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Wedge prism vs cruising prism vs angle gauge

Started by elduderino260, July 28, 2014, 01:00:13 PM

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elduderino260

Hi all,

I am a student researching ecological restoration in the tropic. One of the metrics I want to record in our experimental plots is stand basal area, but was wondering about which tool would be best.

Historically, I have used a wedge prism, but apparently this is distinct from a cruising prism (they are different items in the Ben Meadows catalog: http://www.benmeadows.com/ecatalog/JB4_Big_Ben_Book/F/18).  How so?  The only difference I see is the shape and the cost.

I also found references in the forums to angle/cruising gauges like this one: www.forestry-suppliers.com/product_pages/Products.asp?mi=13951&title=JIM%2DGEM%AE+Cruz%2DAll&itemnum=59795.  Again, they are half the price of the cruising prisms (probably because they are easier to manufacture), but the only real difference that I can see is that they will likely not be quite as delicate and that you need to keep them at a set distance from your eye.  Is there any downfall that I am not seeing?  Are they significantly less accurate than prisms? 

Thanks!

beenthere

Welcome to the Forestry Forum.

Where are you located now? And do you have either the wedge or the cruising prism in hand? or needing to order one or the other?
How large are your experimental plots?
south central Wisconsin
It may be that my sole purpose in life is simply to serve as a warning to others

Silvanus

If your plots are fixed area, then you dont want to use a prism or angle gauge because the radius of these plots are variable radius and may be too small or extend beyond your measurement area.

Comparing the two types, prism vs gauge, the angle gauge may be easier to use.  It's simpler to pivot in place, keeping you eye over plot center and its easier to determine borderline trees if youre not measuring out to the limiting distance.
"There are some who can live without wild things and some who cannot."  -AL

BradMarks

elduderino:  I went to Ben Meadows website. What they are calling a wedge prism is just a "full size" with rounded top, and what they call a cruising prism is just a "half prism". Both work the same, just the full size gives you a larger view of the dbh area.  And yes, if you are looking for accuracy, the prisms are far superior for the task. If you want a "down and dirty" ballpark estimate, the angle guage will work. And some people say they have a 20 BAF thumb! for what it's worth. Brad.

elduderino260

Quote from: Silvanus on July 28, 2014, 01:51:20 PM
If your plots are fixed area, then you dont want to use a prism or angle gauge because the radius of these plots are variable radius and may be too small or extend beyond your measurement area.

Comparing the two types, prism vs gauge, the angle gauge may be easier to use.  It's simpler to pivot in place, keeping you eye over plot center and its easier to determine borderline trees if youre not measuring out to the limiting distance.

Thanks.  Is there something that you would recommend for fixed radius plots? The issue is that the veg down here is very thick and often the radius is small (>30 m)

elduderino260

Quote from: beenthere on July 28, 2014, 01:40:01 PM
Welcome to the Forestry Forum.

Where are you located now? And do you have either the wedge or the cruising prism in hand? or needing to order one or the other?
How large are your experimental plots?

I can order anything and have my research assistant bring them when he comes down in Aug (I am currently in Mexico).  The plots are 600 m^2 (20x30m).

elduderino260

Quote from: BradMarks on July 28, 2014, 01:54:29 PM
elduderino:  I went to Ben Meadows website. What they are calling a wedge prism is just a "full size" with rounded top, and what they call a cruising prism is just a "half prism". Both work the same, just the full size gives you a larger view of the dbh area.  And yes, if you are looking for accuracy, the prisms are far superior for the task. If you want a "down and dirty" ballpark estimate, the angle guage will work. And some people say they have a 20 BAF thumb! for what it's worth. Brad.

Thanks for the info.  I have been looking everywhere to references regarding the accuracy of wedge prisms vs gauges.    I don't doubt you, but do you know of any resources that I could cite in a proposal?  Also, does the fact that visibility is quite small (<30 m in most places) affect which you would choose?

BradMarks

The wedge prisms are ground to a stated 1% accuracy. Just using the BAF factor of lets say a 40, simple math says it is ground in the range of 39.6 to 40.4. An angle guage has no stated standard of accuracy and is dependent on distance from the eye and the guage being over plot center. A Prism only needs to be over plot center regardless of distance from eye. But PLEASE REMEMBER, the accuracy of any instrument is only as good as the OPERATOR. And finally, if you cannot see the bole of the tree, neither prisms nor guages will work(as they are visual instruments) and you need to consider the ultrasound instruments available for cruising. And also, in a fixed area plot is there a need for variable plot cruisng? Are you doing both from the same plot center?

