[Proj] proj=undef ??

[Jose Gonçalves]

Isn't the "undef" projection the same as "+proj=longlat"? You do a datum
transformation of geographic coordinates with a command line as:

cs2cs +proj=longlat +ellps=bessel +towgs84=123,456,789 +to +proj=longlat
+ellps=GRS80 +towgs84=1,2,3

Regards

Jose Gonçalves



2011/6/23 <support.mn at elisanet.fi>

> Mikael,
>
> if the projection is "undef" then you can not enter any
> projected points, since there is no known projection!
>
> But you can still enter latitudes and longitudes, and
> that is exactly what people are doing.
>
> Janne.
>
> ----------------------------------
>
> Mikael Rittri [Mikael.Rittri at carmenta.com] kirjoitti:
> > Janne, you wrote:
> >
> > > simply since you don't need any projection to do datum calculations.
> >
> > Do you mean that datum shifts can be done in longlat? True, but it
> > was you who said that the coordinates were not in longlat but in
> > some "undef" projection.  I though "undef" meant that you had no
> > idea of how those coordinates are related to longlat.
> >
> > Let't take a concrete example:
> >
> > proj=undef
> > ellps=bessel
> > towgs84=123,456,789
> > no_defs
> >
> > Now, I say that the input point is
> >
> > 53479811      97344703
>
> this is clearly a projected coordinate since
> the values are  too large to be lat or lon. As
> said if the proj="undef" then the coordinates
> are passed through and are earth angular
> (lat and lon).
>
> You should here enter:
>
> 65.12312N 34.56W
>
> for example.
>
> >
> > Can you tell me how this point could possibly be translated to
> >
> > proj=undef
> > datum=WGS84
> >
>
> With datum shift that might give output such as:
> 65.2342N 34.52344W
>
> > Or maybe you can tell me under what circumstances you want to
> > use such a feature?
> >
>
> Many point file formats give values earth angular (lat and lon) and
> never bother to project them to anything. They can still be datum shifted
> but mostly in WGS84 nowadays.
>
> My idea is that it is better to let the proj.4 library to handle this case
> than
> to program an additional layer over it just to check if the projection is
> undef and do nothing in that case.
>
> So as the proj.4 library now can short circuit the case when one of the
> datums is "undef" it should also be able to short circuit the case when
> one of the projections is "undef" and it should pass all the values from
> input to output assuming that the projection (or inverse projection) is
> not required.
>
> This feature would not affect any other calculations and mechanisms at
> all it is just a short cut.
>
> Janne.
>
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