technique for changing map projections within a display??

[Stuart Maclean <stuart@xxxxxxxxxxxxxxxxxx>]

Using the visad/examples/MapProjectionDisplay as a template, I have 
learned how to do map projections is VisAD.  My preferred projection 
classes are the ones from IDV, i.e. ucar.unidata.geoloc.projection.*, 
with the ucar.visad.ProjectionCoordinateSystem the bridge to the visad 
classes.

Now I'd like to change map projections within a display, say from a 
Lambert to a Mercator.  Trying this, I first remove scalar maps for 
RealType.Latitude and RealType.Longitude from my DisplayImpl (J2D).  But 
then when I try to build a new DisplayTupleType with DisplayRealTypes 
from the example visad app, I get errors.  Seems like a DisplayRealType 
must be uniquely named.  To get around this, I just make sure that the 
N'th time I set the display coord sys, I name the DisplayRealType with N 
in the name.   I did think about just catching the TypeException and 
continuing regardless but was unsure of the ramifications.

I also tried keeping the display real types  and just building a new 
tuple type for each map projection, but that failed with a 'display real 
type already in a tuple' error.

Can anyone explain why such types must be uniquely named?  On a related 
note, is it a waste to be building 3 display real types and a 
displaytupletype every time I want a new display side coordinate system, 
i.e. a new map projection??  Is there a better way to achieve my goal. 
I note that what I really want is a setCoordinateSystem on my 
DisplayTupleType, then I wouldn't even have to remove the scalar maps at 
all (??)

Hmm, it's just dawned on me that IDV allows in-display map projections 
on the fly, can anyone enlighten me there?

Any help appreciated.

Stuart