Re: Worldmap mercator projection - Latitude to Y calculation. - last try

John,

> There are three stages to this
...
> 1. The mathematical conversion from latitude (and longitude) to the
> position on the plane map *relative to its centre*

Nope.  There is no rule that that center must be taken as the origin.   Take 
the formule Paul provided for instance.  It returns a result in the range of 
0 to 1 .

> 2. The _scaling_ required, depending on the size of the map image.
> Which again will depend on the size of the image (and may be different
> for X and Y).

Worse : Mercator maps go from +85 to -85 latitude.  That must be part of the 
formule (I've found several maps where the south-pole is cut off, and a bit 
of the north too. iow, useless without further information).

But again no.   The result of the formule *is* the scaling (in your and my 
usage ranging from +1 to -1, in Pauls formule case, from 0 to +1).  You just 
apply it on whatever size Mercator-style map you have handy.

> I think _most_ of those contributing to this discussion know that,
> but have not been making it very clear which bits of their formula(e)
> do what.

I do not need to know what all the parts of a car do, as long as I can drive 
it.  The same goes for these two formules.   Latitude goes in, something I 
can apply comes out.


Though the whole problem isn't that nobody understood what you said there, 
but that nobody was willing to compare the (intermediate) results I posted 
with what they got themselves, allowing me to locate where I made my 
mistake(s?).

Not when I asked for it in my first post, and not when I rather explicitily 
asked for it a few days back. :-(

Regards,
Rudy Wieser