Reply to post by Russ Carlson, on May 15, 2000 at 22:59:22:
Russ wrote: "We're back to accuracy, and to expertise."
I think required accuracy is the starting point and expertise follows. As always, we have to start with one of Russ's favorite things... the definition of the assignment. Why are we doing this?
If we want to know if there's absolute and permanent movement the plumb bob and a carpenter's expertise seem fine. I agree, within an inch or maybe a quarter inch (say at 4' above grade with plumb bob hung 3' above that) will tell us there's movement and is within the sensitivity of the instrumentation and the operator.
If on the other hand you run out your angle calculations to 5 decimal places as if the accuracy matters and as if the amount of movement over tree height and the changing center of gravity is something you need to know then maybe you're setting yourself up for needing more expertise.
Maybe it's not rocket science, but if you bother to do the math at all rather than relying on visual observation you're creeping into a context where maybe accuracy does matter. And talking about your engineering ability to predict or even suggest the stability of a mechanical system is a different animal than measuring DBH for an appraisal or height for a record book.
I think it's a little paradoxical to say "my math is cool and it's cooler than visual observation but the accuracy of it doesn't matter." It begs the question, "what level of accuracy does matter and why?"
I'm not sure I agree with your wind solution. A 6 oz. plumb bob on a 3' string will swing a lot in a very light breeze.... let's say 1.0" off vertical. I doubt a 45" DBH tree with a 7'6" diameter flare at grade will move perceptibly at 4' above grade in the same breeze.... so that theoretical variable is not really a problem. But what are you averaging? The maximum plumb bob offset from Point B each time you visit the tree? Or each time the wind gusts? Or the average offset of six different wind gusts each time you monitor? It's all meaningless if you don't know the wind speed, the plumb bob weight and cross sectional area, length of arc, atiabatic lapse rate and price of tea in China.
It's a PITA, but based on a week or so of trying out this system you a) read a book while waiting around and for a break in the wind to take your observation or b) block the wind with your body (or I suppose some other barrier). If accuracy is not a big deal it's OK, though you don't get a straight on view of the plumb bob and Point B.
The point I think Russ, is that the plumb bob system IS a useful tool for the kit. But it's because of it's simple efficiency NOT because of the mathematical exercises that might be employed with it.
Reply to post by Scott, on May 15, 2000 at 22:59:22:
Nice one guys. Have I been missing something on this one. Isn't this where I came in on the 24th January nearly four months ago in posting 425? Oh well! they say that everything that goes around comes around, now where is my plumbob, this has gone round so much that I am giddy and may no longer be upright!!!!
It has been good reading and I have enjoyed watching. As Adam Tom would say "Good on yer".
Reply to post by Scott, on May 15, 2000 at 22:59:22:
Scott wrote "The point I think Russ, is that the plumb bob system IS a useful tool for the kit. But it's because of it's simple efficiency NOT because of the mathematical exercises that might be employed with it."
Scott finally understands what I've been trying to say all along. [g]
The system is useful because the math works. But the same is true if you're using a laser transit or any other device. It is simply the basis for the system. Without the trigonometery, we'd never be able to determine change in lean. But that's not what makes the system so elegant.
Reply to post by Colin Bashford, on May 16, 2000 at 07:21:43:
Good on yer, too, Colin.
I hope other readers have enjoyed this long and twisted thread as much as you and I have.
How's life 'cross the pond these days?
Reply to post by Russ Carlson, on May 16, 2000 at 07:21:43:
Sorry to be the "equipment geek" in the mix, but I dont find the plumb bob method as useful as I do interesting. By the way, few machines measure distance with laser, most use modulated infared light which won't harm your eyes. The basic principal employed in its use is triagulation, and the foundation of triangulation is an objective, uneffected standard against which to compare. This is the basis for everything from simple mapping on a level plain to ballistics. The only objective standard in the plumb bob system is gravity. Although the string appears to be an objective standard, it is only so on account of gravity holding it on the zenith azimuth. Where the system begins to fall apart as a geometric tool is that the index of the objective standard (the string) is moving with the object being measured.
