[Proj] proj=undef ??

support.mn at elisanet.fi support.mn at elisanet.fi
Thu Jun 23 05:16:59 EST 2011


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 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.


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