[Proj] Cosmetic latitudes, and the diacritics of Josef Krovak and others

Gerald I. Evenden geraldi.evenden at gmail.com
Fri Mar 20 09:26:49 EST 2009

On Friday 20 March 2009 5:34:00 am Mikael Rittri wrote:
> Hello,
> I lump together some unrelated light-weight questions here:
> A) For some projections, a parameter for central latitude can be given,
> but it does not really affect the shape of the map - it just creates
> an extra offset in the northing values (in addition to the offset
> due to the FalseNorthing).
>     My question: is there an established terminology for such
> a central latitude?
>     When documenting projections, I would like to write something
> like "the central latitude can be specified but is just cosmetic",
> with a link to a glossary entry for "cosmetic central latitude".
>     But is "cosmetic" the best word?

This is a case of deja vu all over again.  ;-)

Not long ago I went through a debate with someone who wanted a "lat_0" for a 
cylindrical projection.  As commented above, I responded that y_0 would do 
the job and that lat_0 was totally unnecessary.  In this case the use of 
lat_0 was simple lazyness or the creation by someone completely ignorant of 
the basics.

Note: lat_0 in the above case had nothing to do with the lat_ts or true scale 

The only legitimate use of lat_0 is when the location of mathematical origin 
is necessary to determine the overall character of the projection such as in 
the azimuthal projections where it determines the point of tangency of the 
projection plane with the earth (when k_0=1).  I would tend to call it 
the "natural" orgin.

In the case of normal cylindrical projections the "natural origin" is *always* 
at the equator and thus lat_0 is always 0 and omitted.  If a lat_0 is present 
in the cylindrical projections, it might be called a "geographical origin 
offset" as contrasted to the the Cartesian origin offset of the false 
eastings and northings.

Much of this argument can also be carried over to lon_0.

I would tend to call it "geographical origin offset."

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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