The whole point of coordinate geometry is to establish standards outside the influence of the moving object against which the object is measured. If the string is attached to the tree and the tree is moving, the principal of an objective standard is violated and no longer valid. If you were to draw a map by using triangulation but moved the location of your baseline points from time to time the resulting map would not reflect accuracy. Of course you could calculate the magnitude of each alteration in the baseline (the objective standard in triangulation mapping) but why do the extra work and risk error. Employ the machine designed for the job, that represents the accepted standard, and use additional professional help if it is needed.
We are quick to point out that the complexities of hazard tree analysis are beyond the average musician because his or her training, education and experiance are in a different field. We would caution the musician against drawing conclusions about a hazard tree without employing someone with the appropriate training, experinace, and expertese. Should we not adhere to our own standard and recognize when we are beyond our own abilities and hire the surveyor, or acquire the required skills to conduct the assessment at the "measurement industries" standard? (I recognize of course that the assignment determines if any of this is of value to the client) I think that although we may know much about hazard diagnosis and analysis, we dont know much about standardized ballistics and that is in fact what we are discussing, the measurable,directional movement of a solid object. There are standards and techniques for measuring the movement and the direction and we will get laid low if we make up our own.
Reply to post by Russ Carlson, on May 16, 2000 at 07:21:43:
Russ wrote "Scott finally understands what I've been trying to say all along. [g] The system is useful because the math works."
What I really said Russ, which you quoted, was "the plumb bob system IS a useful tool for the kit. But it's because of it's simple efficiency NOT because of the mathematical exercises that might be employed with it."
It's simple and easy to install and monitor (if there's no wind) to see if there's absolute movement. Using your ruler (remember, first grade as you said) and recording readings you can tell if there's been movement between readings. Yes that math's OK, and it's first grade math to go along with the ruler. The
trig is another story. Yes it works on paper (if you know how to use it). Yes it might be accurate enough for some situations. But that's not the main advanatge of the plumb bob. The main advantage is you can use it without knowing the trig or relying on it and without obtaining or learning how to use the transit.
I have to weigh in with Wayne (at #522). If the angle REALLY matters and you have to REALLY rely on it you really want to rely on expertise and instrumentation that are bulletproof.
Reply to post by Scott, on May 16, 2000 at 22:39:14:
We don't disagree on the simplicity vs math issue. But the two cannot be separated. It is useful because of simple efficiency, and it is simply efficient because of the math.
You can hang a bob twice, get different measurements and know the tree moved. No math, no-brainer. You can hang it twice, and measure the difference, and calculate the change in angle of lean, using the basic trig stuff.
You can also go out with the optical equipment, take you measurements, and do the same thing. You hustle back to the office, plug your data into the computer and hit enter. It whizzes, whirrs and grinds, and spits out some numbers that tell you the tree moved, and by how much. Do you know what it did? The same thing I explained in the trig, more or less. The basic concept is the same. It may have arrived via a different path, taking measurements based on known geographic points rather than the vertical alignment, but it is still the basic trig that it comes down to.
The optical approach is simple because it does the work for you. You set it up, you read the numbers, you get the result. When you cut to the basics, it is the same for both systems. The difference is the degreee of accuracy, and I still maintain that in the vast majority of cases an arborist comes across, a plumb bob method will more than suffice, and provide an accuracy sufficient to the assignment. And if you can't explain exactly what the optical equipment measured, and how it arrived at the result, you are no better off.
As to credentials, anyone can testify to common knowledge, and since reading a tape measure is taught in elementary school, and basic trig is just high school math, I don't think a court will throw you out on those grounds. It goes back to the level of confidence necessary to the assignment, and the accuracy needed.
Reply to post by Wayne, on May 16, 2000 at 22:39:14:
Thanks for the explanation of the workings of the optical equipment. I haven't had any direct involvement with the modern equipment in many years (I guess that shows!)
