Limb Length Using Monocular w/reticle and Rangefinder

General discussions of measurement techniques and the results of testing of techniques and equipment.

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#1)  Limb Length Using Monocular w/reticle and Rangefinder

Postby dbhguru » Fri Mar 29, 2013 6:14 pm

Length Using Monocular w/reticle and Rangefinder

The length of a limb can be measured using a moocular w/reticle and a Lase Rangefinder.  The length of a limb is measured using a monocular w/reticle aligned along the orientation of the limb, the distance to either end of the limb segment, and a calculated scaling factor to determine limb length.  Essentially the apparent length of the limb at each end as using the distance to that point and the scaling factor for that distance as if the limb were perpendicular to the observer.  These lengths are considered to be the top and base of a regular trapezoid with a height equal to the difference in the distance between the two points.  The true length of the limb can then be calculated by treating it as a diagonal of the trapezoid.

               
                       
reticle_limb.GIF
                                       
               

               
                       
derivation.GIF
                                       
               


Compute limb or limb segment length using reticle and laser rangefinder                                                                
NOTES:                                                                                        
1. Assumption is that measurer cannot always get oriented 90 degrees to the axis of the  limb.                                        
2. From position of measurement (P0), measurer shoots distance to the ends of the limb, P1 and P2. These distances are the variables L1 and L2.                                                                                                        
3. M is the reticle reading for the limb or limb segment.                                                                        
4. F is the reticle factor.                                                                                                        
5. The equations shown below are used to compute W, the length we want.                                                                
6. Note that L1 is used as an approximation of the distance of the solid brown line.                                                
7. L2 is used to approximate the length of the solid and dashed brown line total.                                                
8. If the ratio of the distance to the target to the length of the limb or limb segment is 9 or greater the distance error will be small.        
9. Limbs over 20 feet should be divided into segments                                                                                
10. The table above implements the process.

               
                       
ReticleBasedLimbLengthMeasurement.xlsx
                                               
(197.25 KiB) Downloaded 7 times
               
               


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Robert T. Leverett
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Native Native Tree Society
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#2)  Trapezoid-Reticle Method for Limb Measurement

Postby dbhguru » Sun Mar 31, 2013 2:16 pm

NTS,
 
  I've been working on improvements to the reticle-based measurement of the length of limb segments. I think I'm there. The idea is to be able to measure a limb that slants toward or away from the measurer and is level, vertical or sloped. I had mentioned to Ed that I was working on a more advanced solution to the problem for possible inclusion in the Wikipedia articles that he has taken the lead on.

  The attachment shows the measurements that need to be taken on the Calculations worksheet and the Trapezoid Worksheet shows the full development of the process. I am obtaining measurement accuracies to within an inch for the targets I've been using. I'll continue testing, but this is a process that can deliver really good results and with a monocular like the Vortex mounted on a tripod, you can easily orient the reticle scale to lie on top of the target regardless of its orientation. Sweet!

Bob
Attachments
TrapezoidReticleProcess.xlsx
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Robert T. Leverett
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Native Native Tree Society
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Friends of Mohawk Trail State Forest
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#3)  Re: Trapezoid-Reticle Method for Limb Measurement

Postby dbhguru » Sun Mar 31, 2013 8:12 pm

NTS,

  Here is a screen shot of the Excel spreadsheet that I previously posted with some more tests.

               
                       
Screen shot 2013-03-31 at 8.07.24 PM.png
                                       
               


   The trapezoid-reticle method definitely has a place in our bag of measuring tricks. The accuracy that I'm obtaining is eye-popping. You may wonder about the distance measurements. They are being done with a Bosch GLR825 red beam laser which has an accuracy of around one millimeter. In order to test the efficacy of the new method, I have to be able to keep out distance errors.

Bob
Robert T. Leverett
Co-founder and Executive Director
Native Native Tree Society
Co-founder and President
Friends of Mohawk Trail State Forest
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