Re: OT land surveying

Posted by PJH on December 21, 2020 at 19:21:10 from (50.45.34.202):

In Reply to: Re: OT land surveying posted by bc on December 21, 2020 at 18:05:30:

bc - the double centering is for alignment only.

If you think about how a transit is built, the scope is supported by a "tower" of sorts on each side of the instrument. If those "towers" are not perfectly built (and they are not) the crosshairs of the scope will not be absolutely perfectly centered on the instrument. So - when you look at your backsight for shot #1, imagine your scope has a built in lean of just .001 inches to YOUR left. That's a minute amount, but it magnifies when you flip the scope to give your helper a point. His #1 mark will be shifted to HIS left a little bit. (Remember - he is facing you) Next you rotate the instrument to take your #2 shot. By rotating the instrument, your built in lean is now .001 inches to YOUR right. When you flip the scope to give your helper his #2 shot, his #2 mark will be shifted a little bit to HIS right. That's what we call a split. You have two marks on the top of the stake, so you divide the distance between the marks by two and drive a tack at the midpoint between his #1 & #2 marks. I must say that this would be a high and unnecessary level of accuracy for most fence work.

It is a little confusing to explain, but basically what you are doing when you double center is you are compensating for the built in error that every transit leaves the factory with. Transits have a lot of adjustment points on them, but I have never seen one that was adjusted so well that it would not split on a double center.

When I staked fences for neighbors and friends, I tried to get them to have the tee posts scattered along the line. Once you have the transit set up, you can align the posts as they drive them. They are always surprised when I ask them if they want the nub side of the post on the line or the back side of the post on the line. They are thinking "within an inch or two", but we have the capability to be right on the money.

I want to stress again - this is THEIR fence line. They chose the points to start and end from. I'm just connecting the dots.

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