[Proj] Terminology for origins

Gerald I. Evenden geraldi.evenden at gmail.com
Mon Oct 6 11:20:48 EDT 2008

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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