I remain unconvinced about optical vs the plumb line, though. The arguements you present are entirely valid- IF we are measuring the lateral or directional movement of the tree related to fixed geographic points. However, the original question, and the premise I have based all my discussions on, is that we are trying to determine if the tree has changed its vertical alignment, if it is leaning more than before, and by how much. For this, we do not need any more of an objective baseline than a vertical axis. Yes, the plumb line is attached to the tree and moves with it. If the tree leans one foot at the point where the plumb is attached, and the bob still hangs free, is that not still vertical? The direction of gravity has not shifted (well, I suppose it migh be different over a 12 inch distance, if you have the equipment to measure it). In fact, it may be more accurate unless your calculations take into account earth's curvature over the distance of tree movement (now we're getting REALLY trivial!). I think most courts will accept that gravity is measurably and reasonably constant over the small distances and short time frames we are considering here.
Doctors have MRIs, CATs, and all sorts of equipment at their disposal. So what's the first thing they do when you go in? Stick a piece of wood down your throat and tell you to say "Ahhh". [g]
Reply to post by Russ Carlson, on May 17, 2000 at 15:01:42:
We agree. As you said, "It goes back to the level of confidence necessary to the assignment, and the accuracy needed."
Actually it goes back even farther to the issue you raised earlier. We don't know ho much accuracy is needed or how much movement is acceptable or is a threshold for action.
I suspect this is much more a "relative scaling of risks" or liabilities of the sort that the ISA Hazard Rating Form provides than it is a science of precise measurement. On the low end the plumb bob - with or without trig - may suffice and provide "common sense" guidance. On the high end - with a high value or high vulnerability target and a large tree or tree part - we may want to be very conservative and careful in estimating the third variable, "risk of failure" and in recommending action or inaction.
I don't know that "...in the vast majority of cases an arborist comes across, a plumb bob method will more than suffice..." If you monitor movement on every tree in the inventory the denominator is big and the expected failure numerator is small, so your suggestion is true. If you determine a need to monitor movement based on a sreening for risk the denominator is much smaller. Maybe you only monitor movement in the very highest risk cases and you want to be very careful in and discuss with the client the selection of methodology and expertise and how they influence liability.
Reply to post by Russ Carlson, on May 17, 2000 at 09:01:11:
Russ wrote "I think most courts will accept that gravity is measurably and reasonably constant over the small distances and short time frames we are considering here."
I agree. And the court will probably accept that if the plumb bob moved out of vertical alignment the tree moved.
The real issure here is liability and responsibility. Why are you monitoring movement and why are you in court?
CASE A. Big old historic tree is in a fenced off field surrounded by a migratory bird nesting ground. You and the manager think the tree is leaning over, you confrim it with the plumb bob, remove the tree carefully before it falls and wipes out half the world's nesting population of an endangerd bird species. Outraged citizen sues saying the tree should not have been removed, it was an historic landmark. "Mr. Carlson why did you act now? The plumb bob system indicated increasing lean. This tree was in the process of failing. Thank you sir." Jury throws the case out.
CASE B. Very big tree next to a busy roadway with a playground on the other side. No obvious decay or above ground hazard. No indication of need for root crown inspection. Someone suggests it looks like it's leaning though. You put a plumb bob system on it and monitor twice a week. You calculate a 0.15 degree lean over the months of May and June and suggest it's becaue of seasonal foliage weight, not to worry. Next day it falls across the road killing a woman and her two children in a passing car. Three kids on the playground are under psychiatric observation they were so scared.
Absent the instrumentation and estimate of lean this sounds like a non predictable failure. Tragic but not negligent. BUT you recommended no action on the basis of a judgment relying on instrumentation and estimate of lean.
