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For my reference heading, I shot a recognizable tree on the Olde Stage ridgeline (about 6km away). I moved the theodolite generally along the bearing to this tree, say within 10cm, so this reference heading should be good to atan(0.1/6000) = 3.5" of arc.
The array obviously was not exactly square, so I shot it three different ways, to be more-or-less representative of the variations that might be used.
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