DBH (diameter at breast height measurement

[Paul Barton]

I've got one moob that's more saggy than the other so am never quite sure.

 

 

Sent from my iPhone using Arbtalk

[Arctostaphylos uva-ursi]

I've got one moob that's more saggy than the other so am never quite sure.

 

 

Sent from my iPhone using Arbtalk

 

Ahhh....but does your milkshake bring all the boys to the yard?

[daltontrees]

 

Forestry on the whole deals with circular trees normally without too many branches so their protocols (which may use a "diameter" tape which actually measures girth and the UK tape rounds down) don't typically cover the more complex tree shapes found in arboriculture; arboriculture has not developed a full protocol but BS5837:2012 has Annex C which covers some of the more common problems encountered but doesn't cover eccentricity: I measured an avenue of oaks the other day - possibly about 5 years since establishment as heavy standards. I was amazed at how oval the stems were - something that a diameter tape would not pick up but calipers did.

 

 

I did a rather complicated exercise last year to quantify the effects of eccentricity on cross sectional area relative to the area rderived froma circumference that assumes a circular cross section. The result was surprising in that the eccentricity needs to be quite pronounced before the difference is significant.

[10 Bears]

The result was surprising in that the eccentricity needs to be quite pronounced before the difference is significant.

 

You may find this publication of interest then Jules, "Comparison of Three Tools for Measuring Tree Diameter in Stands of Different Age and Tree Size"

 

In short, discrepancies between calipers and dbh tapes are minimal, where dbh tapes run over a 'gap', for instance over a stem depression or flute, then the effect is minimal and essentially averaged out, and as you say, eccentricity is not as significant a factor as you would imagine.

 

As for saggy moobs or measuring diameter at breast height at your throat, I always tell surveyors to measure from the ground where 1.3m is on their body (and 1.5m for BS5837 surveys) and to just put the tape in that equivalent area on the tree. 1.5m is at my throat, and I always thought that measuring breast height there meant that I must be incredibly short by comparison!

 

That FC document posted earlier states all the common conventions which are absolutely correct and should be followed rigorously... but the only item it misses out is how to measure a multi-stemmed tree that bifurcates close to the ground. One method is to measure the entire stool at a 'representative' point and record the diameter and height taken from the ground (and estimate the number of stems if it is a hazel coppice stool for example). Another method for the same situation is to estimate the number of stems eg 25 and estimate an average diameter and record these details. Personally, I don't like this method as it relies upon too much guesswork.

 

A mathematical solution to the low multi-stemmed measurement problem is to sum the squared dbh of the stems like this:

 

Multi-stem dbh = (dbh1^2 + dbh2^2 + ...+dbhn^2)^0.5

 

Example: 3 stem with dbh's of 10, 12, 14

 

((10^2+12^2+14^2)^0.5) = 20.9

 

Hope the info is of use...

[Arctostaphylos uva-ursi]

For multiple stems I measure the diameter of each stem work out the total basal area and then work out the equivalent diameter for that basal area.

[daltontrees]

You may find this publication of interest then Jules, "Comparison of Three Tools for Measuring Tree Diameter in Stands of Different Age and Tree Size"

 

In short, discrepancies between calipers and dbh tapes are minimal, where dbh tapes run over a 'gap', for instance over a stem depression or flute, then the effect is minimal and essentially averaged out, and as you say, eccentricity is not as significant a factor as you would imagine.

 

 

A strangely interesting article, that. My own effort dealt with 'eccentricity' in the pure mathematical sense, the relationship between the long and short axes of an ellipse. But the article uses the word 'eccentric' to mean all ins and outs, which is a different problem. That form of eccentricity will always result in an overestimate of the cross sectional area of a stem.

 

Must get a life...

[10 Bears]

 

Must get a life...

 

Well, you and me both it seems...

 

Sent via the arbtalk app, hence the brevity...

[Wendelspanswick]

Can someone explain DBH (diameter at breast height) measurement to me. or is it as simple as it sounds?

 

Is this to determine whether you need a felling licence?

[Lazurus]

Is this to determine whether you need a felling licence?

 

No, I had seen it mentioned several times in various posts and was just being nosey, there are some many acronyms in the world. LOL, TTFN

[Jon Heuch]

In short, discrepancies between calipers and dbh tapes are minimal, where dbh tapes run over a 'gap', for instance over a stem depression or flute, then the effect is minimal and essentially averaged out, and as you say, eccentricity is not as significant a factor as you would imagine.

 

It does of course depend what you are trying to do; the paper you referred to states an 8% "maximum mean difference" (for diameter) between methods - if you are only interested in rough sizes and don't intend to estimate growth, compare treatments such variability may be of no concern. The trees I referred to had diameters (typically) of 10 cm one way and 12 cm at right angles to the first diameter. I diameter tape might have suggested the tree had a diameter of 11 cm. If you are trying to estimate growth over time or compare individual trees of this size this issue requires a bit of thought.

 

If you are interested in cross sectional area rather than diameter an 8% variation could translate into a 16% variation in CSA.