Quilting Trans Merc Charts

[jonasaberg]

I know we spent a lot of time grinding the topic of quilting transmerc chart back in the spring during the 2.1 development.

I think Dave and others (temporarily?) gave up, but I would like to bring the subject back up and either get it declared as dead duck, impossible to do, or maybe lift it up as a possible enhancement for the next version.

Dave wrote back in April:
http://www.cruisersforum.com/forums/...tml#post437629

a. Quilt for TMerc: This needs some discussion. We cannot accurately include a TMerc chart in a Mercator quilt without a costly reprojection of the TMerc chart into Mercator. On the other hand, if all the charts in the quilt are TMerc, then we can build a reasonable TMerc quilt. This will be the case in some Scandinavian areas. This needs coding...
So, how do we handle the mixed-projection case? Ideas?

'

I suggested

Why not use a combination of quilt and not quilted?
Use Tmerc quilted and show the Mercator charts on the chart selection bar.
This would show there are Mercator charts showing the same area while having the possibility to quilt the Tmercs.

I think it would be real nice with quilted tmercs, at least to have the tmercs quilted with other tmercs.

I wrote a small notice on a Swedish site about the release and the first reply was "nice with quilting" which of course will not work for most Swedish charts.

Thoughts?

[cagney]

I have added this issue to FlySpray FS#315 : Quilting Transverse Mercator Charts

OpenCPN must have the ambition to quilt TMercs - full stop.

But personally I have the feeling that using Tmercs as a marine chart projection is the work of "desk jockeys". I can't see any good reason to use it. What is the advantage to use this, non intuitively warped, projection? I fail to find any good arguments, from the point of view of practical navigation, to do so.
Maybe this will get the discussion going!!


Thomas

[jonasaberg]

Thomas,
Tracker addition: Good! It is better if OpenCPN behaves similarly using different projections and it will make CPN better in areas with a lot of tmerc charts, like in Scandinavia.

I guess all different projections solve different problems...
A Mercator projection is true along a horizontal line through the chart, a tmerc along a vertical. I think the reason that tmercs are used in Sweden simply is that the country is "slim and tall" and to get a better map of large areas tmercs make sense. Tmercs at sea might then just be from tradition from land cartographers... maybe they just used what they where used to use...
I will ask around, would be good to know why...

I kind of doubt the OpenCPN community will get the projection changed though... ;-)

/Jonas

[cagney]

In the upcoming 2.4 beta, quilting between tmercs will be introduced.
Quilting between tmercs and other chars will not(yet!) be supported.
This is good news for, for example Swedish users, were all inshore charts use a version of tmerc.

Thomas

[jonasaberg]

That is really good news for Scandinavian sailors!
Looking forward to it.

/Jonas