Clark

It seems your questions have been answered but I will add that I find prisms to be much more accurate in determining in/out trees. Whether you pull the limiting distance or not I would not rely on an angle gauge to determine borderline trees.

Clark
SAF Certified Forester

WDH

Using a higher Basal Area factor prism will help if visibility is limited. 
Woodmizer LT40HDD35, John Deere 2155, Kubota M5-111, Kubota L2501, Nyle L53 Dehumidification Kiln, and a passion for all things with leafs, twigs, and bark.  hamsleyhardwood.com

elduderino260

Quote from: BradMarks on July 28, 2014, 04:36:00 PM
And also, in a fixed area plot is there a need for variable plot cruisng? Are you doing both from the same plot center?

I was initially doing a fixed area plot survey, but either the subplot at the center of the whole plot was too small and I missed some huge trees which would have completely changed the BA of the plot or it was too big to be manageable both logistically (getting to each tree) and time (this is only one of many metrics we are recording); especially with dense understory.  I can see the boles however, so I thought that I could use a wedge prism from the plot center to get a more accurate measurement for the plot...is that not valid?

clearcut

Either a prism or an angle gauge lets you know quickly whether a tree is in or out. With either device, you should measure the horizontal limiting distance for borderline or difficult to visualize trees when sampling.

I prefer an angle gauge as I only have to carry one to get several Basal Area Factors.

For an upcoming project we are exploring the use of the Big BAF or a double sampling technique where you take a lot of basal area plots with a small factor prism or angle gauge. You supplement this with a few carefully measured plots using a larger BAF. The idea is that height, diameter, volume, etc. is closely related to basal area locally. By taking many basal area counts, quick and easy, you are sampling a large area for basal area, and relating this to the variable (usually volume) that you really want to estimate that is difficult or expensive to measure.

More on the technique here:

    http://www.proaxis.com/~johnbell/guest/guest58b.htm

The author developed and used the technique in New England mixed forests. I don't know if it is viable for tropical conditions. 
Carbon sequestered upon request.

Silvanus

I think using a variable radius measure (prism or gauge) would be fine if you're using it as a stand level data. Are the other variables you are measuring plot level?
"There are some who can live without wild things and some who cannot."  -AL

elduderino260

Quote from: Silvanus on July 29, 2014, 12:58:19 PM
I think using a variable radius measure (prism or gauge) would be fine if you're using it as a stand level data. Are the other variables you are measuring plot level?

That is correct.

beenthere

QuoteThe plots are 600 m^2 (20x30m).

With a plot that size, am wondering why not measure each tree? Probably a good reason, but am wondering also if you find the plot center to use the tools or do you go to a random location?

A short explanation of how you set up these plots and how you extract or extrapolate the information would be helpful to me, and maybe to others as well.
south central Wisconsin
It may be that my sole purpose in life is simply to serve as a warning to others

elduderino260

Quote from: beenthere on July 29, 2014, 01:16:42 PM
With a plot that size, am wondering why not measure each tree? Probably a good reason

My reason, whether good or not, is one of logistics.  Being a tropical forest, there is tons of very thick understory which makes it very, very difficult to physically reach the actual tree to measure it (without destroying the understory herb layer, which we are also measuring).  Second, measuring each tree would be extraordinarily time consuming (both because of the difficulty in getting to it and because of the sheer number of trees in the plot), and this is only one of many metrics we are recording.  I only have so much field time and need to budget it accordingly.   

Quote from: beenthere on July 29, 2014, 01:16:42 PMwondering also if you find the plot center to use the tools or do you go to a random location?

I was initially using four 2m radius subsample quadrats in each plot that were selected randomly throughout the plots; recording the species, and DBH of individuals DBH>=5cm who were at least partially in the quadrat (allowing for large trees to be considered).  However, if I were to switch to a wedge prism, then I would find the center of each plot so I would stay within the plot as much as possible.

Quote from: beenthere on July 29, 2014, 01:16:42 PMA short explanation of how you set up these plots and how you extract or extrapolate the information would be helpful to me, and maybe to others as well.

Sorry, I wasn't sure how much info was necessary, but I suppose more is always better.  I do a more detailed writeup here: https://www.researchgate.net/post/Assessing_ecological_restoration_How_to_measure_basal_area_and_vegetation_species_richness_in_20x30m_experimental_plots?_tpcectx=profile_highlights

Thanks!

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