[kh99]

I'm really a js beginner so I don't know what's going on, but you only have to google "getElementById in ie" to find that it has issues. One suggestion I found was to get all elements of a given tag name and check the id yourself, something like

function myGetElementById(tag, id)
{
   var elements = document.getElementsByTagName( tag );
    for var i = 0; i < elements.length; ++i )
    {
        if( elements[i].id == id )
        {
            return elements[i];
        }
    }
    return null;
}
 ...

var basediv = myGetElementById('div', 'basediv').offsetTop;

(I haven't tried this code at all so it could have typos or other problems, but you get the idea).