Suunto PM-5/1520 Handbuch Seite 3

On level ground, the tree top readings is usually sufficient (see fig. 3): 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. Note that the nomogram meant
for PM-5/1520 or vice versa because of different way of establishing the
basic 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.