💾 Archived View for republic.circumlunar.space › users › johngodlee › posts › 2020-02-16-rebar.gmi captured on 2023-04-19 at 23:51:32. Gemini links have been rewritten to link to archived content
⬅️ Previous capture (2021-12-04)
-=-=-=-=-=-=-
DATE: 2020-02-16
AUTHOR: John L. Godlee
Last year when I did terrestrial laser scanning (TLS) in Bicuar National Park in Angola, I used lengths of threaded bar which can be driven into the ground to mount the reflective targets which are needed to align multiple scans. Conventionally each reflective target is mounted on its own tripod, which allows flexibility in the height of the target, but it was unfeasible to carry 6 tripods on the plane. The lengths of threaded bar I used had been manufactured previously by a colleague who had also used them to mount reflective targets for laser scanning on a tidal mudflat. I found however, that although the threaded bar was a step in the right direction, they were still very heavy, weighing about 8 kg each. This year I designed an even more minimal system to hold the reflective targets, which utilises local materials where possible. My chosen method might sound like its very simple but it took me a long time thinking about this to achieve the optimal setup.
The tripod which would normally hold one reflective target
A reflective target with a screw base
While shopping around I had noticed that it was possible to buy shorter lengths of threaded bar with the thread pitch I needed for the targets (5/8" Whitworth - BSW, 11 threads per inch), commonly 1' or 8". I decided that I could buy one of these small pieces of bar for each target and then find some way of attaching them to a pole, which I could buy in Tanzania, rather than taking the entire pole on the plane.
A length of threaded bar in use as a target mount
The local material I decided to use for the pole was steel reinforcing bar (re-bar), which is used in poured concrete construction to strengthen walls and foundations. Re-bar is found easily around southern Africa. As well as being used for building, it's commonly used as a welding material to make gates, fences, construct carts and repair vehicle chassis. In my work we often used re-bar to mark plot corners, which can be partially buried at the end of the plot census and recovered with a metal detector at a later date.
Cutting lengths of re-bar to 1.5 m and 2 m length in Tanzania
Now I had to figure out how to attach my short lengths of threaded bar to a longer length of re-bar. In construction, a common way to attach lengths of re-bar is to tie them with an overlap using wire. Unfortunately when I tried this there was too much wiggle. I need the target to remain completely still during the measurements so that the GNSS measurement lines up with the location of the centre of the reflective target. Continuing my research I found a company called UniTec, who make lots of re-bar jointing methods. One that seemed promising was called a mechanically bolted coupler, which has a long clamp that you put both pieces of bar inside, then tigthen screws to clamp the bar tight. I had planned to buy these until I realised that a) each coupler weighed over half a kg, b) they only ship from the USA, and c) they cost about £25 each. I imagine they are so heavy and expensive because they have to adhere to a number of regulations surrounding the weight-bearing potential for the construction of very large buildings. Another issue that made the coupler less than ideal was that I would be relying on finding re-bar that fit the coupler. I did some research into common sizes of re-bar but the standard sizes differ a lot between countries. I could have used bits of wire as shims to pad out the coupler, but this would likely have introduced wiggle.
A UniTec mechanically bolted coupler
I looked briefly at manufacturing a set of metal backing plates which I could attach wire rope grips to, sort of like a DIY mechanically bolted coupler, but it would have taken a lot of work to get the metal backing plates manufactured to the correct specification and I was running out of time at that point.
I looked also at buying boss head clamp stands like they have in chemistry labs, but this was too expensive, as I would have needed at least 10 clamps, two for each target to avoid lateral wiggle. It might also have been difficult to get them tight enough. An advantage to this method however is that the clamps are easily removable and adjustable.
One option which I probably didn't give enough thought to was whether I could weld the threaded bar to the re-bar. I could have found out what material the threaded bar is to see if it could be welded to re-bar. My worry with that method is that the weld may have broken due to poor workmanship, and it wouldn't be repairable in the field.
In the end, I was wandering around B&Q trying to find a hammer and I came across a set of hose clamps (aka jubilee clips, worm clamps) which are used to attach a rubber hosepipe to a tap fitting. They are adjuatable with a flathead screwdriver and accommodate many different sizes of re-bar, they are also very lightweight. I chose 18-25 mm diameter clamps. In the UK I performed a test using two hose clamps to join a piece of threaded bar to a piece of re-bar, which I was kindly given by the local scrapyard. It creates a very tight connection, the only compromise being that it is slightly more difficult to hammer the re-bar into the ground as it is easy to accidentally strike the threaded bar, which should be avoided in case the thread gets messed up. To solve this problem I bought a spare short length of re-bar about 8" long that can be used with the hammer to transfer all the force to the re-bar as it is hammered.
A test of the hose clamps with re-bar and threaded bar
There are a couple of advantages to the hose clamp method over the original large lengths of threaded bar. Because the threaded bar lengths are considerably shorter I can take 10 rather than five, allowing me to leapfrog between subplots, scanning the first while I GNSS the next, this could make the work considerably faster once I get on a roll. Also because the re-bar is cut near to the fieldsites, I can get many different lengths cut, allowing me to achieve the variation in target height which is desirable for calibrating the scan rotation, and also allowing me to place the targets well above the thick long grass, allowing them to be more easily identified by the software in the scan alignment stage.
