question on projections

[Chris]

Hi everyone,

I'm wondering how I can change from one projection to the other one the fly,
depending on the area of the map I'm in.

For example, if I'm using UTM projections, and I pan from one UTM zone to
ther other, how can I switch my output projection let's say from UTM 16 to
UTM 17.  It looks like I can only define one output projection in a map
file.

Regards,
Chris