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Sticky targets on top of timber posts – good or bad idea?
Hi guys
I’m involved in a large housing construction project in a city center which is at the initial ground works stage. There is some primary control around the perimeter of the site from the original survey but there is now plywood fencing around the site blocking line of site to the stations.
The site foreman want me to put sticky targets on top of the 4″ x 4″, 8ft high timber posts holding up the plywood fencing and use these for the site control. The bases of the posts are concreted into concrete cast blocks.
My feeling is that its a bad idea. I know from doing carpentry jobs that rain and sun make this sort of timber move like crazy (as much as a couple of eight of an inch).
What do you guys think? and where would you put the site control?
Cheers, John
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14 Replies
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Just say no.
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anything ambulatory is not a suitable for any kind of reference point.
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where would you guys put them or would you set prisms over nails instead?
There are many tall buildings surrounding the site where targets could be mounted
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Transfer control to reflectors on nearby buildings?
If not that, you probably need to transfer redundant control to inside the plywood walls for easy access during building layout.
You will lose a little accuracy in the process of transfer, but can allow for that by siting the building with a little safety margin, not up against the limits.
Then the layout of building features can be done conveniently and with your best accuracy relative to their other features.
To get inside the walls, maybe tall prism poles carefully plumbed over the existing control, set up multiple times for redundant checking and averaging and remeasured from inside?
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I dunno, I’d maybe try to scribe an X on a transformer pad or footing inside the fence if there is one. Something concrete is what I look for first.
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Cheers Bill
I can take the odd sheet of ply off and put back after afterwards. After transferring control into the site.
Do you agree with the others about putting the targets on top of timber post?
“ambulatory” – great word, love it!
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Rather than taking sheets off and replacing afterwards, see if you can cut small squares in them, say 12″, in positions where you can see an external station. You can then establish new stations inside the site. If you cut neatly the square will go back in the hole to keep the site secure – fit a batten across the back and you can then lift it out whenever you need to observe. You can retain the accuracy of the external control and check from the distance if the inside station is moving (but do this in several places so you have redundancy in the checks you make).
Targets on posts are a no-no. I once saw a site where the guy had put targets on hinged lighting columns. Every time they changed a bulb his reference points changed position.
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Buildings can swell and contract with temperature as well. Nevertheless that would be preferable to posts that are within the construction zone.
Perhaps you could cut some windows in the plywood wall.
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Why not set new control rods just inside the fence posts?
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There are many tall buildings surrounding the site where targets could be mounted
We have a winner..
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thanks for the help guys. Good idea about putting holes in the ply and sighting the control outside
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I wouldn’t do what the contractor suggests. Those fences get bumped or obscured so you better have a workaround or alternative solution in your tool belt. You don’t say what you are staking or the precision required or expected but anything off-site is always best. I’ve used targets affixed to buildings after permission is granted and gift cards exchanged. Put it up high out of reach where it will be visible if a beer truck parks in front of it. Around here anything more than 500 feet away would be risky; fog rolls in and you are out of bullets. On-site ground control in a tight site is a risky endeavour as there are too many ways for your investment to become worthless. Perhaps something outside of the gate(s) of the project that you can carry onto the project when you need it. Now that I’ve written this, it all seems too obvious, go with your gut and hope for the best.
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Paint stick to concrete or other solid surface and apply sticky target.
If it makes him happy, set some on the posts and then use the best control that you can.
I prefer control to be as low to the ground as possible and still see it.
If you are staking overhead, place control checks overhead.
The more you set, the more you can check into as the project grows.
I have hand painted grade markings on 1×4 boards and attached to power poles and light poles around projects. When project is completed, removed them and took them to next project.
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“The bases of the posts are concreted into concrete cast blocks.”
If the concrete post anchors are poured in place why not use them as control? Of course some will be destroyed during construction but they can be relocated to a convenient place of your choosing. When doing layout etc. check in to as many control points as you can see. ????
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