GozdV 75 (2017) 1 21
Znanstvena razprava
1 Dr. A. Đ., Department of Forest Engineering, Faculty 
of Forestry University of Zagreb, Svetošimunska 25, 
Zagreb, Croatia; aduka@sumfak.hr
2 Dr. D. V., Department of Forest Engineering, Faculty 
of Forestry University of Zagreb, Svetošimunska 25, 
Zagreb, Croatia; dvusic@sumfak.hr
GDK 377.22+114 (045)=163.6
Ocena grobosti terena za spravilo lesa z vlačilci hlodov z vitlom 
Terrain Roughness Evaluation for Timber Extraction by Cable Skidder
Andreja ĐUKA1, Dinko VUSIĆ2, Tomislav PORŠINSKY3
Izvleček:
Đuka, A., Vusić, D., Poršinsky, T.: Ocena grobosti terena za spravilo lesa z vlačilci hlodov z vitlom, Gozdarski 
vestnik, 75/2017, št 1. V angleščini, z izvlečkom in povzetkom v slovenščini, cit. lit. 46. Prevod Breda Misja, 
pregled slovenskega besedila Marjetka Šivic.
Ta članek podaja analizo talnih ovir v Zalesini, eni od gozdnogospodarskih enot (GGU) prebiralnih gozdov 
v hribovito-goratem predelu Hrvaške, kot del opisa terena in klasifikacije, potrebnih za načrtovanje gozdnih 
del. V predelu visokega krasa je grobost terena izražena z naklonom, različnimi smermi njegove oblikovitosti 
in s talnimi ovirami, saj je stanje tal zaradi skeletoidne zgradbe manj pomembno za mobilnost vozil. Podatki, 
ki so na Hrvaškem na razpolago (pedološki zemljevidi, uradni gozdnogospodarski načrti) in se tičejo talnih 
ovir (kamnitost/skalnatost, druge ovire pa niso omenjene), so še vedno podani kot delež na področje in kot 
taki dejansko ne razlikujejo med seboj območij, neprimernih za različna gozdarska vozila. Glede na smernice 
iz literature so bile talne ovire zabeležene na 319 vzorčnih ploskvah, velikih 10 × 10 m na razdalji 100 m. Na 
vsaki vzorčni ploskvi sta bili izmerjeni višina in pogostost talnih ovir, ki so bile glede na pogostost razdeljene 
v štiri skupine: 1) posamezne, 2) redke, 3) zmerno pogoste in 4) pogoste; glede na višino so bili določeni štirje 
višinski razredi: 1) H20 (10-30 cm), 2) H40 (31-50 cm), 3) H60 (51-70 cm) in 4) H80 (> 71 cm). Na osnovi 
analize deleža skupin talnih ovir, kotov zmožnosti manevriranja in polmera manevrskega prostora vlačilca, ki 
opisujejo značilnosti njegove mobilnosti, so bile določene tri nove kategorije grobosti terena: 1) les je mogoče 
vleči preko talnih ovir (20,14% GGE območja), 2) lesa je mogoče vleči, če obidemo ovire (24,54% GGE območja) 
in 3) potrebna je gradnja vlak (54,16% GGE območja).
Ključne besede: talne ovire, prevoznost terena, vlačilec, grobost terena 
Abstract:
Đuka, A., Vusić, D., Poršinsky, T.: Terrain Roughness Evaluation for Timber Extraction by Cable Skidder. Goz-
darski vestnik (Professional Journal of Forestry), 75/2017, vol 1. In English, abstract and summary in Slovenian, 
lit. quot. 46. Translated by Breda Misja, proofreading of the Slovenian text Marjetka Šivic.
This paper gives analysis of ground obstacles in one Management unit of selective forests in hilly-pre-mountain-
ous part of Croatia, Zalesina, as one part of terrain description and classification required for planning forestry 
operations. In the area of high karst, terrain roughness is defined in terms of slope, its various direction forms 
and ground obstacles as soil condition is of minor importance for vehicle mobility due to highly skeletoid soil 
composition. Available data (pedological maps, official management plans) in Croatia regarding ground obstacles 
(stoniness/rockiness while other obstacles are left unmentioned) is still given in terms of share per area and as 
such does not really differentiate areas unsuitable for various forestry vehicles. According to literature guideli-
nes, ground obstacles were recorded on 319 sampling plots, each of size 10 × 10 m and with 100 m of distance. 
