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Author Topic: Measuring DBH  (Read 5601 times)

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Offline Gilman

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Measuring DBH
« on: May 07, 2006, 01:04:44 PM »
When measuring a standing tree, is the DBH calculator have a figure for bark already worked in?  I'm measuring Circumference and then calculating diameter.  Do I need to subtract for bark?  Is there a rule of thumb for subtracting for bark?

Thanks in advance,

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Offline wmrussel

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Re: Measuring DBH
« Reply #1 on: May 07, 2006, 01:25:41 PM »
DBH is always outside bark diameter.  Volume tables account for this fact in their calculations. 

I would imagine that there are certain instances where diameter inside bark (dib) at DBH may be needed.  In that case, it (dib) should be specifically requested.

Hope this is the info you needed!

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Offline Frank_Pender

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Re: Measuring DBH
« Reply #2 on: May 07, 2006, 03:20:12 PM »
DDavid, inside the first page of the Scribners scale boo you will find a table to determine the volume in a 40' log, I believe. :-\ :P
Frank Pender

Offline Ron Scott

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Re: Measuring DBH
« Reply #3 on: May 07, 2006, 03:38:46 PM »
What are you using to measure the  circumference with? Use of a tree diameter tape will give you the tree's direct diameter measurement outside bark (dob).
~Ron

Offline Ron Wenrich

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Re: Measuring DBH
« Reply #4 on: May 07, 2006, 08:15:34 PM »
Volume tables are developed using what is called a Form Class.  This is the ratio of diameter outside bark at DBH and the diameter inside bark at 17', which is the end of the first 16' log. 

Two factors are measured.  One is the bark thickness, the other is taper.  Taper after the first log is dependent on tree diameter and the location of the log.  A third cut log will have a different taper than a second cut log.

You will end up with a number that is taken to a decimal point.  So, the volume gets bumped up a little for each increase in form class.  We have a form class built in to our tree volume calculator.   
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Offline woodmills1

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Re: Measuring DBH
« Reply #5 on: May 07, 2006, 10:24:12 PM »
since circumference is equal to Pi times diameter you can make a DBH tape easily.  Use any flexible material and make marks 3.14" apart and lable them 1  2 3  etc and they will directly convert to the diameter in inches.
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Offline nyforester

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Re: Measuring DBH
« Reply #6 on: May 07, 2006, 11:54:00 PM »
I like using a biltmore stick.
It measure DBH - outside bark,
height in 16 foot logs,
log volume from 8 foot to 20 foot logs,
and also has a ruler.
[img width=300 --Photos MUST be in the Forestry Forum gallery!!!!!--.com/albums/y166/nyforester/114215.jpg[/img]
It much faster than tapes or calipers.
I do use tapes on trees over 30 inches.
I have found that the sticks error increases with size.

Offline Ron Wenrich

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Re: Measuring DBH
« Reply #7 on: May 08, 2006, 05:48:01 AM »
I remember one of the first jobs that I did was to go out with a consultant to see why his figures were so far off from our mill figures.  In order to make up for the difference, he was going to mark more trees. 

He was using a Biltmore stick.  He would call out his measurements to an assistant, who would record them.  I went and checked some of those measurements with my diameter tape.  He was consistantly off by 1 or 2 diameter classes.  He also measured midslope instead of topslope on a hillside.  We never bought from him again.

Biltmores may be quicker, but the accuracy is very dependent on the user.  It is wise to check yourself periodically to make sure you are still doing accurate work.
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Offline Gilman

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Re: Measuring DBH
« Reply #8 on: May 08, 2006, 11:07:30 AM »
Thanks for all the great replies, now I have more questions.

Quote
1 or 2 diameter classes

1) What's a diameter class?

2) What's form class?

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Offline Ron Wenrich

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Re: Measuring DBH
« Reply #9 on: May 08, 2006, 04:44:06 PM »
Diameter class is just a group of trees that are relatively the same diameter.  Here on the East coast, we generally use 2" diameter classes.  12", 14", 16", etc.  Sawtimber classifcation generally starts at 12".  The West coast used to use 4" classes when I worked there. 

For the 18" dbh class, all trees from 17.01" to 19.00" are considered in the 18" class. 

Form class I covered above.  Its the ratio of diameter inside bark at 17' / dbh *100.  Its usually expressed as a whole number and it gives the amount of taper.  In the northeast, most species use either a 78 or an 80.
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Offline SwampDonkey

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Re: Measuring DBH
« Reply #10 on: May 08, 2006, 06:06:07 PM »
Also, on the west coast when measuring DBH, make sure you scrape the moss and loose bark from the tree at the dbh height. It can get pretty thick in some locations.
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Offline Phorester

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Re: Measuring DBH
« Reply #11 on: May 08, 2006, 07:45:04 PM »

A Biltmore stick or a tree scale stick is the least accurate device to measure tree diameters and height.  Accuracy using one can be developed, but it takes a lot of practice and use, comparing it to more accurate devices, to get accurate with one.
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Offline wmrussel

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Re: Measuring DBH
« Reply #12 on: May 08, 2006, 09:33:27 PM »
Not trying to promote a company, but I know where you can order diameter tapes & loggers tapes with diameter tapes on the back side.  Construction safety products is who I use.  I go through roughly 5 or 6 d-tapes a year and a couple of loggers tapes.  Of course, I also cruise several hundred or thousand acres (depending on the month).  So I'm measuring thousands of trees a month.  If you want their info, drop me a line. 

My name is William, but people call me Pete.  Long story......


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