[Proj] Cosmetic latitudes...

Mikael Rittri Mikael.Rittri at carmenta.com
Tue Mar 24 04:19:09 EST 2009

> Mikael wrote:
> >   [-- text deleted --]
> >
> >	"Latitude of origin" sounds fine, but I am not too fond of using

> >	many different names for what is called lat_0 in Proj.4.  The 	
> > 	EPSG Guidance Note 7.2 has gone too far in this direction, I
> > 	I like "central latitude" as the only name.
> Please note also that "Latitude of origin" is the  name in the Well
Known Text 
> format specification.
> E.g.,
> PARAMETER["latitude_of_origin",49],
> Regards,
> - daan Strebe

For some projections, yes. Perhaps for the majority. But for some
it is "latitude_of_center".
And some projections have "central_meridian", while others have
"longitude_of_center" instead.

I have tried to figure out the reasons why, by looking at the OGC WKT
at http://www.remotesensing.org/geotiff/proj_list , and first I thought
"central_meridian" is used if and only if the central meridian appears
as a straight vertical line.  When the meridian is curved, it seems that
"longitude_of_center" is used instead, and the "center" suffix spills
and renames "latitude_of_origin" to "latitude_of_center", even when
is no particular reason that it should spill over. 
   However, things aren't that simple.  The central meridian of Krovak
as a straight vertical line, but it is called "longitude_of_center"
And the central meridian of the New Zealand Map Grid is called
even though it appears as a curved line.
   I just can't see the logic. And aren't there plenty of WKT flavours
with different 
parameter names? 
   If one prefers a single name for all versions of lat_0, then
is not a good name, since it would be misleading for the Hotine Oblique
(the origin is near the equator, not on the central latitude), and for
the Krovak 
(the origin is at the oblique cone apex near Finland, not on the central

Mikael Rittri
Carmenta AB

More information about the Proj mailing list