Height and frequency of ground obstacles were measured on each sample plot and according to frequency were 
divided into four groups: 1) isolated, 2) infrequent, 3) moderately frequent and 4) frequent; as well as in four 
height classes: 1) H20 (10-30 cm), 2) H40 (31-50 cm), 3) H60 (51-70 cm) and 4) H80 (> 71 cm). Based on share 
analysis of ground obstacle groups, manoeuvrability angles and clearance radiuses of skidder which describe its 
mobility characteristics, three new terrain roughness categories were defined: 1) skidding timber across ground 
obstacles is possible (20.14% of MU area), 2) skidding timber while by-passing ground obstacles is possible 
(24.54% of MU area) and 3) construction of skid roads is necessary (54.16% of MU area).
Key words: ground obstacles, terrain trafficability, skidder, terrain roughness
3 Prof. dr. T. P., Department of Forest Engineering, Fa-
culty of Forestry University of Zagreb, Svetošimunska 
25, Zagreb, Croatia; tporsinsky@sumfak.hr
GozdV 75 (2017) 122
1 UVOD
1 INTRODUCTION
Terrain trafficability is its ability to support vehicle 
movement during which terrain characteristics 
(slope, ground obstacles, soil bearing capacity) 
affect vehicle mobility (Eichrodt and Heinimann 
2001, Suvinen 2006). On the other hand, vehicle 
mobility is its ability to move in space from point A 
to point B while retaining its purpose for example 
timber harvesting or timber transport (Eichrodt 
2003, Lubello 2008, Đuka 2014).
Skidders and forwarders are still dominant for 
timber extraction in most European countries such 
as Germany, Finland, Sweden, Norway, Slovenia, 
Italy, Austria, Ireland, Switzerland, Greece, Croatia 
(Schwaiger and Zimmer 2001, Beuk 2007) and 
vehicle choice for timber extraction depends on 
terrain characteristics i.e. terrain slope (FAO/ECE/
ILO 1971, Heinimann 1999), ground obstacles 
(Horn et al. 2007, Olund 2001, Visser and Berkett 
Đuka, A., Vusić, D., Poršinsky, T.: Ocena grobosti terena za spravilo lesa z vlačilci hlodov z vitlom
2015) and soil bearing capacity (Amishev et al. 
2009) as well as primary and secondary forest 
openness and as such is an important parameter 
in the whole timber supply chain. Pentek et al. 
(2010) highlight the importance of shape, position 
and density of secondary forest network which is 
critical for basic vehicle-to-timber access during 
timber extraction (length of winch rope or boom 
reach). From the strategic level of planning timber 
harvesting, terrain slope is the most important 
factor that directly affects the choice of timber 
harvesting system (Đuka 2014, Đuka et al. 2015) by 
affecting vehicle stability where all vehicle wheels 
(tracks) collide with the same macrotophograhic 
conditions. Same authors conclude that ground 
obstacles are defined as microtopographic terrain 
characteristics independent in size and position 
of macroptopographc terrain parameters and 
that they affect one or more wheels (tracks) 
during vehicle movement in forest stand. Ground 
Slika 1: Raznolikost talnih ovir v oddelku št. 5 študijskega območja (14,85 ha) (foto: A. Đuka) 
Figure 1: Variety of ground obstacles in compartment No. 5 of researched area (14.85 ha) (photo: A. Đuka)
GozdV 75 (2017) 1 23
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roughness is described independently of slope, 
and as such is a well-known terrain parameter 
in forestry classification systems (Eriksson et al. 
1975, Rowan 1996, Mellgren 1980, Berg 1992, 
Owende et al. 2002). Steep terrain slope and/or 
ground obstacles affect longitudinal and lateral 
vehicle stability, with that their mobility, produc-
tivity and work safety (Visser and Berkett 2015, 
Visser and Stampfer 2015).
An obstacle is generally defined as any natural 
or man-made terrain feature that slows, diverts, 
or stops the movement of personnel or vehicles. 
