Finding the center of a possibly scrolled image (stand-alone SVG)

For stand-alone SVG, I am trying to reposition an SVG element in the middle (or top or bottom) of the visible part of the screen, even after scrolling.
For stand-alone SVG, IE 10, chrome, and Safari do not correctly report window.pageYOffset or window.scrollY.  Firefox 19.0 does work.
Can anyone suggest what I might be doing incorrectly or another way to find the center/top/bottom of a (possibly) scrolled stand-alone SVG image?

thanks,
steve



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