Measuring Trees

Many trees have low branches, burrs, or multiple stems that affect where a measurement should be taken. When this happens, the aim is always to find the smallest meaningful measurement below 1.5 metres and make a note of where that measurement was taken.

Not every tree fits neatly into a category. Ancient, damaged and heavily layered trees (like Lime) can present unique challenges. In these cases, record as much information as possible and include notes about what you’ve measured. Other examples of difficult to measure trees include phoenix trees which have fallen over but grown upwards again.

Because trees of the same species may grow differently they are not always comparable.

We record their tree form as A, B, C, D, E or U

See these definitions on our Information page here

The tree heights presented in the Tree Register have been recorded in various ways. In an approximate order of descending accuracy, these methods are:

Climbing to the top of the tree and dropping a tape measure to the bottom (using a pole to reach the true top if this seems too slender to bear the climber’s weight). The only limit to the accuracy of this method, assuming the hanging tape is not deflected by branches, lies in deciding on the exact level of the tree’s base. (The Tree Register does not endorse this method, except when a tree needs to be climbed for a safety inspection or safety work, since climbing can potentially damage the tree. The exact locations of many of our tallest trees are withheld, at the landowners’ request, to discourage recreational climbs.)

Using a laser range-finder (the sine method). The distance from the observer to the tree’s highest point is measured, and the distance from the observer to the tree’s base. The angle above the horizontal to the tree’s highest point is also measured, and the angle above or below horizontal to the base, allowing the vertical distance from the base to the top to be calculated. Modern devices incorporate in one body the range-finder, a clinometer to measure the angles, and a computer to calculate the resultant height. Devices such as the Nikon Forestry Pro can achieve an accuracy of about  25cm, particularly if a tripod is used or repeated measurements are averaged; more expensive devices can be accurate to within 10cm. The shortcomings of this method are that the tree’s true tip may not be visible, leading to an underestimation of the height if the laser-beam only hits a point on the tree’s near side. The tree’s base may also be obscured by vegetation. A refinement of this method, which has been used to measure the heights of slender trees in dense plantations, is to fly a drone exactly level with the tree’s top, and to measure the vertical distance of the drone from an observer exactly level with the tree’s base.

The external baseline method. Until the advent of laser technology this was the most precise way of measuring trees, but was seldom used because of the complex calculations involved. The bearing and distance to the tree’s base from two fixed points on the ground are measured, along with the angles to the tree’s true top from both fixed points; a 3D model can then be calculated to give the vertical distance from the tree’s apparent top to its base, even if the top is not above the base.

Using LiDAR data. A plane-mounted laser beam bounces off the contours of tree crowns and the ground beneath them to create a three-dimensional point-cloud. Professional software allows the vertical distance to be calculated between the highest points of features recognisable as trees and the apparent ground level at their base. The laser beam however will miss the slender leading shoot of some fast-growing conifers, and in some instances a leaning crown or a steep slope at the base will make it hard to determine the level of the tree’s actual base. If the data is recorded in winter, the laser beam will also tend to miss the uppermost twigs of deciduous trees.

Photo: LiDAR imagery provided by Håvar Bunes when helping serach for the tallest tree in Cumbria in 2024

Using Google Earth. Some LiDAR point-clouds are visualised, in combination with aerial photographs, as the 3D imagery that is available within Google Maps for parts of Britain and Ireland (mostly cities and National Landscapes). The Google Earth app allows users to measure tree heights by hovering a cursor over the top of a tree’s crown and over the ground at its base, noting the altitudes that are displayed, and subtracting the second altitude from the first. This method is only reliable when the ground is fairly level or the base of the tree is visible.

Using a drone. The drone’s onboard altimeter can be used to measure the tree, by subtracting the altitude at the tree’s base from that of a point level with its top. The altimeter is likely to be accurate to within about a metre, although sudden changes in the ambient barometric pressure can influence this.

Using a clinometer or hypsometer (the tangent method). Until the advent of laser range-finders, this was the only straightforward method available to measurers on the ground. From a measured horizontal baseline, the angle is measured to the tree’s highest point, and the angle to the base, and simple calculations are made to record the apparent vertical distance from top to base. (Hypsometers are calibrated to show the height directly.) Except in the case of a perfectly straight spire-tipped tree, this method can grossly over-estimate trees’ heights; the measurer needs to use a combination of experience, spatial awareness and common sense to work out the displacement of the tree’s top relative to its bottom, or the likely highest point of a broad-crowned tree when this is hidden by branches arching out towards the observer. However, this method is still extensively used when the tree’s true top cannot be seen from the ground, and measuring devices that incorporate lasers usually still offer a tangent (or ‘3-point’) option for calculating heights, sometimes by default.

Sine and tangent methods can be used in various combinations, and very many heights on the Tree Register have been made via such hybrid methods. Very often, if the true top of a tree is visible, the base is hidden by vegetation, and the sine method can be used to calculate the height of the tree’s top from the observer’s position, while the tangent method is used to estimate the height from the tree’s base to the observer’s position. The inaccuracies inherent in the tangent method will only become significant if the slope to the tree’s base is steep and the tree also leans.

Experienced observers can learn to estimate trees’ heights with considerable accuracy, and recorders may use this method if the tree is small and its height is not considered critical.

If the tree has been recorded using the most accurate of these methods, its height may be given to the nearest centimetre on the Tree Register (with the caveat that the highest soil-level around the roots can change by several centimetres from year to year). Generally, if the height is presented to the nearest 10cm, this will indicate that a reliable method is known to have been used, such as the sine method. However, the heights of trees recorded using the tangent method are sometimes also presented to the nearest 10cm, even though the true height may be several metres less than the reported figure. Generally, the comments section will note if the presented height is suspected to be inaccurate.