Tree Measuring Methods
  Nov 07, 2003 16:03 PST 


   Our discussions on tree measuring methods periodically make the rounds. I think we've circled back and are ready for a new round.

   In some ways the errors that other, otherwise qualified, people make is frustrating and in other ways, pretty funny. Understanding why so many forest-savvy people screw up so badly on tree measuring amounts to a study in human psychology. Lots of emotional ingredients: turf issues, personal pride, arrogance, foolishness, lack of mathematical maturity.


   In May 2000, a number of us met at Sweetbriar College in VA for an old-growth conference. As part of the post-lecture entertainment, Will climbed a southern red oak near a campus building. There was quite a group watching the event. While he was climbing, I scampered up a hill and shot the broad crown several times. It looked like the top was going to be about 93 feet. However, when Will got near the top he called out to me. I told him I couldn't see the branch he was measuring, so he wiggled it. I hadn't measured that branch. It was farther back in the crown and looked lower. So, I took three separate measurements of the branch and averaged the measurements. The height came out to exactly to 95 feet. I thought about it and started to add a decimal position (like Mt Everest's original 29,002-foot measurement, which had actually been an average of 5 that came out exactly to 29,000), thinking that 95 wouldn't sound accurate. Then I thought better of it - had to be honest and faithful to the cause. When I went down hill and told Will that I'd done my best and got 95 feet, he called out to Rick Van de Poll who was holding the tape to the ground to read the tape aloud. Rick called out 95 feet! I let out an ape call.

   There's more to the story - the humbling part. An old horse logger who was attending the conference had eye-balled the tree and told those standing close by that he estimated the tree at 95 feet. So Will's actual taped height, my laser-clinometer measurement, and the logger's eye-ball all agreed. So the logger's field experience proved its value. Will and I had plenty of measuring experience and the old logger had felled many trees and had developed a keen eye for height.

   I had hoped the lesson to all would be that when it comes to tree measuring, ENTS measurers are THE experts. End of story. Actually, we were happy to share the accolades with the horse logger. He reminded us of what a well trained eye can do without equipment.



RE: Coast Redwoods in the South & more Champion Tree Bull_ _ _
  Bruce Allen
  Nov 09, 2003 05:36 PST 


I hear your story, and I see a 2' height error (without the benefit of
Will's climbing effort) on an open grown hardwood with good site
lines. That does not sound substantially better a clinometer and tape.

Re: Coast Redwoods in the South & more Champion Tree Bull_ _ _ _   Colby Rucker
  Nov 09, 2003 09:50 PST 


Someday I hope to venture down from Maryland to the Congaree, meet you, and
see some of your super trees. In the meantime, let's look at Bob's
measuring adventure at Sweetbriar.

To simplify, let's suppose Bob was standing downhill so that the base of the
tree was exactly at eye level. That makes for a simple diagram with one
triangle. Let's suppose the 93' elevation was shot at 45 degrees, and was
on a "shoulder" of the broad crown, 20' out from the centerline of the

Now, Bob's elevation of the shoulder twig (93') was accurate. You or I
might sight by clinometer or stick to the same prominent point. If we took
care to find the spot directly under the twig (20' from the trunk), we could
pace the distance from the sighting point to the said spot and it would be
93'. Indeed, as you suggest, the laser, stick and 45 degree clinometer
methods could all be accurate in determining the height of the twig on the
shoulder of the tree.

Unfortunately, many practitioners of the stick method assume the spot they
see (a spot on the shoulder of the crown) is synonymous with the top of the
crown, and they pace a baseline right up to the trunk. With Bob's tree,
that would be an extra 20', for a total of 113', or 18' above Will's actual
top. I call this "false-top triangulation;" it's the triangle formed above
the crown by an extension of the vertical line above the base, and the
oblique line of sight; it's just air. If the measurer is using a
clinometer, the angle can be steeper than 45 degrees, and the amount of
false top increases rapidly. That's how that swamp chestnut oak in Alabama
got to 200 feet on the national list. A white oak near here was listed on
our state list as 143', but proved to be 104'; it had a broad crown, causing
false-top triangulation.

