Measuring up

HEIGHT

When people see a tall tree, the first question they ask is "How tall is it?". You would need a long ladder to take a tape measure to the top of many trees but the problem can be solved with the use of basic trigonometry.

People who work with trees normally talk about two types of height - total and timber heights.

• (a) The total height of a standing tree is the vertical distance from its base to the uppermost or highest point or tip. Care is needed to make sure the person doing the measuring really can see the true top of the right tree and in woodland that can be difficult if the crowns interlace.
• (b) The timber height which is the vertical distance from the base of the tree to the point on the main stem where the diameter is at least 7cm including the bark.

Timber height is really from ground level up to the highest point on the main stem or trunk which is usable for conventionally saleable timber - and 7cm diameter is the minimum size for a trunk to be useful for timber.

Exactly where that point is, is easier to determine in theory than practice. Things like side branches, just where ground level is at the base of a tree growing on a slope and other imponderables need taking into account.

Tree height can be found using a device called a hypsometer - or a builder's plastic clinometer or angle gauge - or sonic and laser technology can be employed - or a DIY method with a rod or bamboo cane shown below.

Choose a rod as long as the distance from your eyes to your finger tips with one arm fully stretched. Walk back for about 1-1½ times the rough height of the tree where you can clearly spot the tree top or timber height. Stretch out your arm holding the stick vertically and walk backwards or forwards until the top of the stick coincides with the point you want on the tree and the bottom at the bottom of the trunk. Then pace or measure the distance from where you are standing to the base of the tree - and that is the tree height.

VOLUME

In paragraphs 5 + 6 of this section there were 4 x "pie" signs which did not copy over.
Foresters are interested in knowing the volume of timber in individual or stands of trees. If you treat trees as tapered cylinders, you can calculate the volume of timber with a good measurement of the diameter or girth of the trunk, the length of the useful timber-yielding section and tariffing tables.

When it comes to measuring trunk diameter, for forestry purposes this is done at breast height - by convention, the dbh (diameter at breast height) is 1.3m above ground level.

But slight adjustments are needed for trees growing on a slope, or leaning or forking low down or with a misshapen trunk.

Callipers are used to measure trunk diameter directly. With callipers, it is best to take two measurements with trunk diameter at right angles to one another and use the average.

Or non-stretch tapes can be used to measure the circumference which is then converted to diameter. Remember from school that the circumference of a circle is x d where (pi) is a constant (3.142). So dividing the circumference you measure by gives the dbh.

Special girthing tapes do the mathematics for you - they are already marked off to take (pi) into consideration so you can read off the diameter in centimetres directly from the tape.

It would be impractical to measure each and every tree to estimate the total volume of useful timber in a stand or compartment of a larger woodland or forest. So foresters sample random plots in a woodland and count the number and species of trees and take their height and dbh. From that they calculate the species composition, size distribution (by dbh), height, basal area and timber volumes.

That is tree measurement in a nutshell but there are many refinements - the posh name for measuring trees is forest mensuration. Measuring felled timber is quite complicated too.

More: FC Booklet 39 - "Forest Mensuration".
FC Booklet 49 - "Timber Measurement: A Field Guide".