MEASUREMENT OF HEIGHT
The actual measurement of the height of the tree should be done from the distance measured in the following way: the observer sights the top of the tree with both eyes open. The object sighted, the hair line and the scale will all be simultaneously visible in the instrument`s field of vision. As soon as the hairline coincides with the top of the tree, the tree height can be read off (in this example, from the 20 m scale on the left of the instrument). The reading obtained is the height of the tree measured from the eye level of the observer. The base of the tree remains to be sighted. If this is situated below the eye level of the observer, then the actual height of the tree is obtained by adding the two readings together. If it is above the observer`s eye level, the tree height is obtained by taking the difference between the two readings. In fact, in the latter case the distance cannot be measured horizontally. Thus, to get 13,5 m 3,25 m 10,25 m 20 m 3,5 m 12,5 m 16 m 20 m 6 exactly correct result you have to proceed as stated below. On level ground, the tree top readings is usually sufficient: one only has to add the height of the observer`s eye level (1,60 m in this case), which is already known.
INSTRUCTIONS FOR USE OF NOMOGRAM
If the distance, because of very uneven ground, cannot de determined horizontally as stated above, the nomogram supplied with the instrument should be used.
ESTABLISHING THE BASIC DISTANCE
Because this instrument does not incorporate a prism, the basic distance e.g. 15 m has to be determined using a tape measure along the ground. Take the top and base readings and add or subtract them to get the apparent height. Of the four nomograms on the card, choose the one corresponding the measuring distance. Locate the apparent height on the right hand scale. On the double scale on the left, locate the reading obtained from sighting the base of the tree. Note that readings for falling and rising gradients should be taken from different sides of the scale. Connect these two points of the nomogram with a straight line. The centre scale of the nomogram now indicates the true height of the tree.