"Mr. Carlson, are you a licensed surveyor. No sir. Mr. Carlson, you estimated the lean. Yes sir. Mr. Carlson could the lean have been 0.25 instead of 0.15? Well yes I suppose, I don't think a licensed surveyor could have been more accurate. Well Mr. Carlson if as you've testified you're not a licensed surveyor how do you know what level of accuracy a licensed surveyor might have had. Well sir I guess I don't know. Mr. Carlson, what change in lean over what time period would have changed your recommendation? Well I don't exactly know. And what was the change in the failure moment of this tree based on your estimate of lean and would the tree have experienced a change in effective load based on the movement you monitored or might have monitored with more accuracy? Well...."
If you had relied only on your arboricultural judgement you might have remained on firm ground. But you introduced the instrumentation as if it mattered. It may have only been one piece, a small piece, of your analysis but you introduced it and used it without legal competence and you have no published guidelines from competent sources to fall back on. It might not have changed your arboricultural judgment at all, but the jury looked from that cross examination across to the grieving husband and surviving children at the same time. Are you squirming? Is your client exposed to more liability?
Reply to post by Russ Carlson, on May 17, 2000 at 09:01:11:
Ah! gravity is one element in establishing a baseline, it holds the plumb bob on the zenith azimuth. Earlier in the thread there was discussion about using a second point on the tree for measuring deflection and the suggestion was made to use a stake in the ground with a surveyors tack in the top also. Russ, I think it was you that suggested the stake initially. The stake represents the second element in establishing a baseline. Some valid information can be gained and and described trigonometrically is there is a solid baseline. As long as the plumb bob is measured against an unaltered point basic trig will work. When you attach the second measuring point to the tree so that it is moving in a rotational manner with less angular volicity then the upper screw, and your tape is measuring a lateral deflection, you now need calculus to calculate the derivitives. Not so simple any more.
If staying with highschool trig and common measurements are the objective of the "keep it simple" method, then the stake or point below the plumb bob is far superior to the two points on the tree version. Essentially the same principal I noted before applies, the calculation must be done against an objective standard, or you run the risk of being outside of "common knowledge" mathamatics.
By the way, there is a move afoot in NY to make the following state law: "Practice of Surveying or Land Surveying within the intent of this act shall mean providing professional services such as consultation, investigation, testimony evaluation, expert technical testimony, planning, mapping, assembling and interpreting reliagle scientific measurements and information relative to the location, size, shape, or physical features of the earth, improvements on the earth, the space above the earth, or any part of the earth, and utilization and development of these facts and interpretations into an orderly survey map, plan, report, description, or project..." Unless I misread this, my method and Russ's method both become the domain of the surveyor, as does making a site map for our reports. They havent out much besides using "MapQuest".
Reply to post by Scott, on May 18, 2000 at 00:29:37:
Hi everyone! I wrote the first post on this subject, and it has been interesting to see what ideas popped up. I never thought it would go on this long!
I wish to pose a question regarding liability in using a measurement of any kind to determine "progressive lean". As an employee of the state, not identified as a professional consultant, but as a "tree trimmer", I give my input to supervisor (not a tree expert). Believe it or not, I was once told that trees falling are considered an "act of god", and that making notice of any awareness of tree fault was an admission of liability. We have progressed beyond that to a situation where action is taken to reduce much of the obvious potential of falling tree parts. My question is this: If we undertake any measurements, with the hope of learning something about predictive use, and later the tree fails with damage or injury, is the simple act of measuring (and implication of our suspecting a potential failure) something that makes any of us more liable, even if no formal recommendation was made from data collected? It is hard for me to accept that in the absence of "more qualified" professional resources applied to the quest, that it is better to not do anything except wait for gross and obvious signs of immanent failure. I feel that any attempt to study a suspected problem should be taken as a good faith effort towards public safety. If no action is taken, it may simply mean that our observations are in the nature of ongoing study, with no established predictive value. Then, if we detect an alarming change, and a recommendation is made to eliminate the hazard, we would be on the same ground as we were without having measured anything.
