1. Summary
The sustainable management of forest ecosystems requires the regular monitoring of the status and changes in renewable natural resources. Data for these criteria are usually provided by forest inventories. The forest inventory data are not only used for decision making in forest management practice, but often serve as an empirical platform for various research activities. In traditional forest inventories with multiple sample plots, tree attributes are manually measured using mechanical or optical instruments, such as calipers, hypsometers, compasses, and measuring tapes [
1,
2,
3,
4]. The most relevant outcome from a forest inventory is the estimate of the growing stock timber volume. For this purpose, average per-area-unit values from the multiple sample plots are up-scaled to the entire survey region. The calculation of the per-area-unit values of the growing stock requires diameter and height measurements of single trees in each sample plot. These diameter and height measurements are used as input variables in stem taper models providing single-tree volume estimates that are subsequently summed to produce the intended per-area-unit values. Besides measuring the tree heights and diameters, forest inventory fieldwork also involves the registration of the tree positions to determine whether a particular tree lies within the boundaries of a sample plot area. In conclusion, measurement of diameter, height, and stem position is mandatory when forest inventory must provide total estimates of growing stock.
Since the beginning of forest inventory practice, the instruments, measurement techniques, and sampling protocols have been continuously enhanced [
5]. Only recently has the precise registration of the complete forest structure in 3D become possible with light detection and ranging (LiDAR) technology. Since terrestrial laser scanners became available about 20 years ago, studies were initiated to determine their possible usage in the forestry context and in forest inventory applications in particular (e.g., References [
6,
7,
8,
9,
10]). The first-generation terrestrial laser scanners required static positioning, which necessitated relocating the scanner and conducting multiple scans from different positions (multi-scan approach (MSA)). Besides the standstill time during the scan process, the MSA consumes extra time and incurs additional labor costs for the repeated transportation and set-up of the scanner. However, even under the MSA, shadowing effects are likely, and non-detections of trees easily occur [
6,
9,
11]. Airborne laser scanning (ALS) (e.g., References [
12,
13,
14,
15,
16]) is able to cover large areas (up to regional level) and provides an opportunity to complement ground-based inventories, which provide more detailed information, but sample only limited areas. However, ALS systems are generally unsuitable for deriving accurate and detailed information of individual trees (tree position at breast height and dbh). Three-dimensional (3D) ALS point clouds represent tree stems too sparsely, and the derived information largely depends on the quality and quantity of field reference data [
17,
18]. Recently, mobile laser scanning (MLS) systems have become available, which circumvent the high standstill costs and the incomplete coverage associated with the static positioning of the terrestrial laser scanning TLS systems (e.g., References [
17,
19,
20,
21,
22,
23]). Hence, as reported by Liang et al. [
17], MLS enables the survey of a larger forest site within a given time budget. However, the 3D point cloud data acquired by MLS are often less precise and much noisier than the TLS point cloud data because the positioning errors of the MLS point cloud data propagate along the walking path [
17,
22,
24]. In modern MLS systems, especially in those constructed as handheld devices, the laser distance measurements in the 3D space are referenced during the travel with the laser sensor using a simultaneous localization and mapping (SLAM) algorithm. Laser scanning systems that provide such SLAM technology are hereafter termed personal laser scanning (PLS) systems in this article.
Much effort is being dedicated to developing novel software routines through which the relevant single-tree characteristics, including diameter, height, and position, are automatically derived from 3D point cloud data, which were collected by LiDAR sensors. The major goal of these approaches is to present more efficient and more precise measurements than the traditional forest measurement techniques that have, thus far, been used in forest inventory. However, in most of the recent research applications, each study uses its own data for training and performance tests, and the respective data are often not made publicly available, which makes it hard to compare the performance of the different algorithms under standardized conditions. Besides these circumstances, fair comparisons are also hindered as the various studies have used different hardware properties, scanning set-ups, and performance criteria.
In Liang et al. [
5], an international benchmark study was conducted with TLS data collected by MSA, showing that the single-tree detection rate of different software algorithms ranged from 20 to 100%. A major conclusion of that study was that a fair benchmark test requires three preconditions to be met: (1) Identical TLS data are used by all candidate approaches, (2) a catalog of the unique plot- and tree-level target variables are defined; and (3) the results from the different algorithms are evaluated in comparison with reliable reference information that is independently measured.
The dataset (doi:10.5281/zenodo.3698956 (version 1.0) [
25]) presented in this data descriptor contains co-registered raw 3D point-cloud data collected from 20 forest inventory sample plots in Austria. The point cloud data were measured using two different laser scanning systems: (1) A PLS system (ZEB Horizon, GeoSLAM Ltd., Nottingham, UK [
26]) and (2) a static TLS system (Focus
3D X330, Faro Technologies Inc., Lake Mary, FL, USA [
27]). The dataset also includes digital terrain models (DTMs) and reference data. The latter was obtained by field measurements and can serve as ground truth data. The reference data are comprised of tree positions, tree species information, and measurements of diameter at breast height (dbh), crown base height, and tree height. Finally, we also included the results of our software routines for automatic tree detection and automatic stem diameter measurement, which were recently published in Gollob et al. [
28].
The entire dataset will enable fair comparisons of different algorithms using both PLS and TLS data collected from the same sample plots. The PLS device (ZEB Horizon, GeoSLAM Ltd., Nottingham, UK [
26]) is a new technology that, to the best of our knowledge, has so far only been used by Gollob et al. [
28] in a forest inventory context. The publication of the PLS point cloud data will help others to enhance their software routines using these novel data.
5. Discussion
Providing datasets is regarded as essential for benchmarking and comparative performance tests of the different algorithms, especially in the field of forest inventory, where novel measurement technology for LiDAR-based sensors is currently being introduced. Only if the same data basis and the same criteria are used, it is possible to reveal the true potential of PLS compared with TLS in particular. Beyond the approaches for tree detection and dbh measurement that were presented in Gollob et al. [
28], the dataset also offers the possibility of automatically estimating tree heights and crown bases from PLS and TLS data, and it also enables the comparison of the achieved results with the provided reference data. Besides creating new DTMs, other developers can alternatively access the DTMs provided with this dataset. This will probably help to avoid possible confounding effects that may occur when different routines are used for vegetation/ground classification or for the spatial interpolations of DTM grids. However, a direct comparison of the DTMs from both sensors (static TLS and portable PLS) is not possible because the DTMs derived from both sensors differ in an offset, which varies per plot. It should be noted that the created DTMs were only valid locally. If DTMs were required for larger areas, it would be beneficial to use ALS data. In addition to the abovementioned possible use cases of the dataset, it is also worth noting that, to the best of our knowledge, it is the first publicly available dataset of the GeoSLAM ZEB HORIZON in a forest inventory context. This enables the reader to assess the data quality provided by the novel device.
Regarding the reference dataset, it is worth noting that these field measurements were recorded with pencils on paper and thereafter manually transcribed into an electronic database. Thus, the existence of possible entry errors cannot be excluded. Other possible errors in the manual measurements may result from imprecise height, diameter, and position measuring, or tilted trees. For the comparison of algorithms, however, this is only a minor problem, since all algorithms are evaluated against the same reference data. Regarding the point cloud quality, it is worth noting that in the presented dataset, instrumental drift and registration inaccuracies were neither a problem for PLS nor TLS.
Initial tests on large experimental stands showed that for PLS, longer recording times (greater than 30 min) yielded instrumental drift and registration problems, while for TLS, an increasing number of scans complicates co-registration and adds additional noise. However, a list of tree positions and dimensions could be created more completely and efficiently with PLS. With TLS, the tree dimensions (especially the diameters) were more precise.