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| Height Method 2 - using a clinometer |
Since buying a clinometer, our method of measuring trees has changed.
You can find a lot of information about using Clinometers on the internet. |
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Briefly, you stand at a point as far from the centre of the trunk of the tree as its estimated height, so that the elevation angle is somewhere around 45°, this gives the best "geometry." Measure the distance (D) from the tree with a tape. Then stand at the mark and sight the top of the tree, finding its elevation angle  . The height (H) of the tree is then:
H = D tan + HI |
| (HI is the measurement from the ground to your eye). See the diagram. |
You could do all the above maths if you are feeling a little masochistic but the good news is: you don't have to!
All you need do is set your clinometer to 45° then walk back until your sight is aligned with the top of the tree, taking care not to fall into any ponds in the process. Mark the point in the ground at which you are standing and then set your clinometer to 0°. Now find the point on the trunk of the tree which is aligned with your sight; this will be the point that is level with your head. Now simply measure the distance from your mark in the ground to the tree and add to this the height from the ground to the 0° point on the trunk of the tree. Hey presto! Height found with no maths apart from a simple addition.
For those of you fortunate enough to have one of the modern compact clinometers shown in the right hand photograph below, here is a useful tip to bear in mind. The internal rotary scale is damped by being sealed in a fluid filled container. You might find that (particularly in cold weather), the dial does not respond to changes of tilt as quickly as you would like. This can be eased greatly by warming the device prior to use. They are usually on a cord, so I have found that by hanging it inside my clothes it is warm enough to respond really well. Painful at first when cold though! A slight tapping of the device also helps to overcome hesitancy on the dial to obtain a spot on reading. |
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There is another type of clinometer with which you simply measure to a predetermined distance from the base of the tree, aim to the top and adjust the rotary part. A scale is then provided which gives the distance reading directly. A reasonable quality model like this tends to be rather expensive, feels like cheating  , and is probably not as accurate as a comparable quality standard type clinometer used at 45°.
Compact, fluid damped modern clinometers are really good, but they do still need careful and experienced use to get really accurate results. It is important to take the time to ease the machine up and down finely several times, watching the scale move appropriately as you gently tap the side. This will ensure that you don't get a false reading owing to the dial having been a little reluctant to move. This is not a matter of being pedantic, simply of understanding and working around the limitations of the instrument in order to get the best results possible. |
| Height Method 3 - using a stick |
If you do not own a clinometer, there is a way of getting a surprisingly accurate measurement of a tree's height using just a stick and a tape measure and your eyeballs! Here's how:
Hold a stick at arm's length vertically, between forefinger and thumb. The important thing is to make sure that you have the same length of stick above your thumb as is the distance between your eyeball and your thumb. The easiest way to make sure that the stick is the correct length above your thumb is to carefully tilt it down towards your face, while keeping your thumb at the same distance. Naturally you will want to avoid poking your eye out but the stick should come level with your eyes.
Now, with the stick vertical once more, align the tip of your thumb with the base of the tree and move back away from the tree until the tip of your stick is level with the top of the tree.
You need to take care not to move your hand or your head whilst checking the alignment of both top and bottom of the stick, because this will introduce errors. Swivel your eyeball instead, keeping your head very still. There will still be a small error factor from the movement of your eyeball but there is little that can be done about that.
At this point we can say with a fair degree of accuracy that the height of the tree is equal to the distance from your feet to the tree. |
There are a couple of sources of inaccuracy that should be borne in mind and which you can minimise once you are aware of them.
- The swivelling of your eyeball and the distance that this effectively moves. I believe the effect of this is fairly minimal though.
- Ideally, for most accurate results, the bottom of the stick should be at the same height as the base of the tree. Even on level ground this will not be the case because you cannot put your eyeball on the floor! The distortion that this introduces to the right angled triangle we are striving to achieve will in practise have a fairly small effect on the accuracy for a good-sized tree but you can minimise the effect by crouching down or even sitting on the ground if it is dry. This problem is compounded, however, if the ground slopes significantly as you move away from the tree. The distortion may become too great to get a reasonable measurement.