Anyway, the real tree in question was pruned by a contractor with ISA credentials, and much weight was reduced that applied towards the direction of lean. Getting back to the question of interpreting observations-- If a baseline measurement and permanent reference points are established, who is qualified to interpret any observed changes, and based on what? Ground level changes might be just as predictive in some situations, I imagine. In the absence of trunk decay or other obvious signs of loss of footing, what information is available to compare to our measurements of progressed lean? Liability-wise, where is the threshold for action?
Reply to post by Wayne, on May 18, 2000 at 00:29:37:
I think you have it backwards, Wayne. When the points of comparison are both on the same tree, the basic trig is sufficient. You are simply rotating a line segment (the line between the two points) around a pivot. Since we do not need to know the absolute position of either point on a time continuum, we do not need the calculus. We only want to know the change in angle of lean between two descrete points in time, compared to the vertical. We don't care how the tree moved in relation to any point on earth, only that it is leaning more. This implies that some part of the structure has changed- uprooting, bending, etc. It tells us there is a risk that is increasing.
If the measurement is made with a stake in the ground, as I did mention many, Many, MANY weeks ago, then it only works if you can safely make the assumption that the stake and the tree did not move in relation to each other. That's why is suggested the second point should be on the tree instead. We KNOW that the two points will not move unless the tree has structural failure at a point between.
Reply to post by Russ Carlson, on May 18, 2000 at 09:27:17:
I think it's quite telling that this has gone on MANY MANY MANY weeks. It ain't that simple as much as any of us might try to make it so.
You're both right, sort of, on this issue (I was going to say point but then you'd ask if it's point A or B).
IF maximum accuracy is desired Wayne's description of fixed and known ground points is probably superior. These would have to be distant enough from the tree not to move with the root flare however, so the plumb bob does not work with them, you're back to transit.
IF accuracy is not at issue and you just want to look at movement, points on the tree might be used. Russ is right the distance between points along the trunk is constant (unless there is physical damage to that trunk), BUT the relationship of the two points to vertical is NOT constant in all respects (I think Wayne said the have different "angular velocities." The angle should be constant but the amount of lateral movement will vary since BOTH points are rotating.
YOU may only want the angle of lean (don't speak for WE please) and IF you want to do the trig you can, but you have to understand the limitations on accuracy and when that is or is not an issue.
IF one wants only to know IF there's movement the trig is not needed.
IF one wants to know the actual amout of lateral movement at either point then the ruler does not work nor does the trig. You need calculus (according to Mr. Cahilly, it's beyond my math) or you need to work it out graphically.
I don't know what exactly WE do with either a calculated angle or a calculated amount of actual lateral movement at a point. In my mind it tells me no more than simply knowing that "it moved from day one to day 5 and it moved again from day 5 to day 10."
I don't know that, as Russ suggests, "It tells us there is a risk that is increasing." If it's uprooting I guess that's true. How do we know? Soil cracks? Noticeably uplifted root plate. Root plate deflection with a sensitve instrument like Wessolly's Inclinomter (if any of us could get our hands on it)? If it's tree failure, butt rot or stem failure, I guess that's true. But what if it's seasonal deflection with foliage weight? Or bending over time as the tree matures and compensates for it internally? Do we know what is normal, or what is risky or what is more risky? Does an angle measurement tell us what the cause of lean is?
The real risk is in making calculations as if the calculated number is a meaningful answer or guide. We can fool ourselves and we can set up false and unreasonable expectations among the people who rely on us and the people who hold us accountable. This is particularly true when we have no competent, controlled studies of what the calculations might mean to refer to.
Reply to post by Mark Goodwin, on May 18, 2000 at 07:39:33:
This is a real and important issue Mark, and it goes beyond this particular exercise with measuring lean.
There's a lot here that's legal and each organization needs to get competent advice in it's jurisdiction, but here's my read.
I hear this idea a lot... "it's better if we don't know because if we're not on notice there's no duty to act if there's no duty there's no breach and no negligence and it will cost us less." In some very limited respects (later) that might be true. But I see two problems with it. First is our moral resonsibility. People get hurt or killed, who cares about the money? Second, that may be a very transparent defense. There was an important 1995 decision from the CT Supreme Court that is most cited for eliminating the rural-urban distinction in responsibility. But also important in that case was that the plaintiff had created it's own duty to discover hazardous cponditions by having a stated policy that it would maintain the highways in a safe condition, specifically noting trees in the equation. If you have a policy you can't ignore it.