The height and spacing of obstacles affect the 
ride and machine stability and consequently the 
practical speed of an off-road vehicle. Eriksson 
et al. (1975) and Berg (1992) state that typical 
permanent obstacles that slow down off-road 
vehicles are rocks, boulders, depressions, rock 
outcrops, soil mounds (higher than 10 cm), cavi-
ties (deeper than 20 cm), undulations, sinkholes 
etc. Boulder quota or block index (stoniness or 
covered boulders and larger rocks present in the 
surface layer and down to 20/30 cm) should be 
field checked or measured on 100 sample plots 
selected randomly or systematically. Authors 
conclude that soil probe (used for determining 
rocks beneath the surface up to 20 cm depth) does 
not have to be used all the time, but is necessary 
when planning for site preparation and choice of 
reforestation method.
In practice the roughness class is usually asses-
sed visually (Eriksson et al. 1975, Mellgren 1980, 
Berg 1992, Suvinen and Saarilahti 2006), and 
experienced operator can classify unit by visual 
assessment while unexperienced operator needs 
supportive measurements. Löffler (1984) and 
Rowan (1996) highlight that ground obstacles can 
be measured on round or squared sampling plots 
of minimum 100 m2 area and then are divided into 
four height classes H20 (10–30 cm), H40 (31-50 
cm), H60 (51-70 cm) and H80 (>71 cm). Similar 
obstacle height division is later done by Owende et 
al. (2002) in Ecowood project protocol (Table 1).
Heinimann (1999) states it is crucial to know 
terrain factors affecting vehicle mobility because 
of their high impact on the whole harvesting 
system. Same author continues that the base of 
scientific research of vehicle – terrain system was 
set by Bekker in 1956 in his book »Theory of Land 
Locomotion«, which was later extended by other 
researchers such as Wong (1989) in »Terrame-
chanics and off-road vehicles« and Mastinu and 
Ploechl (2014) in »Road and Off-Road Vehicle 
System Dynamics Handbook« (Poršinsky et al. 
2016). Mechanics of interaction between geometry 
of terrain and geometry of vehicle must be defined 
Class Ground condition
Roughness category
Slope
Description Obstacle height, cm
Obstacle height, cm
20 40 60 80
Average distance 
between obstacles, m
1 Good Even
H 20 1.6-55-16 > 16 > 16 > 16
Gentle < 8°, 14%
H 20-40 < 1.61.6-5
> 16
5-16
> 16
> 16
> 16
> 16
2 Average Uneven
H 40-60 < 1.6< 1.6
1.6-5
1.6-5
5-16
1.6-5
> 16
> 16
Intermediate 8°-14° 14-25%
H 40-80 < 1.61.6-5
< 1.6
1.6-5
5-16
1.6-5
5-16
5-16
3 Poor Rough H 40-80 < 1.6< 1.6
< 1.6
< 1.6
1.6-5
< 1.6
5-16
< 1.6 Steep
> 14°, 
25%
4 Very poor (not trafficable)
Preglednica 1: EcoWood klasifikacija terena (Owende in sod. 2002)
Table 1: EcoWood terrain classification (Owende et al. 2002)
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in any characterization of terrain roughness from 
the viewpoint of: 1) obstacle crossing, 2) vehicle 
controllability and 3) ride comfort (Bekker 1969).
Mobility of forestry vehicles used for timber 
extraction can be described through these para-
meters: 1) dimensional features: turning radius, 
weight, center of gravity position, longitudinal 
and lateral vehicle stability angle, ground clea-
rance, angle of articulation, front axle oscillation, 
unloading of front axle, payload of rear axle, tires 
payload (Šušnjar et al. 2010); 2) locomotion system 
(Uusitalo 2010, Gregov 2012, Marenče 2014, 
Poršinsky et al. 2016); 3) the ability to overcome 
ground obstacles (clearance, lateral stability of 
vehicle) and orientation and slope of terrain 
(Macdonald 1999, Kühmaier and Stampfer 2010, 
4) traction performance: dependence of slip curve, 
power and actual speed on drawbar pull and soil 
bearing capacity (Bojanin et al. 1988, Horvat 1993, 
Šušnjar 2005) and 5) environmental soundness 
(Košir 1994, Košir 1995). While other authors 
also refer to: 1) angle of longitudinal and lateral 
vehicle stability (Alexandrovich 2013, Gibson 
and Biller 1974), 2) critical load considering 
longitudinal vehicle stability (Horvat 1990), 3) 
load distribution on vehicle axles considering 
terrain slope and direction of movement (uphill 
or downhill) and load size (Đuka 2014, Đuka et 
al. 2016), 4) outer and inner vehicle turning radii 
(Sever 1980) and 5) manoeuvrability angles and 
clearance radii (Sever and Horvat 1985).
