[Proj] Terminology for origins

Clifford J Mugnier cjmce at lsu.edu
Mon Oct 6 12:34:50 EDT 2008

Not a separate Grid system but a "work around" when working South of the False Origin.  No other parameters change, and this is considered the SAME ZONE, SAME GRID.  Grid boundaries in North Africa use boundaries defined either by certain Ellipsoidal Bonne Grid Coordinates (for Lambert Zones) and/or Ellipsoidal Loxodromes.  Quite different from UTM.
The Lambert math models use the French Army Truncated Cubic and NOT the fully conformal Lambert Conformal Conic up through the middle 1950s.  Then the equations change with the date legislated by the individual former French Colonies.  After some encouragement, the EPSG now reflects this curiosity for the equations, but I suspect no recognition on the exotic Grid boundaries ...
Same also goes for Morrocco (for False Northing at False Origin), as well as Algeria  (of course not for Tunisia).
C. Mugnier


From: proj-bounces at lists.maptools.org on behalf of Gerald I. Evenden
Sent: Mon 06-Oct-08 10:20
To: PROJ.4 and general Projections Discussions
Subject: Re: [Proj] Terminology for origins

On Monday 06 October 2008 10:05:06 am Mikael Rittri wrote:
> 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.

You are confused?  You have my head spinning.  :-)

A far as the Algerian system(s) you are discussing, I finally guessed that you
are referring to the transition from one grid system to another as one treks
southward.  So?  What is new about that?  In a little postage stamp size
state like Massachusetts we have two grid systems: one for the mainland and
another that covers the islands of Marthasvinyard and Nantucket.  California
has 6 grid systems with irregular boundaries determined by county borders. 
At least the Algerians and UTM have regular grid boundaries.

The situation in Algeria is the rule not the exception.

Welcome to the world of grid systems.

The whole religious complexion of the modern world is due
to the absence from Jerusalem of a lunatic asylum.
-- Havelock Ellis (1859-1939) British psychologist
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