Defining crown spread

General discussions of measurement techniques and the results of testing of techniques and equipment.

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dbhguru
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Defining crown spread

Post by dbhguru » Mon Oct 28, 2013 8:14 am

NTS,

Don Bertolette and I are starting to work on the crown-spread dimension for the AFs National Register. We have been thinking about the traditional method of measuring average crown spread. Using present AF guidelines, the measurer attempts to locate and measure the longest spread. This creates a long crown-spread axis. Then the measurer attempts to locate the narrowest spread at 90 degrees to the long axis. An average is taken of the two measurements to arrive at an average crown-spread. This method is problematic if the crown shape, looking down from above, is roughly circular or elliptical. Spreads at right angles to the long axis become parallel secant lines that grow narrower and narrower as the ends of the long axis are approached. The minimum spread is theoretically zero at the ends of the long diameter. Well, that obviously makes no sense.

If the rule were changed to take the longest crown-spread to establish the long axis and then the longest spread at right angles to the absolute longest, that method works for circular and elliptical crown shapes. It is debatable what the average means in the case of an elliptical shape, but it is a better statistic than the first method. But the second method isn't the AF rule.

The NTS method for determining average crosn spread is the basically spoke method. It is better than either of the preceding methods, but by how much better remains to be determined if time spent becomes an issue.

An important question is: What do we really mean by average crown spread? If we think of a tree as seen from a distance, the tree's silhouette provides us with a sense of the crown-spread. If we circle the tree observing the silhouetted form, some kind of average of these spreads of the silhouettes seems to make sense as an overall crown average as seen by for the onlooker. This latter point is important. If a measurement is taken for which the measurer has no visual cues in judging a champion, the purpose of the big tree program would seem to be questionable. The appearance of big becomes as important as a complicated measurement of big.

Can we measure the silhouetted form? Yes, but not simply, and the result is a different kind of measure from the spoke method. Then there is Google Earth. It has possibilities where photos are clear.

Well, as you can see, we have our challenges. For the advanced measurer, I expect that we will stay will the spoke method. We'll have some diagrams with questions in the coming weeks as we explore the concept of average crown spread.

Bob
Robert T. Leverett
Co-founder, Native Native Tree Society
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edfrank
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Re: Defining crown spread

Post by edfrank » Mon Oct 28, 2013 10:40 am

Bob, Don

To do the traditional crown spread measurement you want the longest axis through the crown - maximum spread and average it with the WIDEST section at ninety degrees from the long axis, not the narrowest. That misstatement has always bothered me.

Ed
"I love science and it pains me to think that so many are terrified of the subject or feel that choosing science means you cannot also choose compassion, or the arts, or be awe by nature. Science is not meant to cure us of mystery, but to reinvent and revigorate it." by Robert M. Sapolsky

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tsharp
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Re: Defining crown spread

Post by tsharp » Mon Oct 28, 2013 12:32 pm

Bob, Don:
I share Ed's comment. The AF guidleines to measure crown spread are stated thusly:

"Two measurements of the crown spread are taken and recorded, in feet, at right angles to one another.

1. Measure the widest crown spread, which is the greatest distance between any two points along the tree’s drip line. The drip line is the area defined by the outermost circumference of the tree’s canopy where water drips to the ground.
2. Turn the axis of measurement 90 degrees and find the narrow crown spread.

3. Calculate the average of the two crown spread measurements using this formula: (wide spread + narrow spread)/2 = average crown spread"

As stated these instructions are contradictory and do not make sense. As stated it seems to me it is assumed the second measurement taken at right angles is the narrowest crown spread.
For the AF big tree program why not keep it simple and go with one measurement - maximum crown spread.
TS

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Will Blozan
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Re: Defining crown spread

Post by Will Blozan » Mon Oct 28, 2013 8:16 pm

Bob, Don, AF

Stop this nonsense. We measure maximum diameter (at given height), we measure maximum height, so why the hell not measure maximum spread? What, exactly, is the point to an average???

