How could I set a base dir for target

[Sergei Valid]

I'm working with Ant and one question have been faced.
It's seems clear to set a base directory to project you need to use
'basedir' property in 'project' tag.
But how could I set the basedir only for 'target'?
I need to write build.xml with a target to launch application
from any directory. That application loads a file with a relative path.
Thus, I should set the base directory to root of that application.
Thank you.

[Marilyn de Queiroz]

How about
project name="foo" default="all" basedir="."