Tomcat's BasePath - how to set it for log4j and for file uploads?

[dave sag]

I start tomcat with startup.sh in my tomcat's /bin folder.
As a consequence all file paths I refer to programatically end up in that bin folder.
I want to configure log4j to put these files in the {tomcat}/logs folder and I want struts' file uploads to put the uploaded files in {tomcat}/myapp/userdata
how can I programatically set this base path (ie an option in my web.xml file for example) to tomcat_home so that i can then place logs and so forth without having to be concerned where my client may startup tomcat from?
we noticed that if she started her tomcat with a different script then the folder where that script lives becomes the new tomcat base path.
dave