[Proj] Terminology for origins

[Mikael Rittri]

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.
 
So, in the Lambert Sud Algerie Grid, there is one False Northing that is 300 km, and an extra False Northing that is 1000 km?  
As one goes south from the northmost part of the map grid, the Northing value would decrease until it reaches 300 km at the Origin, 
but there the Northing would wrap around and become 1300 km? And then it would decrease again, if one would go further south. 
 
Tricky.  Reminds me of UTM.  I can see that it works on a paper map, but I suppose most GIS systems would have 
to implement such a map grid as two separate projections.   
 
Regards,

--
Mikael Rittri
Carmenta AB
Box 11354
SE-404 28 Göteborg
Visitors: Sankt Eriksgatan 5
SWEDEN
Tel: +46-31-775 57 37
Mob: +46-703-60 34 07
mikael.rittri at carmenta.com
www.carmenta.com 


________________________________

From: Clifford J Mugnier [mailto:cjmce at lsu.edu] 
Sent: den 6 oktober 2008 15:33
To: PROJ.4 and general Projections Discussions
Cc: Mikael Rittri
Subject: RE: [Proj] Terminology for origins


Sorry you find it contradictory, but it remains a fact.  When you get to zero and still need to go further south to use the same grid system, they add an additional number to that zero for a significant "jump."  I have not ever seen it used for False Easting; just for False Northing.  
 
The French Lambert Sud Algerie Grid has a False Northing of 300 km at the origin of 33º 18' North.  ("South of the False Origin add 1,000,000 meters to the northings").  The original grid tables are Tables de Projeciton Systeme Lambert Nort Maroc.  The EPSG has an interesting database for such things, however I consider my personal holdings as authoritative in most cases that differ.  This is one of those common cases.  
 
Regards,
 
Clifford J. Mugnier, C.P., C.M.S.
Past National Director (2006-2008),
Photogrammetric Applications Division
American Society for Photogrammetry and Remote Sensing 
and 
Chief of Geodesy,
CENTER FOR GEOINFORMATICS 
Department of Civil Engineering 
Patrick F. Taylor Hall 3223A
LOUISIANA STATE UNIVERSITY 
Baton Rouge, LA  70803
Voice and Facsimile:  (225) 578-8536 [Academic] 
Voice and Facsimile:  (225) 578-4474 [Research] 
Honorary Life Member of the 
Louisiana Society of Professional Surveyors 
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Member of the Americas Petroleum Survey Group
======================================================
http://www.asprs.org/resources/GRIDS/
======================================================

________________________________

From: proj-bounces at lists.maptools.org on behalf of Mikael Rittri
Sent: Mon 06-Oct-08 02:03
To: PROJ.4 and general Projections Discussions
Subject: RE: [Proj] Terminology for origins


Clifford J Mugnier wrote ( http://lists.maptools.org/pipermail/proj/2008-October/003865.html ):
 
> In North Africa, several of the Grids also have a False Northing at the False Origin in order to avoid negative northings.
> Really!
> :-) 
 
Sorry, I don't understand.  In what sense are you using the term "False Origin" here?  
 
If you are using it as EPSG does (at least for Lambert Conformal Conic 2SP 
and Polar Stereographic Variant C), then your statement is not surprising, since 
that's how such a False Origin works: it does have a False Northing and a 
False Easting. 
 
If you are using it in the other sense, as the point where False Easting and False Northing
are both zero, then your statement is contradictory (that's a little too surprising). 
 
So, you must be using the term "False Origin" in some third, more general way.  
Could you give a concrete numerical example? 
 
Best regards,
 
--
Mikael Rittri
Carmenta AB
Box 11354
SE-404 28 Göteborg
Visitors: Sankt Eriksgatan 5
SWEDEN
Tel: +46-31-775 57 37
Mob: +46-703-60 34 07
mikael.rittri at carmenta.com
www.carmenta.com 
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