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Accuracy Assessment of Digital Terrain Models of Lowland Pedunculate Oak Forests Derived from Airborne Laser Scanning and Photogrammetry

volume: 39, issue: 1

Digital terrain models (DTMs) present important data source for different applications in
environmental disciplines including forestry. At regional level, DTMs are commonly created
using airborne digital photogrammetry or airborne laser scanning (ALS) technology. This
study aims to evaluate the vertical accuracy of DTMs of different spatial resolutions derived
from high-density ALS data and existing photogrammetric (PHM) data in the dense lowland
even-aged pedunculate oak forests located in the Pokupsko basin in Central Croatia. As expected,
the assessment of DTMs’ vertical accuracy using 22 ground checkpoints shows higher
accuracy for ALS-derived than for PHM-derived DTMs. Concerning the different resolutions
of ALS-derived (0.5 m, 1 m, 2 m, 5 m) and PHM-derived DTMs (0.5 m, 1 m, 2 m, 5 m,
8 m) compared in this research, the ALS-derived DTM with the finest resolution of 0.5 m
shows the highest accuracy. The root mean square error (RMSE) and mean error (ME) values
for ALS-derived DTMs range from 0.14 m to 0.15 m and from 0.09 to 0.12 m, respectively,
and the values decrease with decreasing spatial resolution. For the PHM-derived DTMs, the
RMSE and ME values are almost identical regardless of resolution and they are 0.35 m and
0.17 m, respectively. The findings suggest that the 8 m spatial resolution is optimal for a
given photogrammetric data, and no finer than 8 m spatial resolution is required. This research
also reveals that the national digital photogrammetric data in the study area contain certain
errors (outliers) specific to the terrain type, which could considerably affect the DTM accuracy.
Thus, preliminary evaluation of photogrammetric data should be done to eliminate possible
outliers prior to the DTM generation in lowland forests with flat terrain. In the absence
of ALS data, the finding in this research could be of interests to countries, which still rely on
similar photogrammetric data for DTM generation.

Application of Black-Bridge Satellite Imagery for the Spatial Distribution of Salvage Cutting in Stands Damaged by Wind

volume: 40, issue: 1

Salvage logging is performed to remove the fallen and damaged trees after a natural disturbance,
e.g., fire or windstorm. From an economic point of view, it is desirable to remove the
most valuable merchantable timber, but usually, the process depends mainly on topography
and distance to forest roads. The objective of this study was to evaluate the suitability of the
Black-Bridge satellite imagery for the spatial distribution of salvage cutting in southern Poland
after the severe windstorm in July 2015. In particular, this study aimed to determine which
factors influence the spatial distribution of salvage cutting. The area of windthrow and the
distribution of salvage cutting (July–August 2015 and August 2015–May 2016) were delineated
using Black-Bridge satellite imagery. The distribution of the polygons (representing
windthrow and salvage cutting) was verified with maps of aspect, elevation and slope, derived
from the Digital Terrain Model and the distance to forest roads, obtained from the Digital
Forest Map. The analysis included statistical modelling of the relationships between the process
of salvage cutting and selected geographical and spatial features. It was found that the higher
the elevation and the steeper the slope, the lower the probability of salvage cutting. Exposure
was also found to be a relevant factor (however, it was difficult to interpret) as opposed to the
distance to forest roads.

Automated Volumetric Measurements of Truckloads through Multi-View Photogrammetry and 3D Reconstruction Software

volume: 40, issue: 1

Since wood represents an important proportion of the delivered cost, it is important to embrace
and implement correct measurement procedures and technologies that provide better wood
volume estimates of logs on trucks. Poor measurements not only impact the revenue obtained
by haulage contractors and forest companies but also might affect their contractual business
relationship. Although laser scanning has become a mature and more affordable technology in
the forestry domain, it remains expensive to adopt and implement in real-life operating
conditions. In this study, multi-view Structure from Motion (SfM) photogrammetry and
commercial 3D image processing software were tested as an innovative and alternative method
for automated volumetric measurement of truckloads. The images were collected with a small
UAV, which was flown around logging trucks transporting Eucalyptus nitens pulplogs.
Photogrammetric commercial software was used to process the images and generate 3D models
of each truckload. The levels of accuracy obtained with multi-view SfM photogrammetry and
3D reconstruction obtained in this study were comparable to those reported in previous studies
with laser scanning systems for truckloads with similar logs and species. The deviations between
the actual and predicted solid volume of logs on trucks ranged between –3.2% and 3.5%, with
an average deviation of –0.05%. In absolute terms, the average deviation was only 0.5 m3 or
1.7%. Although several aspects must be addressed for the operational implementation of SfM
photogrammetry, the results of this study demonstrate the great potential for this method to be
used as a cost-effective tool to aid in the determination of the solid volume of logs on trucks.

Testing the Applicability of the Official Croatian DTM for Normalization of UAV-based DSMs and Plot-level Tree Height Estimations in Lowland Forests

volume: 40, issue: 1

The Airborne Laser Scanning (ALS) technology has been implemented in operational forest
inventories in a number of countries. At the same time, as a cost-effective alternative to ALS,
Digital Aerial Photogrammetry (PHM), based on aerial images, has been widely used for the
past 10 years. Recently, PHM based on Unmanned Aerial Vehicle (UAV) has attracted great
attention as well. Compared to ALS, PHM is unable to penetrate the forest canopy and, ultimately,
to derive an accurate Digital Terrain Model (DTM), which is necessary to normalize
point clouds or Digital Surface Models (DSMs). Many countries worldwide, including Croatia,
still rely on PHM, as they do not have complete DTM coverage by ALS (DTMALS). The
aim of this study is to investigate if the official Croatian DTM generated from PHM (DTMPHM)
can be used for data normalization of UAV-based Digital Surface Model (DSMUAV) and estimating
plot-level mean tree height (HL) in lowland pedunculate oak forests. For that purpose,
HL estimated from DSMUAV normalized with DTMPHM and with DTMALS were generated and
compared as well as validated against field measurements. Additionally, elevation errors in
DTMPHM were detected and eliminated, and the improvement by using corrected DTMPHM
(DTMPHMc) was evaluated. Small, almost negligible variations in the results of the leave-oneout
cross-validation were observed between HL estimated using proposed methods. Compared
to field data, the relative root mean square error (RMSE%) values of HL estimated from DSMUAV
normalized with DTMALS, DTMPHM, and DTMPHMc were 5.10%, 5.14%, and 5.16%, respectively.
The results revealed that in the absence of DTMALS, the existing official Croatian DTM
could be readily used in remote sensing based forest inventory of lowland forest areas. It can
be noted that DTMPHMc did not improve the accuracy of HL estimates because the gross errors
mainly occurred outside of the study plots. However, since the existence of the gross errors in
Croatian DTMPHM has been confirmed by several studies, it is recommended to detect and
eliminate them prior to using the DTMPHM in forest inventory.

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Web of Science Impact factor (2018): 2.258
Five-years impact factor: 2.197

Quartile: Q1 - Forestry

Subject area

Agricultural and Biological Sciences

Category/Quartile

Forestry/Q1