ISO Standards 13861 (2000) define basic 
dimensional characteristics of skidder, but do 
not however explain manoeuvrability angles and 
clearance radii. On the other hand, literature gives 
only approximate expressions for their calculation 
due to various construction parameters and vehicle 
applications (Poršinsky et al. 2016).
The purpose of this research was to give guide-
lines for terrain roughness evaluation considering 
timber extraction by a winch skidder, since in 
Croatia there are no available data bases of ground 
obstacles from the aspect of forestry vehicles 
mobility nor does the official Forest Management 
Bylaw (NN 79/15) regulate the methodology for 
estimating ground obstacles. Forest management 
plans do contain descriptions of each management 
unit and its compartments (sub-compartments) 
where rockiness is given in percentage i.e. share 
per area. The reason of such representation of 
ground obstacles is probably due to the fact that 
rockiness values are taken from the General soil 
map in 1:50,000 scale which differentiates six 
categories of stoniness share per area: 1) < 2%, 2) 
3–10%, 3) 11–25%, 4) 26–50%, 5) 51–90%, and 
6) > 91% (Bogunović and Rapaić, 1993).
2 MATERIALI IN METODE
2 MATERIALS AND METHODS
Research was conducted in the area of beech and 
fir selective forests of Gorski kotar (hilly and pre-
-mountainous part of Croatia), in management 
unit »Kupjački vrh« at the Training and research 
forest center »Zalesina« which is managed by the 
Faculty of Forestry, University of Zagreb. Mana-
gement unit »Kupjački vrh« is located at 45° 26' 
N latitude and 14° 53' E longitude of Greenwich. 
It consists of 278.80 hectares of forests, of which 
274.87 ha is stacked forest area. Management 
unit is divided into 16 compartments and the 
average growing stock is 446 m3/ha, with annual 
increment of 6.25 m3/ha. MU »Kupjački vrh« 
is a typical representative of high karst (Anon. 
2004) with a centrally located peak from which 
terrain descends in all exposition types. Terrain 
is rich in karst phenomenon without a developed 
hydrological network and with highly skeletoid 
mechanical soil composition.
Ground obstacles were determined by setting 
a systematic network of sample plots consisting 
of 319 measuring areas, with 100 m distance, 
and 10 × 10 m in size (Figure 2). On each sample 
plot, according to the methodology of measuring 
ground obstacles (Eriksson et al. 1975, Löffler 1984 
and Rowan 1996) their height and frequency were 
measured for the purpose of defining categories 
according to literature guidelines (Mellgren 
1980, Löffler 1984, Berg 1992, Rowan 1996). Five 
surface structure classes (Table 2): 1) very even 
terrain, 2) slightly even terrain, 3) uneven terrain, 
4) rough terrain and 5) very rough terrain were 
defined depending on ground obstacle frequency 
and height class (Đuka and Poršinsky 2016) and 
former classification systems by Löffler (1984) 
and Rowan (1996). To be more precise, if H20 
GozdV 75 (2017) 1 25
Đuka, A., Vusić, D., Poršinsky, T.: Ocena grobosti terena za spravilo lesa z vlačilci hlodov z vitlom
obstacles are infrequent or if there are obstacles 
higher than 31 cm (H40 to H80), but isolated – 
class 1 is chosen; if H20 obstacles are moderately 
frequent, but no other class is present – class 1 of 
surface structure is chosen again. However, if H20 
obstacles are moderately frequent and other height 
classes are present (H40 infrequent, H60 and 
H80 combined) class 2 is the chosen one. Similar 
type of assessment continues in class 3. Class 4 
is defined by similar frequency of obstacles as in 
class 3, with one exception – H80 obstacles are 
more common i.e. infrequent rather than isolated 
(which would redirect assessment to class 3).
