A dataset of tree heights in mangrove and non-mangrove trees in Malaysia derived from multiple measurement methods

The dataset contains tree height data collected in 200 mangrove and non-mangrove trees sampled in various sites in Malaysia. Different height measurement methods were performed, including visual measurements (stick, thumb rule) and precision field instruments (clinometer, laser rangefinder and altimeter), which were compared against benchmark values obtained using an unmanned aerial vehicle (UAV) and a Leica distometer. The core data have been analysed and interpreted in the paper by Saliu et al. ‘’An accuracy analysis of mangrove tree height mensuration using forestry techniques, hypsometers and UAVs ’’ [1], in which the accuracy of each method for tree height measurement was discussed.


Value of the Data
• Due to limited scientific investigations on this subject, the present data provides a valuable information on mangrove and non-mangrove tree height measurements obtained from different forest inventory techniques. • The data can be used to develop tree height-diameter allometry in mangrove and nonmangrove species in Malaysia, which can be used for further forest inventory applications, considering the difficulty of tree height measurements in mangrove species. • The present dataset comprises 200 tree height measurements considered to be beneficial to other tree height studies in Malaysia for statistical validation, accuracy assessment and forest biomass derivations. • The outcomes of this study would be able to help the researchers elsewhere to acquire most reliable tree heights by selecting appropriate tools, along with less labour and time saving benefits.

Data Description
The data reported in Table 1 , that was analysed and discussed in the paper by Saliu et al . [1] (173 out of 200 trees were used for the analysis due to some missing data), represents a description of the sampling locations where the individual tree height measurements were obtained through different forest inventory techniques i.e., thumb rule, stick method, clinometer, laser rangefinder and altimeter. Control heights were obtained with the help of an Unmanned Aerial Vehicle (UAV -DJI Phantom 3 Professional) and a distometer. It is also noteworthy that the trees being sampled in the UMT campus and at the MMFR cafeteria were largely isolated individuals, while in the compartment 19A the trees were located adjacent to a closed canopy.

Experimental Design, Materials and Methods
Tree height measurements in the UMT and MMFR sites were acquired through different forest inventory techniques i.e. , thumb rule, stick method, Suunto PM -5/360 PC clinometer (Finland), Nikon 550 Forestry Pro laser rangefinder (Republic of Ireland) , and Blume Leiss BL 60 altimeter(Germany), along with an Unmanned Aerial vehicle (UAV -DJI Phantom 3 Professional, China) and a Leica Geosystems Distometer D2 Bluetooth (Switzerland), the latter considered as control measurements. All trees were randomly selected and marked (with permanent marker) for cross checking, if necessary.
For the thumb rule, the tree height was measured by stretching the arm out such that the top of the thumb aligned with the top of the tree and the base of the fist aligned with the base of the tree. While maintaining the same position, the observer rotated the thumb horizontally such that the base of the fist still aligned with the base of the tree. The tip of the thumb on the ground was marked and its distance from the base of the tree was measured as the tree height ( Fig. 1 A).
By following the stick method, the observer held a ruler while stretching out his arm and standing at a distance from the tree such that the top of the tree is aligned with the top of the ruler and the base of the tree is aligned with the hand holding the ruler ( Fig. 1 B). Once aligned, the distance from the hand grasping the base of the ruler to the observer's eye ( = armlength), the distance from the hand to the top of the stick and, the distance from the observer to the base of the tree were all measured to estimate the tree height by following equation ( Eq. 1 ) - where, HS is the length of stick from observer hand to its tip, d is the distance between observer and tree, and AS is the armlength. The clinometer was operated by looking at the top of the tree with one eye and at the degree scale inside the device (angle α coinciding with tip of the tree) with the other eye simultaneously. The height of the observer's eye above the ground ( H2 ) and the distance between the observer and the tree ( d ) were also recorded using a measuring tape. This method was carried   showing results from measurement between two points (adopted from https://www.nhbs.com/ nikon-forestry-pro-laser-rangefinder ). E) BL 60 Altimeter. F) UAV and Leica distometer and monitor display of the UAV camera at the canopy layer of the tree; the red circle indicates the tip area of the tree. When the tip is visible on the display monitor, the Leica distometer is shot to the drone its distance to the drone is measured as H1. The distometer is also used to measure the height above the ground level (H2). Height of tree = H1 + H2 (Adopted from Saliu et al ., 2020 [1] ). the device was shot at the treetop while the lower button was pressed when shot at the base of the tree. Holding these white buttons releases the corresponding needles and releasing the buttons fixes the needles. The height between the observer and the treetop/tree base can be read directly on the scale. The height measurement was done with the observer standing at the same ground level as the tree ( Fig. 1 E). Each tree height measurement was made from a specific distance, either from 15 m, 20 m, 30 m or 40 m depending on the visibility of treetop and base, for the clinometer, laser rangefinder and altimeter. However, for thumb rule and stick method, the distance of observation went farther than 40 m (in the case of compartment 19A at the MMFR).
We flew the UAV from the base of a tree to the tip of the canopy vertically and recorded the altitude of above ground or sea level [2] displayed on-screen of a video tracking device (mobile phone: Huawei Nova 2 lite). The tree height was measured by pinpointing the UAV when it reached the top of the tree, visible from the UAV camera. This height was further confirmed by pointing a distometer to the base of the UAV ( Fig. 1 F). Saliu et al. (2020) discussed the implications of camera position, camera angle, tip recognition, wind and distometer accuracy on error in this approach.
Tree diameter at 130 cm ( D 130 sensu Brokaw & Thompson [3] ) above the ground or along the stem was measured for all trees using a diameter tape [4] . The angle of inclination was used as a proxy to measure the leaning nature of trees. This was done by placing a steel protractor at the base of the tree, and the angle at which the tree deviates from straightness (90 °) was considered.

Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships which have, or could be perceived to have, influenced the work reported in this article.