Will

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edfrank
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Re: Defining crown spread

Post by edfrank » Mon Oct 28, 2013 8:29 pm

Will,

Trees have only one height, so there can't be an average height, maximum height is what there is. I would say that tree trunks are generally close to round so a girth is a fair proxy for diameter at eh height it is measured. Crown spread is basically a proxy for crown area. Crowns can be very irregular or elongate in form. Average crown spread is a shortcut or proxy to represent crown area with a minimum of measurements. It isn't all that great, but it is much better than implying a crown area based solely on maximum crown spread, which would always over estimate the crown area, and often would drastically overestimate the crown area.

Ed
"I love science and it pains me to think that so many are terrified of the subject or feel that choosing science means you cannot also choose compassion, or the arts, or be awe by nature. Science is not meant to cure us of mystery, but to reinvent and revigorate it." by Robert M. Sapolsky

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Don
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Re: Defining crown spread

Post by Don » Tue Oct 29, 2013 1:46 am

You guys are on to it...Bob and I have had discussions along similar lines, and will be advocating, I think for the Spoke Method, with preliminary thoughts to varying the number of spokes to reflect the added complexity brought by crown shape eccentricity/irregularity, for Levels 1-3 (up to and including State Coordinators).
For the National Expert Level, it's my inclination to introduce ongoing remote sensing work (like that performed by Michael, Zane, BVP and others, as well as Google Earth (and other satellite/airborne platforms) where coverage permits (surprisingly rich in larger cities, parks with high resolution imagery). It's not hard to imagine a Quadcopter navigating the crown edge, gathering GPS data...: > }
-Don
Don Bertolette - President/Moderator, WNTS BBS
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Will Blozan
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Re: Defining crown spread

Post by Will Blozan » Tue Oct 29, 2013 7:10 am

Guys,

It was never my impression that crown spread was a surrogate for crown area (it says "crown spread" not area). So it appears I have misunderstood. In that case, the spoke method is easy and a better idea of area and has no ridiculous 90 degree component that no can do without further equipment. I have always paid little mind to crown spread since in the AF formula it is degraded to being hardly meaningful to gather accurately.

Still, if we are gathering max data on superlative trees then why not measure the longest spread only? Ed, a tree does have multiple heights and a shortest and longest height.

Will

Joe

Re: Defining crown spread

Post by Joe » Tue Oct 29, 2013 8:13 am

Just a curious aside from someone not in this debate- but how effective might aerial photography be for a measurement of crown area? If it's high quality photography, you could draw the exact shape of the crown, then measure that. I presume, though, that most aerial photography just isn't good enough for this purpose, but if it was.... would this method solve the problem?
Joe

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Larry Tucei
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Re: Defining crown spread

Post by Larry Tucei » Tue Oct 29, 2013 3:56 pm

NTS- Why not measure crown spead at Maximum spread on one plane then turn 90 degrees and again measure Maximum spread. This to me makes more sense than AF's measure maximum on one plane and minumum on the other plane for an average spread. To measure the true crown spread of a tree, both Maximums should be measured. If you want the average crown spread then the spoke method should be used at 3 or 4 planes. So should we measure Maximum Crown Spread or Avg. Crown Spread. Like Will pointe we are not measuring the Crown Area. I do both on my Live Oak Listing. Just my thoughts.. Larry

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Matt Markworth
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Re: Defining crown spread

Post by Matt Markworth » Tue Oct 29, 2013 8:17 pm

Hi All,

When reading the 3 bullet points regarding Average Crown Spread on the AF site, the way I'm reading it is that "Narrow Crown Spread" is simply what they're calling the spread that exists at a right angle to the "Widest Crown Spread." My impression is that the word narrow doesn't imply that the narrowest spread should be measured. So, for example, if there are two notches in the tree's drip line that just happen to be across from each other on the tree's "Narrow Crown Spread" axis, those notches would not be used to determine the spread on that axis. Based on the way the guidelines are currently written, I think it would be appropriate to average the "Widest Crown Spread" with the maximum spread on the "Narrow Crown Spread" axis.

The way I'm viewing the two diagrams (http://www.americanforests.org/bigtrees ... s-3111-0-2) on the site are that they just show a general concept of what they mean by "Widest Crown Spread" and "Narrow Crown Spread." The diagrams are simply the same image with the "Widest Crown Spread" image being stretched out for effect, so I'm not certain how much can be inferred from them. However, the arrows for "Narrow Crown Spread" show what appears to be the maximum spread on that axis.

- Matt

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