[Proj] Define custom proj4 CRS

[Jose Gonçalves]

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>

> 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:
>
> Latitude 19° 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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>
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