Paul Ramsey ha scritto:
We have discussed this in the past, and would not be surprised if
there is something written down somewhere, if only in Jesse's notes.
This is just one of those things which has several cases, so instead
of spending effort on doing it right, we did the simple thing and use
WGS84 all the time.
- Use the CRS of the first layer added, and stick with that.
This is sensible, considered that usually all layer are in the same CRS.
- If the first layer does not have a CRS, then we have to "guess" and,
- If the extents fall in the lat/lon range, it is probably lat/lon,
so use that
Hum... yes, should be working. I'm not fully convinced about using this
special treatment...
but it's probably sensible. It's just that in my experience I've never
seen maps in WGS84,
they're all projected in the practical cases I've seen here in Italy,
and sometimes the
coordinates small because people translated them in order to have small
numbers.
Yet, to have a -180/180 extent you must be looking at a building map,
more a CAD application
than a GIS one.
- If not use something generic, like CARTESIAN_2D
CARTESIAN_2D _is_ the only generic that can be used, because every time
you try to reproject
it you end up again with CARTESIAN_2D (the only sensible approach).
Which leads to the next set of cases, given a map in CARTESIAN_2D,
what to do when layers that have valid CRS information are added.
Probably ignore the CRS information and just draw them directly in.
Again, I believe this is a sensible approach. But in this case, allow
the single layers to be reprojected (what happens
when you add a layer in lat/long to a set of layers without CRS but in,
say, UTM? You need a way to escape).
Moreover, speaking of uDig, you need a way to enforce a CRS on layers
that don't have this information (how many
people can write a .prj file by themselves?).
Best regards
Andrea Aime
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