[Proj] Terminology for origins

Mikael Rittri Mikael.Rittri at carmenta.com
Tue Oct 7 05:43:53 EDT 2008

There is a picture of the Sud Algerie Grid on page 4-25 in
but the resolution is bad.  One can see that the grid extends farther south than the zero Northing line, though. 
I wrote: 
> Let me see if I got it right.
but I didn't; the wrap-around occurs where the Northing becomes zero.  
Clifford Mugnier wrote (to me): 

From: Clifford J Mugnier [mailto:cjmce at lsu.edu] 
Sent: den 6 oktober 2008 16:13
To: Mikael Rittri
Subject: RE: [Proj] Terminology for origins

No.  From the Northing being 300 km at the Latitude of Origin, 33º 18' North, going South from there the ordinate would decrease to zero.  At that point, it would then have a "jump" value of 1,000 km and then decrease South from there.  Such things require a logical "IF" to work properly in GIS software.  With most of GIS software being programmed by programmers and not Cartographers or Geodesists, it remains a conundrum.
Likewise, using ellipsoidal Loxodromes for boundaries between various Grids ...  :-)
C. Mugnier


From: Mikael Rittri [mailto:Mikael.Rittri at carmenta.com]
Sent: Mon 06-Oct-08 09:05
To: Clifford J Mugnier; PROJ.4 and general Projections Discussions
Subject: RE: [Proj] Terminology for origins

Thanks for explaining.  I did not understand that the extra False Northing should be added only south of the False Origin. 
Let me see if I got it right.
[ the rest of my post can be found at http://lists.maptools.org/pipermail/proj/2008-October/003876.html ] 
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