TIMBER MEASURING
To those who are not practically acquainted with the measuring of
home-grown timber the following brief remarks in elucidation of the
subject will be useful. It may, however, be well to mention that timber
measuring is rather a vexed question, some following what is known as
Hoppus’s system, and others advocating that of Horton. The former being
that generally in use amongst timber merchants in this country, and
consequently of greatest value to the forester, the following details
of this system may prove serviceable.
Regarding the timber-measurer’s equipment it may first be necessary to
say a few words. This consists of a 66 ft. Chesterman’s tape-line, or
instead of this a 5 ft. wooden rod, standard girt-strap, or fine cord,
scribing knife, and bent piece of iron, with eye at end for drawing the
girt-strap beneath such trees as the arm cannot readily pass under.
For girthing timber a piece of thin whipcord or string is frequently
used, but as the elasticity of this varies greatly, and has in many
instances led to dispute, a much fairer plan and one that is liable
to no abuse is to use the 12 ft. girt-strap, upon which every inch in
length is reckoned ¼. As the proper quarter-girth can be seen at a
glance on this strap, its adoption will at once remove any chance of
trickery, which may be possible in the use of the string and rule.
For entering measurements the most convenient book is that 9 in. by 4
in., with stiff pasteboard covers, ruled with horizontal lines, and
divided into four vertical columns. To measure proceed as follows:—Mark
a number with the scribe on the butt end of the tree, and enter a
similar number in the first column of the book; this will not only
serve to identify the particular log, but prevent any risk of measuring
twice. Should the taper throughout the whole length of the tree be
tolerably gradual, set down the length in the second column of the
book, opposite the number already entered. At exactly one-half of
the length of the portion measured, take the girth by passing the
girt-strap tightly around the stem. Put this down in the third column
on the same horizontal line as the number and length. Should, however,
the trunk taper not be fairly uniform throughout the entire length, as
frequently happens, several measurements may require to be taken.
For example, a tree may be 36 ft. in length, running with regular taper
for perhaps 12 ft., after which it branches out, reducing the size of
the remaining part very considerably for, say another 12 ft., where it
again branches and leaves the last 12 ft. of a relatively small size.
With such a tree it would be quite impossible to obtain anything like a
correct measurement by taking only one length and girth. The difficulty
is, however, readily got over by first measuring the lower 12 ft., then
the second, and then the third, marking the respective lengths and
girths in the vertical columns as already described.
The measurement of these trees, so far as the field work is concerned,
is now completed, the contents of each tree being found by referring
to “Hoppus’s Measurer”—a book with which every forester should be
supplied. By squaring the quarter-girth in inches, multiplying by the
length in feet, and dividing by 144, the same result will be obtained.
This is, however, a tedious method, especially where large numbers of
trees have to be dealt with, and should only be adopted when Hoppus is
not at hand.
By committing to memory the following short table of quarter-girths
much time in calculating and consulting authorities will be saved.
6-inch quarter-girth will give contents equal to ¼
the entire length in feet.
7 ” ” ” ⅓
8½ ” ” ” ½
10 ” ” ” ⅔
12 ” ” ” 1
13 ” ” ” 1¼
14¾ ” ” ” 1½
16 ” ” ” 1¾
17 ” ” ” 2
19 ” ” ” 2½
21 ” ” ” 3
22½ ” ” ” 3½
24 ” ” ” 4
27 ” ” ” 5
29½ ” ” ” 6
31¾ ” ” ” 7
34 ” ” ” 8
36 ” ” ” 9
The sliding rule is also useful for determining contents.
The proper allowance to be made for bark is half an inch for every
foot of quarter-girth for oak and elm under 12 in. quarter-girth, and
an inch for all beyond, but it is quite impossible to fix upon any
one uniform scale that will meet even the majority of circumstances.
Trees growing in exposed situations will frequently have bark almost
double the thickness of those of a similar size in the woodland. My
plan has been always to allow for the bark of each tree at the time of
measurement.
=Measuring Standing Timber.=—For this a pliable pole 18 ft. long,
marked in feet, and the girt-strap already referred to are the
necessary equipment. In estimating the number of feet of timber upon a
large area, it is not always necessary to measure each tree separately,
particularly when the whole situation is composed of one species, and
the individual trees are about the same age and size, as by multiplying
the total number of trees by the average content of those selected and
measured a very just calculation will be arrived at. Great care in
their selection, and considerable judgment in taking the average will,
however, be required.
[Illustration: MEASURING HEIGHT OF TREES]
When each tree is to be measured separately two assistants will
be required, one to carry the 18-ft. pole, and the other the
girting-strap. Sometimes, when the timber is of great height, jointed
bamboos are used, and a light ladder brought into requisition.
In carrying out the work in this way, the man with the pole declares
the height of the tree, and the one with the tape the quarter-girth.
=Measuring the Height of Trees.=—There are several methods of
ascertaining the heights of trees, but the two following are, perhaps,
the most simple, and the appliances necessary quite inexpensive:—
No 1.—Take three laths, such as bricklayers use for tiling, and nail
them in the shape of the frame shown; _a a_ must be of equal length;
_a_ and _b_ being placed on the ground, the eye must follow up the
larger lath _d d_ until it is in a line with _e_, the top of the tree
or object you wish to measure.
The frame must be placed as level with the bottom of the tree as
possible. Should the ground be very uneven you must give and take
accordingly.
It will be seen that _b_ to _c_ is the same length as _c_ to _e_, and
this gives the height of the tree.
No 2.—Suspend the triangle between the thumb and forefinger of the
left hand, knuckles down, upon the point _a a_, allowing it to swing
freely. The edge _b c_ will then fall perpendicularly, and _c d_ will
be horizontal. The remaining edge _d b_ will then lie at an angle of 45
to the horizon. On this edge are two sights, _e_ and _f_. Look through
_e_ until _f_ is aligned with the tree-top, advancing or retiring till
the sights point exactly to it.
Then, if the observer’s feet are level with the tree root, the height
of the tree is the distance from his feet to the root, plus the height
of the eye from the ground.
The dendrometer is perhaps the most useful instrument for taking the
height of a tree and can be procured from some of our nurserymen.