Skidder maneuverability angles (Figure 3A and 
3B) impact its mobility not only in movement 
through the forest stand, for example through 
bigger terrain depressions (descending or ascen-
ding), basins or sinkholes, but also during turning 
of vehicle, backing skidder for loading/unloading 
timber, stacking roundwood on landing site 
etc. Long (frequent), uneven ground obstacles, 
perpendicular to skidder movement direction 
will affect its mobility for longitudinal clearance 
radius value (Figure 3C), while the lowest vehicle 
point – transverse clearance radius (Figure 3D) 
should therefore be connected with height of 
ground obstacles (Poršinsky et al. 2016). Authors 
further conclude that due to skidder construction 
parameters they can either drive over ground 
obstacles (roughness class 1), drive around them 
(roughness class 2) or by-pass ground obstacles 
(Krieg et al. 2010, McEwan et al. 2013) while in 
uneven, rough and very rough terrain conditi-
ons (classes 3-5) skid road network should be 
constructed.
Depending on skidder Ecotrac 120V dimen-
sional features: 1) front axle oscillation ±11º, 2) 
β1 approach angle 38º, 3) β2 departure angle 28º, 
4) β3 break-over angle 50º, 5) R1 longitudinal cle-
arance radii 806 mm, 6) R2 transverse clearance 
radii 734 mm, 7) ground clearance 470 mm and 
8) clearance circle 5.1 m, a new reclassification 
of ground roughness was made for the entire 
management unit.
Reclassification on new three classes was based 
on the fact that the highest/lowest possible ground 
obstacle which Ecotrac 120V can cross is up/down 
to 35 cm (based on front axle oscillation ±11º and 
wheel tread 1.8 m). Approach angle (β1) of 38º 
allows obstacle up to 135 cm height/depth, so 
one can conclude that ground obstacles in height 
class H20 (10-30 cm) do not represent a threat 
for vehicle mobility. Descending or ascending 
of skidder in basins, sinkholes, depressions or 
similar, is connected to its break-over angle (β3 
50º). Sides of such terrain formations shouldn’t 
have slopes over half of skidder break-over angle 
i.e. 25º, or vehicle could encounter a so called 
hang-up failure (Bekker 1969), also sides and 
bottom of such a basin or depression should 
Preglednica 2: Klasifikacija površinske strukture (Löffler 1984, Rowan 1996) 
Table 2: Surface structure classification (Löffler 1984, Rowan 1996)
Surface structure 
classification
Obstacle height class 
H20 (10-30 cm) H40 (31-50 cm) H60 (51-70 cm) H80 (> 71 cm)
1
Infrequent
(41-400 ha-1, 6-16 m)
Other classes combined = Isolated
(4-40 ha-1, 17-50 m)
Moderately 
frequent
(401-4000 ha-1,
1.6-5 m)
No other classes present
2
Infrequent
(41-400 ha-1, 6-16 m)
Other classes combined
Isolated (4-40 ha-1, 17-50 m)
Frequent 
(> 4001 ha-1, <1.5 m)
No other classes present
3 Moderately 
frequent 
(401- 4000 ha-1,
1.6-5 m)
Infrequent
(41-400 ha-1, 6-16 m)
Isolated 
(4-40 ha-1, 17-50 m)
4 Infrequent(41-400 ha-1, 6-16 m)
5 All combinations more severe than Class 4
GozdV 75 (2017) 126
Đuka, A., Vusić, D., Poršinsky, T.: Ocena grobosti terena za spravilo lesa z vlačilci hlodov z vitlom
Slika 2: Vzorčne ploskve na študijskem območju (lastnik vektorskih podatkov: A. Đuka, lastnik topografskega 
zemljevida: Hrvaška geodetska uprava)
Figure 2: Sampling plots in research area (vector data owner : A.Đuka, topographic map owner: Croatian Geodetic 
Directorate)
GozdV 75 (2017) 1 27
Đuka, A., Vusić, D., Poršinsky, T.: Ocena grobosti terena za spravilo lesa z vlačilci hlodov z vitlom
Slika 3: Vpliv kotov zmožnosti manevriranja in polmera manevrskega prostora na mobilnost vlačilca
Figure 3: Impact of maneuverability angles and clearance radiuses on skidder mobility
be at least 2.4 m in length which correlates to 
wheelbase of this skidder. Departure angle (β2) 
affects skidders’ movement more during backing 
up and finding position before loading timber, 
organizing roadside landing and similar as well 
as during transition from secondary to primary 
forest traffic infrastructure, than in driving over 
ground obstacles (Poršinsky et al. 2016). Bekker 
(1969) claims that vehicles that have approach 
and departure angles from 30° to 40° have good 
off-road characteristics, but Sever and Horvat 
(1985) state that for cable skidder these values 
should be between 35° and 50°.