So does that mean don't have a policy? Some lawyers and managers seem to think so. I think that's transparent too. If you have skilled and competent staff there may well be a presumption that you know what you're looking at and if someting is obvious and predictable you should do someting about it policy or not.
There has to be a reasonable limit on what you discover, how often you try. Budgets are not unlimited. But from everything I've read you need a rational program with objective criteria and implement it a some reasonable fashion.
Now here's the other side of the coin. You have to avoid creating duties there's no way you can fill. You can't guarantee safety. You can't do everthing. Don't suggest that what the policy means.
The bigger issue for me is the "professional" standard of care we create for ourselves. I think we have gone overboard in some respects in our own idea of what we can and cannot reliably predict. I'm told ISA got very close to being a party in a case involving a fatality because of a presumption that each and every certifed arborist is by definition a "hazard tree expert" and has a duty to instantly recognize and deal with any tree that might qualify under that definition, whether you present youself that way or not.
This is the tie in to this "measuring lean" thread. We have to be very cautious - as an industry not just individuals - in establishing phantom standards the create duties that can't be met. Who knows what that angle means?
So, I guess I mean we can't responsibly do nothing, but there are real limits on what we can do and what we know and we should not go out of our way to or inadvertantly create duties in those uncharted areas.
Way way back there was a thread I think Tom Dunlap started on this question, maybe that's worth a look too. I certainly hope others will weigh in on this question.
Reply to post by Scott, on May 18, 2000 at 23:07:47:
Now I'm confused.
What are you trying to measure with all this?
Did the tree move?
What state is it in?
Define the assignment first. Do you want to know if the tree is leaning, or where it is in relation to fixed geographic points. One or the other- you can't do both with the same measurements. We started out talking about the lean (vertical angle) of the tree, and somewhere surveying to geographic location creeped in. Angular momentum? Calculus? Irrelevant! We need discrete points in time. Calculus determines derivatives along continuous dimensions or changing base points. To measure the change in lean, we take two measurements (points in time), and we have a constant base point (vertical alignment, or as Wayne calls it zenith asimuth). No calculus needed.
What confuses me is the long argument that accuracy is so important, that a plumb line system yields suspect results, then you say you don't know what to do with it.
My opinion, from the start, has been simple: Apply basic, easily collected data, and apply simple trigonometry to yield a result that can be understood and that yields reasonably accurate information. It is still just a tool! We have walked all the way to the Supreme Court and back with this, posing all sorts of hypothetical what-ifs and scenarios for all kinds of exceptions. And it still comes back to one thing- What information does it provide to the arborist or decision maker?
Using the plumb line method I described will give you all the accuracy you need to determine if the tree is moving, and how much it has moved over a period of time. You can expand the accuracy with more precise measurement methods, and by extending your base line (AB line, in the diagram). No, you can't say what the tree will tolerate or how much/how little lean is "safe". we don't have the data on all of that. But we daily go out and apply Shigo's or Mattheck's theories without more proof than that. And this is a whole lot better than guessing.
Perhaps it's time to put this one to rest (Did I hear a collective sigh of relief?) I think the the best points to learn from all this discussion are that you must define your assignment first, and know what the job demands in terms of accuracy and credibility. This applies not just to plumb bobs and lean, but to any aspect of the job. Know your tools, and select the one most appropriate to the assignment.
Reply to post by Russ Carlson, on May 16, 2000 at 17:22:19:
Hi Russ, as you will note from this delay in replying life is good but hetic. Have spent last three weeks in Courts and have had at the same time been preparing proofs of evidence for cases to be heard this coming week.Yesterday got called out to a fatality (human), never nice but part of our instructions.
Looking forward to Baltimore and a rest!!
See you soon
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