If the ground slopes upward from the tree and you are good at judging the level point from your measuring distance, you could simply visually mark the level point (with your thumb) on the trunk and add the height from the base of the tree onto your distance to the tree.
If the ground slopes downward from the tree, you could be quite lucky and your head could be level with the base of the trunk, in which case you would not need to crouch down nor make any adjustments. If it slopes a little less, this is not a huge problem (crouch a little) but if the slope is so steep that your level line of sight is well below the base of the trunk, there is not much you can do without purchasing a clinometer.
Thanks to Chris for sending the idea for this method. |
| Height Method 4 - using a stick and the sun |
Place a stick in the ground making sure it is standing vertically. The length of the stick needs to be around a metre or so for best effect. Draw a semi-circle on the shadow side that is precisely as far from the base of the stick as is the height of the top of the stick from the ground.
At this point you need to unfold your comfy chair and open a nice cool drink, because you could be in for a long wait! The idea is to wait until the tip of the shadow of the stick touches the semi-circle drawn on the ground. Now you must rush over to the tree (every second will count) and mark the tip of the shadow that the tree casts on the ground. Measure the distance from this mark to the base of the tree and that will be the height of the tree.
As with Method 3 there are a few sources of inaccuracy to bear in mind.
- The ground around the tree and the measuring stick must be level. You could in theory compensate for slopes by holding a piece of paper at the same height as the base of the stick when the time is close, allowing the shadow to be cast on the paper instead of the ground, and then doing the same for the tree. This involves a bit of judgement on your part, or the use of a simple spirit level that you may have about your person(!)
- If the stick is too short, say a foot or so, any minor inaccuracies in your measuring of the stick and its shadow will be multiplied to a significant error when measuring the tree's shadow. A metre or so should suffice, but two metres would be fantastic.
- The stick needs to be mounted vertically in the ground reasonably accurately. If you have a keen eye for these things you should be ok, otherwise the use of a spirit level or a simple plumb line may be advantageous.
- Because we are relying on a shadow rather than a line of sight, this method will not work for a tree that has a very broad, flattened top. Most Redwoods are reasonably narrow at the top and so well suited to this method, but it can be a problem for damaged Redwoods or other types of tree (such as oak).
Thanks to Ricardo Gomes for sending this simple yet effective method.
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| Summary - which method should I use? |
For a truly accurate measurement of a tree's height to the nearest few centimetres, as required by researchers checking for tallest tree status, there is of course no substitute for climbing to the top with one end of a tape measure between your teeth. For the rest of us one of the above methods should more than suffice. A few points should be borne in mind however, the most significant of which is that we need to be wary when a tree is leaning heavily to one side as this will have an adverse effect on accuracy unless the tree is measured perpendicular to its lean.
Also, please bear in mind that when the ground around the circumference of the base of the tree is uneven, you must use the highest point as the ground level (as when you are deciding the 1.5m height above ground for taking a girth measurement.
Which Method is most accurate? This will generally depend upon the circumstances. Method 2 (clinometer) is best in my view. It is usually the most accurate and is certainly the most versatile, with experienced use it is possible to make adjustment for difficult conditions such as heavily sloping ground or inability to measure from a location that provides a 45° angle to the tree top.
On perfectly level ground, and provided sunshine and timing permit, Method 4 (stick and sun) will be the next most accurate as long as a big enough stick is used. It is a great way to measure a tree in your local park, when you have plenty of opportunity to stroll down and do the measuring when the conditions are right, but not ideal when you are out and about and chance upon a tree on your travels.
Otherwise, I suspect that Method 3 (the stick at arm's length) will be the next most accurate. I cannot say for certain that this would yield better results than Method 1 (camera and steel rule) as there are too many variables to consider, for example camera lens and perspective distortion.
If you send tree measurements to Redwood World, please remember to tell us the date and the method that you used to determine the height.
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