Prisms

[Texas Ranger]

yup

[Tillaway]

I think I lost mine. :  They work pretty darn good and the price is right.

[SwampDonkey]

Hey, Jeff and Tom. I expect to see the results of a 'mini' cruise on an under explored corner of the Brokenclaw (Ian's name, not mine) woodlot. Not one of them trail side cruises though, won't do. ;D  ;) :D

[Riles]

I can vouch that they're still teaching dot grids. A lot quicker to pick up than the GIS software.

[SwampDonkey]

 

I suspect that they do as I see them used from time to time. They are or were and maybe still are used in more disciplines than just forestry. Some aren't even used to find area, they are used to layout elements in a publishing program. ;)

[SwampDonkey]

Just a review of the formulas used in calculating Density and Basal Area in a spreadsheet layout of your cruise data by prism sample.

Density = tally x BAF/ (0.005454 x Diameter²)
                              # points sampled

BA = 0.005454 x Diameter² x Density

The constant of 0.005454 comes from converting inches² to feet².
       pi feet²        
4x144 inches²

A stand with basal area of 100 ft²/acre determined by sampling with a prism of basal area factor 10 ft²/acre has an average of 10 trees per sample point.

[kczbest]

I refuse to Prism Cruise any more. I will use a prism to get basal area for thinnings. I have not done a line plot cruise in years. I prefer a 10% strip cruise or even a 20% in small stands or when a stand has rapid changes in timber type. I no longer carry a tally book or a hand-held. I use a tape recorder with a microphone with on/off function. I make my tree "calls" into the recorder as well as every ditch, creek , fence I cross. At the end of every line I check to ensure the recorder is working (never hadf a problem yet). I can cover a few more acres per day and do my take off in the air conditioned office. When marking timber it is really handy not to keep up with a tally book or hand-held. The recorder fits perfectly in the left inside pocket of a filson vest. (In a ziploc bag for weather resistance)

[SwampDonkey]

I figure it's enough to go through the paces during the day and right it all down then, rather than live it all over again that evening or the next day listening to a tape recorder. I know the whole day isn't recorded, but what time did you save? You might as well write it down (or type into GPS/Windows Mobile datasheets) and it's more reliable. Just sit the unit in the charge cradle and hit send, open the cruise program and click and print the reports. It will only take once, maybe twice for the recorder to malfunction to test your enthusiasm. ;D

Each timber type gets a minimum of 3 or 4 points, if the stand is quite variable, usually more points are added (closer spacing). A map is usually sketched on paper along strip lines or GPS'ed position  (some have camera to geolocate a photo of a stream), aerial photo or GPS used to set up preliminary stand perimeters, ground features and "truthed" when walked.

[sunflower4dayz]

Could someone please tell me how to calculate the tree units once I've done the work in the field. I've done it before but I forgot....and may I ask for the very simple version...

Thank you!!!

Sunflower4dayz

[SwampDonkey]

What are you asking for? The density and basal area per acre? Volume would depend on your local conditions and standardized volumes for the site, species and crown class (dom,int, sup).

The tree factor equation is built into the density formula above. Finding density is your first step to get to the others. And you need your prism BAF of course. If you don't know it, you can find it by measuring as accurately as possible the distance to a bordline tree and the diameter of that tree at breast height with calipers pointing to point centre. Just use a yard tree and step away until it is borderline. Formula is in this thread earlier.

[SwampDonkey]

sunflower4dayz , hope you found your answer.  ;)

Welcome to the forum anyway.  ;D

[SwampDonkey]

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

[SwampDonkey]

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.

[CJ5]

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.

[SwampDonkey]

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.

[clearcut]

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. 

[CJ5]

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.

[SwampDonkey]

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.

[clearcut]

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 tree's 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. 

[Legacy]

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.