ABNEY LEVEL

noun: a surveying instrument consisting of a spirit level and a sighting tube, used to measure the angle of inclination of a line from the observer to another point [ETYMOLOGY: 20th Century: named after Sir William Abney (1843--1920), British chemist and physicist]

The Abney Level is an engineering instrument which can be used to determine height. It is moderately expensive and of medium size and weight. Although the Abney Level is relatively robust, the bubble tube can be knocked out of alignment during rough treatment. General instructions Using an Abney Level to determine tree height: Measure the horizontal distance from the base of the tree (or position directly beneath the tree tip) to a location where the required point on the tree (e.g. tree tip) can be seen. Sight at the required point and move the index arm over the scale until the bubble tube is level. Read the percentage scale (or the degrees and minutes of the angle). Calculate the height by multiplying the percentage read by the horizontal distance (or by multiplying the horizontal distance by Tan of the angle). Site to the base of the tree and repeat steps 2 - 4. Combine the heights from steps 4 and 5 to determine total tree height: Add the 2 heights together if you looked up to the required point in step 2 and down to the base of the tree in step 5. Subtract the height to the base of the tree from the height to the required point if you are on sloping ground and had to look up to both the required point and the base of the tree. Check all readings and calculations.