I've been working at mapping my wife's farm land, which consists of several parcels that do not align with the roads or streams.?ÿ I wanted to have repeatable points on fence corners and splits, so used a couple 5/8-11 bolts drilled to fit headless nails.?ÿ It could be done in a drill press but I had a friend do them on a lathe for better centering. I cut the heads off 16d nails and chamfer them to allow for a little mushrooming when driven. If the nail doesn't come out plumb you can still use the bolt to bend it a little without binding in the hole.
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An alternative - get a 5/8" bolt 3 or 4 inches long and cut off the head. Drill a 9/16" hole in the top of the fence post and drive the bolt in.?ÿ
That would give perfectly repeatable centering for frequently used points, whereas my method allows a mm or two of slop.?ÿ But I didn't think it was worth the extra work, cost, and damage to the posts to do a whole bunch your way.
And I would rather carry the weight and bulk of a hammer than a drill and battery around the fields.
We've occasionally done the same with a short bolt welded to the top of a steel gatepost. Cattle proof and you do know if the point ever gets knocked. Only thing to remember is that the coordinated point is at the head, so if you use an extension rod at the start you always need to use an extension rod (fully screwed down). It might be slightly off plumb, but with exactly the same set up it is always out by the same amount, so it doesn't matter.