Isn't the "undef" projection the same as "+proj=longlat"? You do a datum transformation of geographic coordinates with a command line as:<br><br>cs2cs +proj=longlat +ellps=bessel +towgs84=123,456,789 +to +proj=longlat +ellps=GRS80 +towgs84=1,2,3<br>
<br>Regards<br><br>Jose Gonçalves<br><br><br><br><div class="gmail_quote">2011/6/23 <span dir="ltr"><<a href="http://support.mn">support.mn</a>@<a href="http://elisanet.fi">elisanet.fi</a>></span><br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;">
Mikael,<br>
<br>
if the projection is "undef" then you can not enter any<br>
projected points, since there is no known projection!<br>
<br>
But you can still enter latitudes and longitudes, and<br>
that is exactly what people are doing.<br>
<div class="im"><br>
Janne.<br>
<br>
----------------------------------<br>
<br>
Mikael Rittri [<a href="mailto:Mikael.Rittri@carmenta.com">Mikael.Rittri@carmenta.com</a>] kirjoitti:<br>
</div><div class="im">> Janne, you wrote:<br>
><br>
> > simply since you don't need any projection to do datum calculations.<br>
><br>
> Do you mean that datum shifts can be done in longlat? True, but it<br>
> was you who said that the coordinates were not in longlat but in<br>
> some "undef" projection. I though "undef" meant that you had no<br>
> idea of how those coordinates are related to longlat.<br>
><br>
> Let't take a concrete example:<br>
><br>
> proj=undef<br>
> ellps=bessel<br>
> towgs84=123,456,789<br>
> no_defs<br>
><br>
> Now, I say that the input point is<br>
><br>
> 53479811 97344703<br>
<br>
</div>this is clearly a projected coordinate since<br>
the values are too large to be lat or lon. As<br>
said if the proj="undef" then the coordinates<br>
are passed through and are earth angular<br>
(lat and lon).<br>
<br>
You should here enter:<br>
<br>
65.12312N 34.56W<br>
<br>
for example.<br>
<div class="im"><br>
><br>
> Can you tell me how this point could possibly be translated to<br>
><br>
> proj=undef<br>
> datum=WGS84<br>
><br>
<br>
</div>With datum shift that might give output such as:<br>
65.2342N 34.52344W<br>
<div class="im"><br>
> Or maybe you can tell me under what circumstances you want to<br>
> use such a feature?<br>
><br>
<br>
</div>Many point file formats give values earth angular (lat and lon) and<br>
never bother to project them to anything. They can still be datum shifted<br>
but mostly in WGS84 nowadays.<br>
<br>
My idea is that it is better to let the proj.4 library to handle this case than<br>
to program an additional layer over it just to check if the projection is<br>
undef and do nothing in that case.<br>
<br>
So as the proj.4 library now can short circuit the case when one of the<br>
datums is "undef" it should also be able to short circuit the case when<br>
one of the projections is "undef" and it should pass all the values from<br>
input to output assuming that the projection (or inverse projection) is<br>
not required.<br>
<br>
This feature would not affect any other calculations and mechanisms at<br>
all it is just a short cut.<br>
<div><div></div><div class="h5"><br>
Janne.<br>
<br>
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</div></div></blockquote></div><br>