Have you considered differential DGPS? I have one and contacted the manufacturers of my YUMA unit here at Ultimate Positioning in Melbourne today to assess your question and they said that once the unit has acquired the necessary data you will get true south as close as you ever will any other way. If you were in Melbourne I would come and do it myself, maybe look into half day hire? All you need to do is turn it on, acquire the spatial data and use the compass. Some units (mine included) project a laser once data is acquired.
Just a thought.
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