With a clinometer, most people use a fixed baseline. They measure out 100'
from the trunk, find a target, shoot up with the clinometer and read the
dial. It's sort of automatic, and they don't consider the true orientation
of the target twig where crown meets sky. Of course, if the tree leans,
things can get worse. Before I had a laser, I spent a day with a forester
measuring trees. We put a lot of faith in the clinometer; technology could
make no mistakes; just go 100 feet and read the dial. We both thought a big
tuliptree was 170 feet; it's actually 143'.

It's easy to go 100' and read the dial; probably no one is going 200 feet
and doubling the reading on the dial. Few use the half-stick method and
halve the baseline measurement. It's even less likely that anyone is
locating a window to the true top, shooting the angle, then measuring the
actual baseline and doing the trigonometry.

Anyway, there's a lot of very decent people out there who have complete
faith in their clinometers. Sometimes they get good readings, sometimes
not, but it's very hard for them to broaden their understanding. Sometimes
we get frustrated. All of us can spot the truly bad readings. Either the
height is far beyond known maximums for the species, or structurally
impractical. Trees with broad crowns just aren't as tall as forest grown

Now, if Bob thought the tree was 93', and another twig went to 95', is that
a 2' measurement error? It's not always possible to locate the highest
point. That's different than getting a bad measurement of a known point.
What's most significant is that he didn't exceed the actual height of the
tree. With laser-based hypotenuse measurements, you don't go over. You
don't get false-top triangulation, and you don't get 200' basket oaks.

Your input is most valuable, because you are in a position to consider all
sides of the situation. I'd enjoy hearing from you and discussing how we
can encourage others to understand the care needed in using the baseline
methods, and why a $199 laser is a good investment.

Best wishes,

A rap on the old knuckles
  Nov 09, 2003 15:17 PST 


   Colby's accompanying explanation is appropriately civil. It adds significantly to the credibility of these discussions on measurement methodology. I'm sure we both thank him for taking the time and taking a measured tone.

   Over the life of this list, we've drawn many diagrams identifying where and why clinometer only measurements lead to significant measurement errors - unless adequate precautions are taken. I just don't know what else we can do. Colby has identified huge measurement errors as have Will Blozan, Bob Van Pelt, Mike Davie, Howard Stoner, Jess Riddle, Jack Sobon, Dale Luthringer, and myself. Others including Lee Frelich, Paul Jost, and Tom Diggins may have also identified them. Incidentally, super scientist Dr. Lee Frelich uses the sin top-sin bottom method for measuring trees. He changed from the clinometer only method of measuring tree height for the same reasons that the rest of us did. It wasn't a case of rolling dice on which method to use. He chose the more advanced one.

   Bruce, it isn't that we are purposely trying to be elitists in ENTS by pushing hard for broader acceptance of more accurate measurement techniques. It is that we have put tons of time into perfecting our methods, which are demonstrably more accurate than the clinometer only methods for the reasons we state. Bob Jones's methods are NOT equal to ours unless he takes into full consideration where the top of the tree is relative to the base and succeeds in doing it accurately, so that he doesn't create a triangle with a false top. It's just the trigonometry that has to be applied to the tree form. Nothing personal. It's just business. We're not measuring vertical telephone poles in level parking lots where a clinometer only and a 100-foot baseline would be completely sufficient.

   With respect to my measurement in Sweetbriar College, I got exactly the same result (95 feet) as Will's measurement in the southern red oak once I knew which twig he was measuring. Could I have done better than achieving 100% in the accuracy of the measurement? The southern red had a broad crown with many potential highest points. The one at 95 feet was not highest apparent point via the angle it subtended because it was well back into the crown and didn't stick up as high as closer points. The point I first measured was a more obvious choice since it stuck up higher and it did measured to 93 feet. That isn't far off the point Will chose, so my first choice wasn't a bad one. The high point was definitely not over the base of the tree and not visible standing beneath the tree looking up through the foliage. Identifying where a plumb line from that point would intersect the ground would require cross- triangulation of the point. Will and I used that technique many times before the days of lasers and hypotenuse-based measurements. I'd be interested in watching clinometer only measurers tackle that broad-crowned tree. The odds are high that they would over-measure it by 10 feet or more.

