[Proj] Terminology for origins

[Mikael Rittri]

There is a picture of the Sud Algerie Grid on page 4-25 in
 
http://www1.nga.mil/ProductsServices/GeodesyGeophysics/WorldGeodeticSystem/Related%20Documents/8358_1a.pdf
 
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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