Questions on TruPulse 200

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

Moderators: edfrank, dbhguru

Post Reply
User avatar
JHarkness
Posts: 249
Joined: Sun May 13, 2018 5:44 pm

Questions on TruPulse 200

Post by JHarkness » Thu Dec 27, 2018 10:58 am

ENTS,

I have a couple questions involving the TruPulse 200, specifically its measurement modes and the accuracy of the same modes.

To start off, on TruPulse units, what measurement modes do you find yourself typically using? Missing Line, Vertical Distance, or taking individual inclination and Slope Distance measurements and calculating height from those?

Which of those modes do you find to be the most accurate/precise?

Lastly, I ran an interesting test on my TruPulse 200, I decided to use it to repeatedly measure an object in my house to get a sense of its accuracy. I first tape measured the object at 3.64', I then took 10 ML measurements and got 3.6' every time while repositioning the laser in regards to its target between shots. I then repeated the process by taking sperate INC and SD measurements and calculating the height from those, my result was an average of 3.59', the highest and lowest extremes being 3.67' and 3.56'. Not bad, only off by 1/20th of a foot. But what concerned me was that all but the 3.67' measurement were consistently below the objects true height. If we say that the unit reads low by a 1/20th of a foot at 18.6' from its target that works out to being 0.26' low at 100' from its target, and at 200' that works out to 0.53'. As I'm sure all of you know, we commonly have to get 100', 200', sometimes even farther from the tree we're measuring to be able to see the true top, I've personally experienced this with a white ash on my property where the top cannot be seen clearly until over 300' away. I thought perhaps that my tilt sensor was off and needed to be calibrated, I did so per the manual and performed the test again with the exact result as before. Here is my question on this, is it normal for the TruPulse 200, or any TruPulse device, to come up with a lower than accurate reading on small objects and that this does not happen on a tree sized object, or could this potentially be an issue with the unit causing it to consistently measure low?


Joshua Harkness
"Be not simply good; be good for something." Henry David Thoreau

User avatar
M.W.Taylor
Site Admin
Posts: 530
Joined: Sun May 29, 2011 11:45 am

Re: Questions on TruPulse 200

Post by M.W.Taylor » Fri Dec 28, 2018 2:49 pm

I like to use the Trupulse 200 in Vd mode. As I hit tops, I then hit the base (if visible) and add the difference. If the base is obscured I will survey to intermediate targets such as snags, top of small trees or something else that be relocated while looking back at it. I work my way to the base. Add/subtracting the differences. To maximize accuracy I would recommend mounting on a tripod, using the remote trigger and a prism/pole survey to the base. You would need to put the laser in filter mode when using the prism. This will also ignore the foliage and only hit the prism so you can survey through bushes and clutter easily. From greater distances, the accuracy of the Trupulse 200 will decrease. See Robert Leverett's post on TP 200 accuracy. I find mine to be accurate to about +,-1 foot or so at 600 feet. It only reads to the nearest .5 foot so precision is limited. You can find the switch point though to increase the precision of the estimate.

User avatar
dbhguru
Posts: 4519
Joined: Mon Mar 08, 2010 9:34 pm

Re: Questions on TruPulse 200

Post by dbhguru » Sun Dec 30, 2018 8:00 pm

Joshua,

The answer to your first question is all the above. I constantly use all the modes and compare results so I can stay sensitive to all routines and returns of the instrument. However, like Michael, I prefer the VD mode for height, performed just as he says. The reason is that the laser and tilt sensor are invoked for exactly the same target. That said, using the sensors separately can help identify which one is causing differences from shot to shot. It is useful to be able to simply evaluate possible errors in one of both inputs.

Here is a simple formula for the Sine Method in terms of total differentials to evaluate small changes in the angle and/or distance. It is an approximation, but close. The changes may be thought of as measurement errors.

[attachment=0]Screen Shot 2018-12-30 at 7.28.01 PM.png[/attachment

dL1 is the change (or error) in L1. dA1 is the change (or error) in A1. So suppose we have L1 = 150 feet, dL1 = 1 foot, A1 = 42.5 degrees and dA1 = 0.5 degrees. The height for these values of L1 and A1 is 101.34 feet [H = 150 * sin(42.5)]. The changes dL1 and dA1 give 1.64 feet. Oh yes, dA1 is in radians so it enters the formula as 0.5*π/180 = 0.000873 as opposed to 0.5. If you have a scientific calculator, the formula is easy to evaluate. So an error of a half a degree and 1.0 feet translate to 1.64 feet too much. The actual error is 1.642. So, the differential formula works quite well.

Bob
Attachments
Screen Shot 2018-12-30 at 7.28.01 PM.png
Robert T. Leverett
Co-founder, Native Native Tree Society
Co-founder and President
Friends of Mohawk Trail State Forest
Co-founder, National Cadre

Post Reply

Return to “Measurement and Dendromorphometry”