Longitudinal and transverse clearance radii 
of skidder show their significance in elongated 
(frequent) ground obstacles transverse in direction 
to skidder movement, but in this case driving of 
skidder around ground obstacles, as supposed to 
driving across them, was established depending 
on its clearance circle. The differentiation between 
new roughness classes 2 (by-passing ground 
obstacles) and 3 (construction of skid road net-
work) was done on the base of ground obstacle 
height/depth. Everything higher/deeper than 35 
cm i.e. classes H40 (31-50 cm), H60 (51-70 cm) 
and H80 (>71 cm), as well as their frequency: 
group 3 – moderately frequent (401-4000 No/
ha) and group 4 – frequent (4000+ No/ha). So, 
if ground obstacles of H40, H60 and H80 were 
moderately frequent or frequent roughness class 
3 was chosen, if obstacles were isolated (4-40 No/
ha) or infrequent (41-400 No/ha) roughness class 
2 was selected.
3 REZULTATI IN DISKUSIJA 
3 RESULTS AND DISCUSSION
Analysis of ground obstacles (Figure 4) showed 
that class 3 (uneven terrain) prevails on most of the 
management unit area (39.59%) and that classes 
3, 4 and 5 together comprise to 65.87% of MU 
area (Đuka and Poršinsky 2016). Compartment 
12 mostly consists of »very even terrain« (class 
1) – 36%, whilst compartment 6 (in management 
plan defined as protective forests) consists in 85% 
of »very rough terrain« (class 5).
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Slika 4: Delež razredov površinske strukture
Figure 4: Share of surface structure classes
When comparing data on rockiness from 
the management program and surface structure 
classes from field measurements, discrepancy is 
noted. Even though surface structure classes do 
give height and frequency of obstacles, in terms of 
vehicle mobility, for example skidder during timber 
extraction, further reclassification was done.
By connecting surface structure classes (1-5) 
with skidder Ecotrac 120V maneuverability 
parameters, three new ground roughness classes 
were defined: 1) skidding timber across ground 
obstacles, 2) skidding timber while by-passing 
ground obstacles and 3) skidding timber is possible 
only on secondary forest traffic infrastructure 
network (Figure 5).
New ground roughness classification for MU 
»Kupjački vrh« showed 54.61% share of class 3, 
20.14% of class 1 and 24.54% class 2, which con-
cludes that skid roads should be constructed on 
more than a half of the management unit area. 
Share of each ground roughness class in every 
compartment of the management unit is shown 
in Figure 6. Compartments 3, 5, 6, 7, 8, 10, 11, 
15 and 16 have more than 50% of their surface 
in roughness class 3, while compartments 1, 4, 9, 
12, 13 and 14 have more than 30% of area in class 
3. The most favourable terrain from the aspect of 
ground roughness classes and timber skidding is 
in compartment 2.
Compartment number zero (0) represents 
private areas within management unit which are, 
in agreement with owners, also used for timber 
transport, and on which research (sampling plots) 
was also conducted.
When comparing ground roughness classes 
with data given in Management plan (Figure 7), 
deviation is again noted. According to Manage-
ment plan compartments 3, 11, 12 and 16 have 
the lowest share of rockiness, the highest share 
per area of rockiness in compartments 5, 6, 9 and 
15, while rockiness in all the rest is rounded up 
to 10(20)-50% of area. What is significant is that 
height of ground obstacles is still unknown, not 
to mention depth of surface formations which is 
not even mentioned.
