Zooming and projection problems (was: Success (sort-of)!)

[Russell Nelson]

Bernhard Reiter writes:
 > >   2) set the projection of the session using Map/Projection...
 > >   3) load the raster map, and set its projection using Layer/Projection...
 > 
 > If you miss 2) then even 3) will not help much
 > and Thuban behaves oddly when zooming around even with 1).

This question will mark me as a GIS newbie, but no matter: Is it true
that a Map must ALWAYS have a projection?

Or is it true that a Map doesn't need a projection if all of the data
you load into it has the same projection?

I think that in any case, the scalebar should say "Unprojected map.
Use Map/Projection to set the projection" if the map's projection is
None.  It's better than leaving the space empty.

Alternately, an unprojected map should have as its base layer "This
map has no projection. If you add layers with different projections,
Thuban will be unable to reconcile them into one map that makes
sense."

Hey, maybe that's the best way to do it?  When a user creates a New
Session (or starts Thuban with no session; same thing), give them a
"Map Information" layer.  They can delete it, or they can hide it.
But as long as it's there, it will emit helpful information like "Yo,
stoopid!  Dis layer ain't projected!"  or "Ya dimbulb, ya just loaded
two layers that don't overlap!  Maybe deir misprajected?"  Or, at
least, those would be the error messages in the Noo Yawker
translation.

 > Probably more tests and thinking are needed 
 > but from a user perspective this is not inuitive.

Yeah, it's not.  At least part of the problem is user naivete, but
the software should let them know 1) the effects of what they've done,
and 2) how to fix it.

Specifically, we need to do something to prevent scale blow-up.  This
causes ZeroDivision errors that should be entirely preventable.

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