Tape drop is considered the definitive measure of a tree's exact height, but climbing a tree to measure its height is usually not feasible. Don Bragg, Lee Frelich, Bob Leverett, Will Blozan and Dale Luthringer published a paper in 2011 (

**) wherein they measured 42 trees (mostly Tsuga conifers) via the NTS sine method with a TruPulse 200 hypsometer and compared the height to that obtained by tape drop. The trees were in the 150 foot range and they found that discrepancy for those 42 trees ranged from -1.9% to +1.4% with a standard deviation of 0.64%. In other words 68% of the time (definition of 1 standard deviation) one could expect that the sine-based measurement would be within 0.64% of the tape drop measurement. On a tree 150 feet tall, that equates to less than 12 inches. Or as the authors themselves put it,**

__http://www.nativetreesociety.org/specia ... g2011D.pdf__**"Hence, with the accurate laser rangefinders and electronic clinometers available today, instrument error when measuring total tree heights with the sine method can be expected to be consistently less than 1 percent for experienced users" (Bragg et al, 2011)**

I was curious to know how accurate my Nikon 440 laser range finder and Suunto clinometer are when compared to a tape drop. To my knowledge there hasn't been a careful experiment done to answer that question for the Nikon 440. The manufacturer-stated accuracy of the TruPulse 200 is +/- 0.1 yards and for the Nikon it is +/- 0.5 yard. It is widely accepted that the accuracy of the Nikon 440 can be improved through instrument calibration. Even so, I embarked on this study with the expectation that the Nikon 440 might only be accurate to within 3% of tape drop accuracy. I'm happy to say that I had underestimated the Nikon's capabilities.

**Study methods.**

I measured the total height of 4 loblolly (Pinus taeda) conifers ranging in height from 126 feet tall to 141 feet tall. The pines were situated on 3 sites near Chapel Hill, NC that I located using LIDAR data obtained from Doug Newcomb, Cartographer at the Raleigh, NC Field Office of the US Fish and Wildlife Service. The first two pines are located in a stately grove of Loblolly pines along Morgan Creek near the NC Botanical Gardens. Here's a 360 panorama of that grove:

**. I used satellite imagery from Google and Bing to narrow the list of sites to those on publicly accessible land that were most likely to contain tall pine trees. Loblolly pines are common in the Piedmont of North Carolina and are ideal for this experiment because they have tall, straight boles and conical tops with easily identified height maxima.**

__http://photosynth.net/view/bbe1eb38-88f ... 51f3244b86__I measured the trees from the ground first using the NTS sine method. Each tree was measured from at least three locations on different sides of the tree, often on 2 different days.

I had previously calibrated my Nikon 440 to determine the needed correction factor (see

**). I recalibrated it in February 2014 with nearly the same results. I should point out that I have found it impractical in a woods setting to always step back or step forward to LRF "click over" in order to take a measurement -- usually there is only a small window through which to point the laser and often taking a step back or forward puts underbrush clutter in the way of a clear view to the tallest point of the tree. For this reason when I calibrate my LRF, I average the reading of 4 measurements for each reference point -- 2 readings walking backward to click over and 2 readings walking forward to click over.**

__http://www.ents-bbs.org/viewtopic.php?f=235&t=4862__Here are the details of my calibration protocol: I calibrate the LRF by measuring the distance to the side of a brick shed at the end of a long, level parking lot. I stake the end of a 300 foot tape measure at the base of the shed and extended it past 80 yards (240 feet). I record the actual distance (tape measure reading) at 10 yard intervals between 20 yards (60 feet) and 80 yards (240 feet) as measured on the LRF. For example, I step backward until the LRF says 20 yards and then record the actual distance from the tape measure. Then I step forward until the LRF reads 20 yards and record the actual distance at that point. I repeat both measurements once more and then average all 4 measurements for the 20 yard distance. Then I repeat that process for the 30, 40, 50 , 60 , 70 and 80 yard reference points. At each point the standard deviation of the 4 averaged measurements was between 0.4 and 0.5 feet meaning that most any measurement returned by the LRF can be trusted within an accuracy of 6 inches or less. Comparing the LRF reading at each 10 yard reference point to the average of the 4 measurements from the tape measure allows me to determine a correction factor for each range distance:

