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A-Frame Calibration

 
Ben Bishop
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Hey everyone,

I've made the classic low-tech A-frames before. I know the method of calibration on a non-level surface. For those who don't know, you build the a-frame with a string and hanging bob down the center. You mark where string rests on the cross beam, then you spin it around so the two legs are reversed and mark the beam again. The spot in the middle of the two marks is where you aim while getting your contour marks. This method works whether or not the legs of your a-frame are even or whether or not you are calibrating on a level surface. But you HAVE to remember to keep one leg on the ground as you swing the other leg over to the new contour point. Otherwise, if you lift the whole thing up and shift it to get the next contour point, your contour line will not be correct. Hope that makes sense!

Anyway, my question for you all is about calibrating with a bubble level. I went to a workshop recently where they built an a-frame with a level and assumed that as long as the bubble level attached to the cross-beam was showing level, then the a-frame was calibrated. BUT when you spun the a-frame around on the same two points, the bubble was not level. In other words, the cross beam was perfectly level but the two legs were uneven. I noticed this issue and built a string/bob a-frame instead. I couldn't figure out how to calibrate a bubble level a-frame if you don't have a perfectly level surface to calibrate on. Any ideas?
 
Jordan Lowery
pollinator
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Location: zone 7
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go with your method.

the bubble works only if you do a few things first. make sure the wood legs are exactly the same length, measure up the legs exactly on both sides and put the horizontal. measure mid point on the horizontal, or find a "level" spot (good luck)and do as you mentioned they did. even still that method still can fail imo. its close but not good enough for some things imo.

the best way to check your level after the A frame, or laser transit, or sight level. is water. water will always level itself, ALWAYS. i use this if i need to fine tune swales, spillways, sills, paddy's, other things that need level or contour.
 
Erin Phillips
Posts: 7
Location: Lander, Wyoming
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We are looking for an answer to this question as well. This is very difficult, and most likely a question for a master carpenter, or perhaps a Geometry professor? Any mathematical geniuses out there? Who can teach us angles: A squared + B squared = C squared, and how to physically adjust an A-Frame to fit these mathematical parameters.

We have built the A-frame with a plum bob, and it works GREAT!, if you don't live in windy Wyoming. Sure, one choice is to wait for a non-windy day, and another option is to wait until midnight, when the wind has died. Let's not stop there! We can get friends out who are willing to hold up plywood all day, against the prevailing wind, and the plum bob must surely be fine, but we are not on the plains, we are working in a valley at the base of the Wind River Range, and a funnel effect nails us from the occasionally, prevailing South-Eastern winds. Of course, this happens when our plywood holders, and the plum bob least expect it; right when we start laying out contours. Days now, we have tried to deal with this Wyoming wind. No contour success.

To be positive and realistic, we are very patient, and hard working, so we can continue on this path of one contour of 700 feet marked in a weekend. However, we are going to find a solution to making the spirit level, A-Frame combo happen, on account of the many acres we are going to swale. This is not only our property we are considering. Other interested parties in our area look forward to participating in these practices, and the winds of Wyoming will continue to slow our momentum.

No matter how this forum turns out, or how the many conversations with local professionals in our area go, we will be diligent in finding a solution to our problem, and will most definitely share our findings with the permies at large. We would like to keep in mind that it is ideal if this design is near free, and can be built with an individual that has a few basic carpenter's tools, and a little such experience. Does that not describe everything that we are trying to do here?

Thanks for any help!

Kyle and Erin
 
Bill McGee
Posts: 185
Location: Southeastern Connecticut, USA
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Zero experience with an A frame level. Just thinking out loud. Could the plumb bob be enclosed with sheets of plexiglass on both sides. Use a hole saw to cut wind vents (and/or an inverted T cut to the plexiglass)

Are there heavier weighted plum bobs? A heavier cable to be more wind resistant? Would a tripod or quadpod help to support it in the wind? Thats all I got
 
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