[Proj] Cosmetic latitudes...

[strebe]

On Mar 24, 2009, at 2:19:09 AM, "Mikael Rittri" <Mikael.Rittri at carmenta.com> wrote:  

>I just can't see the logic.

It is a mess, isn't it?

>And aren't there plenty of WKT flavours with different parameter names?

Yes, presumably any projection description is technically free to name its parameters whatever it likes.
>If one prefers a single name for all versions of lat_0, then "latitude_of_origin"
>is not a good name, since it would be misleading for the Hotine Oblique Mercator 
>(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 latitude).  
If the latitude of origin and the central meridian do not, in fact, project to the Cartesian origin, then the term really ought not apply. It is unfortunate that Snyder has used the same symbology (Φ₀) to denote the latitude of the map center in the Hotine as he has elsewhere to denote the latitude of origin. But that is a conflation of symbology, not terminology. His annotations for that projection call Φ₀ the latitude at the center of the map. I can't speak about the Krovac, since I have never studied it.

"Latitude of center" seems even more problematic for the same reason: unless the projection is vertically symmetrical, the "latitude of center" generally is not at the center of the projection. There are two different principles at play here: a latitude of origin and a latitude of center.. They should be distinguished by terminology. It is unfortunate that we must live with legacy conflations and confusions of terminology and symbology, but I do not think the situation would be helped by simply adopting a single term for distinct things.

Hence, if it is the latitude of the projection's  "center", then should we not call it the latitude of center (or central latitude)? (In point of fact, since Hotine is infinite in extent there is no mathematical "center", but at least symmetry allows a reasonable choice for a center..) If it is the latitude at the origin, then should we not call it the latitude of origin?

Regards,
— daan Strebe


On Mar 24, 2009, at 2:19:09 AM, "Mikael Rittri" <Mikael.Rittri at carmenta.com> wrote:
> 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
think.  
> >     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
projections,
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
names 
at http://www.remotesensing.org/geotiff/proj_list , and first I thought
that
"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
over 
and renames "latitude_of_origin" to "latitude_of_center", even when
there 
is no particular reason that it should spill over. 
   However, things aren't that simple.  The central meridian of Krovak
appears 
as a straight vertical line, but it is called "longitude_of_center"
anyway.  
And the central meridian of the New Zealand Map Grid is called
"central_meridian",
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
"latitude_of_origin"
is not a good name, since it would be misleading for the Hotine Oblique
Mercator 
(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
latitude).  

Regards,
--
Mikael Rittri
Carmenta AB
SWEDEN
www.carmenta.com

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