Measurement Errors Jess Riddle Nov 14, 2006 03:10 PST

Ed,

when you say our methods under estimate heights you are referring to
the potential ways a rangefinder could return a too-short reading:
the rangefinder is not at click over, the rangefinder unintentionally
hits an intervening twig, the actual high point isn't being measured.
Further, obtaining click over for both the base and top measurements
in forest conditions is extremely difficult, so slight underestimates
are to be expected. However, some random errors are still involved.
The clinometer could be slightly misread too high as easily as it
could be misread too low. The rangefinder is still subject to smaller
random errors, and maybe other small errors from changes in ambient
lighting. Hence, assuming a cluster of measurements indicates the
same top and base are being measured each time, I would expect the
average of a cluster of measurements to be slightly low due to the
systematic click over error, but the highest measurement could be
slightly high due to random errors. The highest measurement could be
the highest of a set of measurements because the systematic errors are
at a minimum, the random errors are at their maximum positive, or some
combination of those two. It seems like we need to now if the
expected systematic error is larger or smaller than the expected
magnitude of the random error to tell if the highest measurement is
likely to be an over estimation. When in doubt, I'd rather be
slightly low than slightly low. Are you seeing some other factor that
makes you certain that the highest measurement won't be an over
estimation?

Jess

On 11/9/06, Edward Frank <ed_f-@hotmail.com>; wrote:
 Dale, Using the ENTS methodology if you are getting a tight cluster of heigt values, that indicates you do not have a significant error. Our methods do not overmeasure height, but under measure it. The best value, that one that represents the most accurate height, for a clustered series of measurements is the highest value. By averaging the values you are not being conservative, but deliberately introducing additional error into the measurement. Therefore in your series, the best height value is the 120.8 foot height for the oak. Ed Frank
 RE: Foundation Ridge Flat update (errors) Edward Frank Nov 15, 2006 08:13 PST
 RE: Foundation Ridge Flat update beth_k-@yahoo.com Nov 15, 2006 08:18 PST
 Ed, Don, and Dale, In my profession of medical laboratory technology when we do corralations between two instruments proforming the tests (ie. Complete Blood Count--CBC) we throw out the highest number and the lowest number and then find the average. Maybe this same thing can be applied in this case. Beth
 RE: Foundation Ridge Flat update foresto-@npgcable.com Nov 15, 2006 12:58 PST
 Beth- In Bob/Ed/Dale's world, numbers are numbers are numbers! I understand your profession's tossing of the highest and lowest, and that is generally employed in larger samples to remove outliers (both tails of the normal curve)which might disproportionately skew the average/mean/median. In Dale's case, the grouping was tight, the tails very short and the removal of highest and lowest would have reduced the sample size to N=2. What Ed was thinking, I think ?, was that Dale had four trees with readings x1, x2, x3, x4 instead of one tree with four estimates of actual height...correct me Ed, if I've misread you! -Don
 RE: Foundation Ridge Flat update (errors) foresto-@npgcable.com Nov 15, 2006 13:13 PST
 Ed- Thanks for the well-thought-out (and patient!) response. I am in complete general agreement, and specifically welcome your points on the loss of accuracy/precision that is involved in the minor adjustments occuring at 1) clickover, and 2)the pivoting up and down of instrument (whether clinometer or laser (since the laser incorporates a clinometer) as a source of error...we can't be calling tree measures accurate to a 0.1 foot, if we are raising and lowering the instrument of measure, 0.1 to 0.2 feet in the process of measuring. Thanks for the added clarity! -Don
 RE: Foundation Ridge Flat update Edward Frank Nov 15, 2006 15:59 PST
 Beth, Don, Thank you for the comments. The situation suggested by Beth is not similar because the measurements made by ENTS are a combination of one type of random error and another type of non-random pseudo-systematic errors, and not just random errors. Throwing out the highest and lowest is a reasonable strategy if you are not sure if there is some other type of error involved potentially creating anomalous high and low values (Its utility can be debated, but the idea is commonly used). The entire question with our measurements is which of the error sources are the predominant factor in errors in height. This is presuming that other errors, such as intervening branches can be distinguished from the  cluster of valid measurements - and I think they can be. As for reducing the sample size, that is an issue because it is often difficult to get a good reading on the very top of the tree there may not be enough measurements to make any average meaningful. This is exacerbated if you toss the upper and lower values. If the grouping is tight, then I am accepting that the values that are high or low in the cluster are not anomalous outliers, but real values. Ed
 Agreeing with Ed Robert Leverett Nov 16, 2006 09:32 PST
 Ed,    I agree with your assessment. As the owner (at one time or another) of seven lasers and five clinometers, I've learned to recoginze the patterns associated with the sources of potential error (instrument driven, shifting position, intervening obstruction).    Averaging in low shots that are often laser clippings of intervening twigs clearly robs the tree. I'll have much more to say on laser testing and error sources in the coming weeks. I'm presently, retesting 3 different lasers with some taped distances as checks. The patterns are interesting, but do not contradict what you've written. Bob