   Bruce, if clinometer only methods worked as well as those we use, we wouldn't be spending all the money that we spend on lasers. What would be the point? Our purpose in going the extra yard is to bring accuracy into the picture for science-based reasons and also for historical documentation purposes. I can't say enough about the importance of the latter especially where important, but threatened, properties like Zoar Valley are concerned. Tom Diggins once wrote an eloquent defense of accuracy for reasons of establishing site significance.

   Perhaps Will and I shouldn't get frustrated when others make extraordinary claims on tree dimensions that we know are wrong and/or don't make sense of the species being measured (179 feet for a red maple in Michigan). We could discuss this point for a long time, but you are correct to call us to task when we get too testy. I'll gladly take a knuckle rap in the proper spirit when I allow lapses to occur in my demeanor.

   You are a valued member of this list and we certainly don't want to lose you. Hopefully, we can agree to disagree at times without either of us taking offense, If my posts on measuring methodology have given you cause for feeling offended, I apologize. That has certainly never been my purpose.

Re: A rap on the old knuckles   Lee E. Frelich
  Nov 10, 2003 07:43 PST 

Bob et al.:

The tangent method and all baseline to the trunk methods for tree height
measurement assume that the high point of the tree is directly above the
base of the trunk. If it is, it yields the correct height.   If it isn't
the formula and resulting triangulation are mathematically incorrect.

The problem is that for old growth forests and record sized trees this
assumption is rarely true. The sine method does not assume that the high
point is above the base of the trunk, so the triangulation there works
regardless of the shape of the tree's crown.

The tangent methods usually yields errors of 5-10 feet in old growth
forests, and to correct those errors by shooting the baseline distance to
the point on the ground directly beneath the high point of the tree is a
lot of work. I know since I measured a thousand trees that way before the
lasers were available.

It is much easier to use the sine method with a laser and commonly be
within 2 feet of the actual height. Then all you have to worry about is to
find the highest twig and a spot where you can see it.

Pushing the envelope   Robert Leverett
  Nov 14, 2003 08:28 PST 


    The flurry of e-mails discussing how we fine-tune our tree
measurements to take into consideration the limits of our instruments
might seem like overkill to those with only a casual interest in tree
dimensions, but our obsession is no more nor less than that of the
members of any group who seek to push the envelope for their occupation
or sport. Consider how far technical climbing has evolved to allow rock
faces to be routinely scaled that once appeared unclimable. We're
pushing the envelope and we know it, but we don't expect those with a
less intense interest in tree measuring to adopt our methods or feel
pressured into seeking ultra precision. There's ample room for lesser
intensity in keeping with the objectives of the champion tree programs -
which we believe are important.

    We will continue to make our expertise available to American
Forests, the state champion tree coordinators, and are quite pleased to
help one of our own - Will Fell develop a measuring guide. More
measuring workshops will be scheduled. However, through it all, we
promise to remain sensitive to those gentle spirits who just want to
admire trees for their beauty.

    As we gain in reputation and accomplishment, our pugnacity will
diminish. We needed to stand our ground early on came to offset the lack
of understanding on the part of others as to what ENTS was all about and
to draw attention to entrenched tree-measuring methods in need of
change. As Russ Richardson stated, we're taking tree measuring to a
higher level, albeit to nowhere near the level that Bob Van Pelt takes
it to in his research. I do sense real progress on both fronts, so the
need to be openly combative/iconoclastic is subsiding. However, we will
still encounter reactionaries on occasion and have to administer a good
verbal spanking. We'll do it with increased sensitivity, though. It'll
be a nice spanking.

    Our site documentation methodology is taking on a life of its own
thanks to the inner group. However, we now need to expand the Rucker
Index concept to encompass both height and circumference indices. We
could even add a third index devoted to circumference x height. The
three indices, taken together, would reveal much more about a site.
Given the large girth cottonwoods out Lee Frelich's way, it makes sense
to add both the Rucker Circumference Index and the Rucker Point Index to
paint a more complete picture of the distinctions between eastern and
mid-western forests of the same species. Differences in forest structure
that can be reflected in the indices that are attributable to the
frequency and intensity of storms and other climate features should be

Well, enough rambling. Someone else's turn.