[Proj] proj=undef ??
jagoncal at gmail.com
Thu Jun 23 06:59:29 EST 2011
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
2011/6/23 <support.mn at elisanet.fi>
> 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.
> 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
> 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.
> Proj mailing list
> Proj at lists.maptools.org
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