The Dendrometer II - nicknamed the Bottle opener because of its looks - is a cheap instrument to measure tree height, stand basal area using Basal Area Factors of 1, 2, or 4 and estimate volume.
Due to difficulties aligning the tree and ensuring that the instrument is at the correct distance and angle, accuracy is relatively poor. No better than an accuracy of about +/- 1 m in a 20 m tall tree (ie about 5%) should be expected.
Measuring height
An indentation runs almost the entire length of the left side of the instrument. Within this indentation, there is a cut out notch exactly 10% up from the base.
To measure the height of a tree, hold the Dendrometer in front of your eye at the same angle as the tree and either walk back/forward or move the instrument close/further from your eye until the base of the tree corresponds to the base of the indentation and the top of the tree corresponds to the top of the indentation. Observe where the 10% notch falls on the tree (get an assistant to mark this point). Measure the actual height from the base to the marked point on the tree - this is 10% of the total tree height, so mulitply this measurement by 10 to get the total tree height.
An indentation runs almost the entire length of the left side of the instrument. Within this indentation, there is a cut out notch exactly 10% up from the base.