded here
>     ftp://geoftp.ibge.gov.br/mapeamento_sistematico/topograficos/escala_50mil/pdf/sao_sebastiao27963.pdf
>
>     Vector (DGN) here
>     ftp://geoftp.ibge.gov.br/mapeamento_sistematico/topograficos/escala_50mil/vetor/sao_sebastiao27963/vetor/
>
>
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