How To Calibrate Your Forestry Laser Using A Level Pond

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M.W.Taylor
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How To Calibrate Your Forestry Laser Using A Level Pond

Post by M.W.Taylor » Fri Oct 03, 2014 3:27 pm

The other day while measuring trees tops with John Montague we noticed a difference in our Impulse200LR inclinometer readings. john's instrument was consistently giving angles about .25 degrees lower than mine. This would indicate at least one of our instruments was out of calibration i.e what the instrument thinks is zero degrees is offset by a constant angle. This can happen after the instrument is jarred or dropped.

We needed to figure out what was going on so we headed to Fern Lake a few days later for calibration. Fern Lake is a more of a pond these days. It is nestled deep in the redwoods above Humboldt State University, my alma mater.

If your Impulse200LR or other forestry laser is out of calibration, you should never adjust the scope unless you know for sure that was what was moved out of alignment with the laser. The scope crosshairs should always be focused to infinity i.e. parallel to the laser, despite the offset. You do not sight in the Impulse200LR scope as you would a riffle scope, which is calibrated to converge with the bullet on a parabolic path. And set at a fixed distance, such as 50m. To change the scope needlessly would cause it to no longer point towards the direction of the laser travel, making locking on specific targets difficult or impossible.

Before adjusting anything, you can verify whether or not your instrument inclinometer is out of calibration and also by how much. Also verify the laser is hitting distant tree tops and is centered. If not centered then the scope is out and needs to be adjusted back to pointing parallel with the laser.

Method: Level off a prism pole at the edge of a glassy pond (no wind) in two locations with pole point just touching the surface of the water and leaving a dimple but not actually protruding into the water. You can use two prism poles or just one pole. If using two poles make sure they are of exact equal length to the nearest mm. After placing prism pole in tripod at pond edge raise measurement stick until level, or approximately level your forestry laser crosshair, which would be reading at 0 degrees to the top of the prism pole. This will require a single person to travel back and forth to adjust so using two people is the easiest and fastest way to do this. Record SD and angle to top of pole. Place same prism pole, or another of exact length at the far edge of the pond. But not so far away that you can't see the top of the prism pole in the magnified scope crossairs. Usually between 100 and 150 feet further away from pole's first position will suffice. Keep the two pole stations approximately inline. Verify after panning instrument to pole station 2 that the inclinometer still reads zero which would indicate your pan axis is level. This axis must be level to perform the calibration.

Now at this point, the crosshair of the instrument will also be lined up with the top of the pole at station#2. If not, then your instrument is out of calibration. See the attachment for procedure to find this offset.
Attachments
Calibration procedure with example
Calibration procedure with example
dimple from pole point. This is easily repeatable to +,- 1mm if pond is glassy.
dimple from pole point. This is easily repeatable to +,- 1mm if pond is glassy.
John shooting to top of pole at station#2 which is about 146.42 fee away
John shooting to top of pole at station#2 which is about 146.42 fee away
Last edited by M.W.Taylor on Mon Oct 06, 2014 2:05 am, edited 1 time in total.

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mdvaden
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Re: How To Calibrate Your Forestry Laser Using A Level Pond

Post by mdvaden » Sat Oct 04, 2014 12:55 am

Appreciate you posting this. This has come to mind several times. I remembered you sharing it once, but couldn't remember all of what you shared.

...
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