This discussion is very helpful for me. It's good to know how much to trust (or not) my measurements and it is also good to know when to obsess about accuracy and when that is futile given the limitations of the instruments.
Is the following an accurate summary of sources of error in the sine method:?
If your LRF is not correctly calibrated you could easily add up to 2 feet of error depending on the angle of measurement.
Ed gives some numbers above to show the error that can occur if your clinometer dial is off, but human measurement error is also an issue. It seems to me the clinometer can only be read accurately to within 0.25 degrees. This human error can lead to many inches of error. For example:
- A 64 yard LRF measurement at 45 degrees or 45.25 degrees is a difference of 0.6 feet over the ~130 foot measured height.
A 50 yard LRF measurement at 64.5 degrees or 64.75 degrees is a difference of 0.3 feet over the ~130 foot measured height
Therefore being closer to the tree (larger angle) is better to decrease clinometer error, but on hardwoods, the closer to the tree you are the less likely that you will be to hit the top most sprig. This is of course assuming that the clinometer scale itself is 100% accurate and the only source of clinometer error is human error in reading the angle.
Not being able to step back to "click-over" in dense undercover adds error. For example, if you only have a tiny window through which to measure you may be stuck with the documented LRF error of 1.5 feet (for a Nikon 440), which, depending on the angle of measurement will add some fraction of that error. Additionally, it may not be possible to step back to click over at both top and base LRF reading points. I believe it was Ed Frank who posted in another thread that it is better to step back to click-over on the top measurement if only one is possible since the crown LRF reading is usually longer. In fact, stepping back at both points in an effort to increase accuracy will actually introduce error if you are not on level ground.
Non-ideal atmospheric conditions such as high humidity or bright sunshine can, according to the Nikon instruction manual, introduce an unknown amount of error.
So, I think it is safe to conclude that even under ideal situations with a seasoned measurer, tree heights taken using the sine method with a Nikon 440 and Suunto clinometer are accurate to within 6 inches or so. That's still very impressive for a ground based measurement! In difficult measuring conditions LRF-derived heights could be off by a foot or two.