Re: Worldmap mercator projection - Latitude to Y calculation.

On 2026/1/15 20:0:31, R.Wieser wrote:
> J.P. ,

[John by the way :-) ]
> 
> First off, FYI, Paul has minimized his posting about the subject to the 
> Win10 group only.  If you're not there already it might be a good idea to 
> take a peek.

I replied to the posting I did, thinking I was the only respondent -
probably in the W7 'group; I saw the other responses when I moved to one
of the other 'groups, but too late then to amend my post.
> 
>> If it _is_ light-source-at-centre, then the Y co-ordinate would just be
>> the tangent of the latitude
> 
> Somewhere along the road I had a similar thought.   But why than do all the 
> formules I now have (four at the moment) all do a tan() and than wrap that 
> up in a log() ?    There must be something to it.

[]

Indeed! But I can't think what.

Did you _try_ using just the tangent on a couple of places? Or perhaps
just a couple of latitudes, to see if the lines come out where they are
on the plot? You'd probably have to do that in the first place anyway,
to see what the vertical scale _is_; if doing that for say the ten and
forty degree lines (assuming that's what the ones on the plot are) gives
different scales, then you'd know it isn't just that.


-- 
J. P. Gilliver