Benchmarks/Reference Marks

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Chris
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Benchmarks/Reference Marks

Post by Chris » Tue Jan 06, 2015 12:36 am

I apologize if this has been brought up before, but has there been any consideration in the use of independent benchmarks or references marks for aiding in highly accurate, repeated measurements of certain trees? Basically, instead of measuring a tree in relation to itself (I measured 115.6 ft 2 years ago, now I measured 116.1 ft), measuring it in relation to a arbitrary, highly stable object (this benchmark is 2.4 ft abv ground level, I measured the tree 113.2 ft higher than it 2 years ago, now I measured 113.7 ft). For 99.9% of trees I see measured, it would be overkill, but for those very few...... it would really reduce some uncertainties and aid in repeatability of measurements.

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Don
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Re: Benchmarks/Reference Marks

Post by Don » Tue Jan 06, 2015 2:07 am

Chris-
In both cases it would appear that the tree grew by 0.5'...how did use of benchmark improve the situation? That said, as a surveyor, referencing monuments is firsthand operation where accurate and permanent measurements were expected.
If I read you right, you're saying that you're very careful in your measuring, remeasuring, and think that where there are discrepancies, having a monument/reference point could isolate where error is?
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tsharp
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Re: Benchmarks/Reference Marks

Post by tsharp » Tue Jan 06, 2015 10:57 am

Chris:
Thanks for broaching this subject. In my short measuring career(5 years) I have encountered large changes is ground level for trees I have previously measured. Mostly associated with bottom lands with flood deposition and erosion. It appears to me that it is such changes may be more pronounced out we west from my observations in Arizona, Idaho). I also believe wind erosion in some areas of the west would be a factor.

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mdvaden
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Re: Benchmarks/Reference Marks

Post by mdvaden » Tue Jan 06, 2015 11:48 am

Chris wrote:I apologize if this has been brought up before, but has there been any consideration in the use of independent benchmarks or references marks for aiding in highly accurate, repeated measurements of certain trees? Basically, instead of measuring a tree in relation to itself (I measured 115.6 ft 2 years ago, now I measured 116.1 ft), measuring it in relation to a arbitrary, highly stable object (this benchmark is 2.4 ft abv ground level, I measured the tree 113.2 ft higher than it 2 years ago, now I measured 113.7 ft). For 99.9% of trees I see measured, it would be overkill, but for those very few...... it would really reduce some uncertainties and aid in repeatability of measurements.
If i deciphered your point correctly, it sounds like how we put a tag on other stable trees and measure in relation to the tag rather than the ground around the tree we are measuring.

I think the landmark method closer to what you mentioned, I've done for only one tree ... the world's tallest living Christmas tree in Ferndale, California. Using the peak of a roof nearby on a house.

There could be flaws with certain landmarks. Buildings settle, ground settles, monuments can tip, etc..

If a landmark was rock solid stable, the benefit may reside in speed of measurement rather than removing uncertainty. If the landmarks remains steady, it would make for a good reference point.
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Chris
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Re: Benchmarks/Reference Marks

Post by Chris » Thu Jan 08, 2015 12:21 am

Don wrote:That said, as a surveyor, referencing monuments is firsthand operation where accurate and permanent measurements were expected.
If I read you right, you're saying that you're very careful in your measuring, remeasuring, and think that where there are discrepancies, having a monument/reference point could isolate where error is?
Yes, I guess I want to bring some "tools" surveyors use to bear with tree measuring. I am not a surveyor (I just pretend to be one on this forum!), but I do some differential leveling for my job; we are measuring to the 0.001 ft accuracy. And even with trigonometric leveling, we can get 0.01 ft. One of the key things is using independent reference marks. Now measuring trees is a completely different thing; trying to find the top most branch on some 200 ft tree is hard enough and has errors. Lets try to reduce the errors we can acquire near the ground. And the person can now spend more time trying to shoot that very top, than wasting it on finding some mid-slope.
mdvaden wrote:If i deciphered your point correctly, it sounds like how we put a tag on other stable trees and measure in relation to the tag rather than the ground around the tree we are measuring....If a landmark was rock solid stable, the benefit may reside in speed of measurement rather than removing uncertainty. If the landmarks remains steady, it would make for a good reference point.
Exactly. Except, you don't have to put a tag in trees. In many places, that simply isn't going to be "acceptable" to the person/entity that owns the trees. It would me more acceptable to but a bolt in a rock, or even just use the top of some very solid feature. Also the tree is growing, so the can move (just very slowly). The hardest would be in some forest, but if you are close to any sort of human construction, you will be able to find bolts in bridges, nails in telephone poles, brass tabs in streets, etc... that are reasonable stable and are unlikely to be gone anytime soon. I hadn't thought of speeding up the process, but it would help, especially when you can see the bottom from the same place you can shot the top.

Turner,there are several sources of error I see in repeatable measurements (I am sure there are others I am not thinking of):
- Actual changing in the ground surface (the 0 for tree height) as you mention. There is deposition, erosion, and even the seasonal accumulation (and decomposition) of duff.
- Root flare that limits your ability to get to ground surface without an angle (if you have more than one person, you could use level and then tape down to the ground surface).
- Different observers assigned different ground surfaces. Even on relatively flat ground different people could measure on different sides. Imagine how different people can pick different locations for their mid-slope on very step terrain. Two people measuring a tree 1 ft difference in height seems entirely possible. This was struck home to me last summer along the Hermosa were we were trying to find the mid-slope on like a 60 deg slope. Now compounded that with measurements being made by someone completely different 10 years from now.

My point is we are all aware there are sources of error. When someone says I measured a tree to 143.3 ft in height, we all know there are some error bars around that number (the size of those error bars vary). If the tree is actually 143.7 ft, well no big deal. But if you are claiming to measure how much a tree is growing, it is really, really important there is repeatability. That means several things (the accuracy of the instrument, how much time to people spending trying to shoot the highest point, etc...), but it means every measurement of that tree needs to start from the same 0.00 ft of the ground surface. That means every measurement of that trees needs to measured the girth in the exact same spot. If not, you simply aren't measuring the same thing.

The idea with using reference marks or benchmarks are you measuring something you care about that can change or grow in relation to something that isn't likely to change. Here is an example (maybe a bit extreme to illustrate the point). There is a tree you want to measure and nearby there is a bedrock outcropping. You put a anchor bolt in the rock (that isn't going to move). You determine the elevation of the bolt from the ground surface at the base of the tree. That bolt is 3.45 ft abv the ground surface. You then measure the height of the tree, but instead of shooting to the 4.5 ft on the tree, or the ground, the measure it to the bolt and add the 3.45 ft to get the height of the tree.

Now someone else can come 5 years later. They measure the height of the tree, the same way (measuring from the top of the tree to the bolt and add 3.45 ft), and they have now measured the tree to the exact same ground surface as 5 years ago, even though, let us say, a river deposited 1 ft of sand. You can use nearly the same process to measure the girth in the same place; you can use the same process to measure the crown spread in the same place; you can have multiple reference marks to prove the other ones aren't moving. A source of error from the equation and we can have more confidence that 0.5 ft of growth that was measured measured is real and not just someone picking a different ground surface from some mid slope.

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