- subtract 0.8 feet for measurements between 0 and 59 feet (0 and 19.9 yards)

subtract 0.9 feet for measurements between 60 and 89 feet (20 and 29.9 yards)

subtract 1.0 feet for measurements between 90 and 119 feet (30 and 39.9 yards)

subtract 1.1 feet for measurements between 120 and 149 feet (40 and 49.9 yards)

subtract 1.1 feet for measurements between 150 and 179 feet (50 and 59.9 yards)

subtract 1.2 feet for measurements between 180 and 209 feet (60 and 69.9 yards)

subtract 1.2 feet for measurements between 210 and 239 feet (70 and 79.9 yards)

subtract 1.25 feet for measurements over 240 feet (over 80 yards)

**Height(feet) = [[[LRF(top) * 3] + CF] * sine of angle(top)] + [[[LRF(base) * 3] + CF] * sine of angle(base)]**

LRF is the reading from the Nikon 440 in yards

CF is a correction factor determined from the calibration process

**and math I don't completely follow) indicates that clinometer error is essentially negated when combining crown and base angle measures.**

__http://www.ents-bbs.org/viewtopic.php?f=235&t=4876__**Results.**

Angle, distance and resulting sine-based height calculations are listed in Table 1 along with the CBH, date of measurement, and rough GPS coordinates of each tree.

Within a few weeks of the ground based measurements I climbed each tree and measured the trees by tape drop. It is best to have a ground helper when performing a tape drop, but I couldn't convince anyone to come out to the woods with me for a few hours at a time in February/March. The person on the ground is helpful because they can position the tape at the correct point at the base of the tree and provide tension while the climber reads off the measurement. A ground person with the right perspective can also let the climber know when a telescoping measuring pole is at the same height as the tallest twig of the tree.

Since I was performing the tape drops myself, I carried a 200 foot construction tape measure into the canopy and lowered the end directly to the ground using a brightly colored, 1 pound throw bag as weight. In each case it was easy to tell when the bag hit the ground both by visual confirmation and because the tension on the tape relaxed. In each tree I could climb safely to within 10 feet of the highest point. When I reached that point I used a metal tape measure to record the remaining distance to the top of the tree. Then I added the numbers from the upward measurement to the tape drop measurement to obtain the total height. Although I did not have a ground helper, I have no reason to believe that my tape drop measurements are off by more than 2 inches. Tape drop measurements, dates, and comparison to sine-based ground measurements are listed in Table 1.

**Table 1(click on the table for a larger view)**

**Conclusions.**

1). The average % divergence was 0.49%, but that value is misleading because some LRF values are high (positive % divergence values) and some are low (negative % divergence values) so the average looks artificially low. For that reason it is more informative to look at the absolute value of the % divergences. The average of the absolute values of all the divergences was 0.88% with standard deviation = 0.57%. The range of the divergences was -1.83% to +1.59%. These numbers are almost exactly what Don Bragg and his coauthors reported for the TruPulse in the paper mentioned in the first paragraph above.

2). I have heard some NTSers state that when taking multiple sine based measurements on the same tree they will throw out all but the highest measurement assuming that in that attempt they found the true highest point in a nested crown and in the others they were not measuring the highest sprig in the crown. Interestingly, I noticed that in 2 out of 4 trees in this small study, the highest LRF measurement I recorded was actually the most accurate measurement (lowest % divergence from tape drop). (The highest LRF measure for each tree is colored red in Table 1 above.) This observation lends some credibility to the practice of throwing out all but the highest sine-based height value. However, even though the tallest measurement was the most accurate in half the cases, it was also an overestimate of the true height in 4 out 4 cases.

3). In 3 out of 4 cases (Loblolly #4 being the only exception), averaging the LRF measures from different sides of the tree resulted in a height value that was closer to the tape drop measure. That leads me to conclude that when possible it is better to average multiple measurements from different locations around the tree.

Overall, I'm happy to learn that the Nikon 440/Suunto clinometer pairing is very good at estimating the height of woods grown trees up to 140 feet tall. I think it is safe to reaffirm that the Nikon 440 is an excellent low-cost alternative to the TruPulse brand hypsometers and that % divergence from tape drop measurements is comparable between the two instruments.

**Please let me know what you think...**

Lastly, here are some pictures of the trees from the ground and from the crown. Enjoy!

Loblolly #1

*Click on image to see its original size*

view form top of Loblolly #1

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Loblolly #2

*Click on image to see its original size*

view from top of Loblolly #2

*Click on image to see its original size*

Loblolly #3

*Click on image to see its original size*

view from top of Loblolly #3

*Click on image to see its original size*

Loblolly #4

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View from top of Loblolly #4

*Click on image to see its original size*