For the times that I need to know how far an element is from the top of the DOM, not the top of it’s parent, I use this helper function.

//Loops through all parent nodes of an element to get it's distance from the top of the document
function getDistanceFromTop(element) {
    var yPos = 0;

    while(element) {
        yPos += (element.offsetTop);
        element = element.offsetParent;
    }

    return yPos;
}

This function returns the number of pixels the top of the element is from the top of the DOM.