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Author Topic: Prisms  (Read 33582 times)

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Offline SwampDonkey

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Re: Prisms
« Reply #60 on: August 24, 2008, 04:17:58 PM »
sunflower4dayz , hope you found your answer.  ;)

Welcome to the forum anyway.  ;D
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Offline SwampDonkey

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Re: Prisms
« Reply #61 on: April 14, 2009, 08:54:42 AM »
Tom here is a way to demonstrate what Ron talked about when using a prism and determining "in" or "out" trees. The yellow circles are trees, the blue circles are limiting distances of trees of different diameters. The red dot in the middle is the point centre where the prism is held for the circular sweep. If you didn't have a prism, you could calculate manually the limiting distance from a tree centre in order to count it. So, if the blue line does not capture the point centre then it's not an "in" tree. The bottom tree is marked borderline because it's just on the outer reach of the point centre.

Ignore the fact I miss labeled the prism point as the "plot centre". My circles are a little wobbly. ;D


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Offline SwampDonkey

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Re: Prisms
« Reply #62 on: April 14, 2009, 08:39:31 PM »
From further reading in Husch, Miller and Beers (1982), beenthere's prisms are termed uncalibrated prisms. They were either home made or done by a lens shop that might make prescription lenses. A calibrated prism would be one manufactured for 10, 12, 14 etc ..ft2/acre and won't have fractions like 9.6 etc... Nothing wrong with them, as long as you calibrate them as the gentlemen obviously did to arrive at those BAF's. The prisms are usually cheaper to get made uncalibrated. The procedure I discussed earlier.
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Offline CJ5

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Re: Prisms
« Reply #63 on: January 28, 2020, 06:57:37 PM »
Good information in this old thread. I am also watching video's. I have not gotten to the point I understand all the math yet but I will.

My question is basic. I understand what I read and see on using a prism or angle gauge. I think I understand the reason for measuring multiple plots to get average. What I am struggling with is how far away from your point are you counting tree's? in other words, I am using a prism and rotating count in or out. How far ahead of me am I looking? Then, I move to another point and do the same. Am I now counting some of the same trees from my initial point? If another measuring point is on the border of the property do I only count what is on the property?

Sorry for being obtuse and I hope I asked that right.

Offline SwampDonkey

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Re: Prisms
« Reply #64 on: January 28, 2020, 07:46:49 PM »
The prism your using or angle gauge determines the plot size. The bigger the trees at DBH (diameter breast height) the further away they can be from the point your at to be counted. If your in big wood you'll have a larger diameter plot than in small wood. It's not really a plot, but based on a 'limiting distance' of each tree diameter based on the angle the prism is shaped and the refracted image seen within compared to the naked eye.

I've been in second growth hemlock that had old growth western red ceder left in it. I had one of those those big cedar in 2 or 3 plots, we used 50 meter spacing on grid.

Personally, I don't count border plots, unless the cover type and development stage of the forest doesn't change. If I'm standing in mature aspen/fir 80 feet tall and on the other side of the line it is now suckered aspen and fir regrowth then no sample if the tree diameters on my side of the line suggest the limiting distance goes beyond the boundary for those diameters.
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Offline clearcut

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Re: Prisms
« Reply #65 on: January 28, 2020, 10:45:01 PM »
Look up borderline tree on Wikipedia or other fine source:


     https://en.wikipedia.org/wiki/Borderline_tree

That is how far you are measuring to with a particular prism or angle gauge, for a tree of a specific diameter. 

Offline CJ5

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Re: Prisms
« Reply #66 on: January 28, 2020, 10:52:55 PM »
Lets assume I want to determine density of a 1 acre stand. I am using a 10 guage angle or prism. I can use 1/10 of an acre plot to measure. I determine the plot center and then measure out 37.2 feet. This would establish the diameter I an measuring. Then from center of the plot I count in and out within this diameter and multiply by 10.

Is that right?

Clearcut, thanks. I wil read that too.

Offline SwampDonkey

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Re: Prisms
« Reply #67 on: January 29, 2020, 06:07:00 AM »
That works for fixed area plots yes. But doing just one plot, there would be no average to account for variability. But for prisms and angle gauge you don't measure out a plot. The size of each plot is variable, depending on tree diameters. You need to make a table of 'tree factors' by diameter class. For 10 ft2/acre (BAF) prism it is BAF/(0.005454 x Diameter^2) then multiply tally of trees in each diameter by this number for that diameter. Then total them up for all diameters for density. The constant 0.005454 comes from pi/(4 x 144 in^2/ft^2), area of circle from in^2 to ft^2.   ^ symbol is to the power of.

The accuracy improves with sample size, stand typing, and known stand area. You can delineate stands and find their area easily in today's GIS software, even Google Earth can measure a polygon. Delineate stands based on texture and color.
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Offline clearcut

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Re: Prisms
« Reply #68 on: January 29, 2020, 10:37:00 AM »
You described a Fixed Radius Plot of 1/10 acre. Each tree measured represents 10 similar trees per acre. 

Plots measured with a prism or angle gauge are Variable Radius Plots, each trees diameter determines the plot size. Plot size is a fixed ratio of tree diameter to plot size. Thus, you get a constant value for Basal Area for each tree counted, depending on the Basal Area Factor of the prism or angle gauge.  ie. 10 sqft of Basal Area per acre.

There are a number of advantages to Variable Radius plots, especially when inventorying timber. Probability is Proportional to Size, here you are selecting larger trees to measure, generally the tree that a landowner can sell, but trees are selected in an unbiased manner, useful for the statistics that describe your stand. Species composition, and a number of other variables can be estimated. 

The biggest advantage is speed. Basal area is closely correlated with volume. You can measure Basal Area with a simple count, and measure a representative sample of trees accurately for volume. Using this information, one can construct a local Volume-Basal Area Ration table, to rapidly and accurately calculate volume. 

Basal Area is also closely correlated with a number of other factors of interest such as habitat value,and fire modeling, among others. 

Your description of a fixed radius plot gives you the density of the plot, number of trees per acre. You would need to calculate the basal area of each tree and summarize to get Basal Area per acre. Basal Area is an expression of density that also incorporates DBH. 

In any statistical sample, the number of sample points n is crucial. More is better for a number of reasons. 

Offline Legacy

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Re: Prisms
« Reply #69 on: February 18, 2020, 02:22:48 PM »
(Image hidden from quote, click to view.)
I like SwampDonkey's visualization of the "variable radius plot".
In this method you measure from the tree to a distance that varies depending on the diameter of that tree, and if the center of your plot is included in that distance, the tree is "in". The prisms have been explained in depth previously, but let's just say until you need to check a borderline tree, it's trigonometry magic that keeps you from needing to physically take this measurement.

In a "fixed radius plot", you measure from the plot center a set distance with no maths involved (on flat ground.)
The fixed plot allows you to easily find trees per acre, whereas the variable plot allows you to easily find the basal are per acre.
Grow a nice tree


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