GozdV 75 (2017) 1 29
Slika 5: Razredi grobosti tal (foto: A. Đuka)
Figure 5: Ground roughness classes (photo: A. Đuka)
Slika 6: Delež razredov grobosti tal po gozdnogospodarskih enotah
Figure 6: Share of ground roughness classes in every compartment of management unit
Đuka, A., Vusić, D., Poršinsky, T.: Ocena grobosti terena za spravilo lesa z vlačilci hlodov z vitlom
GozdV 75 (2017) 130
Slika 7: Zemljevid grobosti tal na študijskem območju (lastnik vektorskih in rasterskih podatkov: A. Đuka, 
lastnik topografskega zemljevida: Hrvaška geodetska uprava)
Figure 7: Map of ground roughness in research area (vector and raster data owner: A. Đuka, topographic map 
owner: Croatian Geodetic Directorate)
Đuka, A., Vusić, D., Poršinsky, T.: Ocena grobosti terena za spravilo lesa z vlačilci hlodov z vitlom
GozdV 75 (2017) 1 31
Đuka, A., Vusić, D., Poršinsky, T.: Ocena grobosti terena za spravilo lesa z vlačilci hlodov z vitlom
4 ZAKLJUČKI
4 CONCLUSIONS
Conducted research showed that available official 
data on ground roughness in terms of timber 
transport is not satisfactory due to the fact that 
ground obstacles are defined solely as rocks and 
only in share per area of each compartment. 
Share of rockiness per area in amount for exam-
ple 10-50% does not mean much, because height 
nor depth (of other ground obstacles) is given, 
so question of vehicle mobility is unknown – will 
vehicle drive across these obstacles freely, will it 
be able to go around them, or is construction of 
skid roads necessary?
The idea of this research was not to state that 
every ground obstacle should be precisely recorded 
and measured, because it is a known fact that in 
practice roughness classes are usually assessed 
visually (Eriksson et al. 1975, Mellgren 1980, 
Berg 1992, Suvinen and Saarilahti 2006). The goal 
was to give guidelines for new reclassification of 
ground roughness in terms of a cable skidder and 
its manoeuvrability characteristics. It should be 
mentioned that, for practical purposes, it would 
be very helpful if manufacturers would give such 
information in their technical characteristics 
booklets which then could be connected by 
practitioners to terrain conditions.
Even though off-road driving of specialized 
forestry vehicles is usually prohibited or not 
recommended in certified forests, researchers 
and practitioners should have information regar-
ding terrain conditions before entering certain 
forest stand which would enhance efficiency and 
promote rationalization (Berg 1992) in forestry 
altogether. Terrain classification systems are a 
necessary tool in forestry, since all activities in 
forests are affected by terrain itself. Recognizing 
the severity of terrain factors will improve plan-
ning and decision making process of all parties 
involved in forest management.
High definition LiDAR images would certainly 
be helpful in determining ground roughness con-
ditions (Dubayah and Drake 2000, Reutebuch et 
al. 2005, Wulder et al. 2008), but this kind of data 
is still fairly unavailable in territory of Republic 
of Croatia, as oppose to other European coun-
tries such as Finland, Poland, United Kingdom 
or Slovenia.
If no data on terrain roughness is available, 
experienced operator can classify unit by visual 
assessment and the inexperienced one should 
together with a skilled practitioner during usual 
operations in forest management determine terrain 
conditions by using explained sample plots and 
classification system. By combining manoeuvra-
bility of forestry vehicles and terrain roughness 
classes derived for those vehicles, planning of 
harvesting operations should be enhanced.
5 POVZETEK
Ta članek podaja analizo talnih ovir v eni gozdno-
gospodarski enoti (GGE) Učnega in raziskovalnega 
gozdarskega centra Zalesina, Hrvaška, kot ene 
od pomembnih značilnosti terena, ki omejujejo 
mobilnost vlačilcev med spravilom lesa. Raziskava 
je bila opravljena na območju bukovih in jelovih 
prebiralnih gozdov v Gorskem kotarju (hribovitem 
in goratem predelu Hrvaške). GGE obsega 278,80 
hektarjev gozdov, od katerih je 4,87 ha območje 
gozda s skladovnicami. GGE je razdeljena na 16 
oddelkov, povprečna lesna zaloga je 446 m3/ha, 
letni prirastek pa 6,25 m3/ha. Ta GGE je značilna 
predstavnica visokega krasa s središčno locira-
nim vrhom, od katerega se teren spušča na vse 
ekspozicijske načine. Teren je bogat s kraškimi 
pojavi brez razvitega vodnega omrežja ter ima 
zelo skeletoidno mehansko sestavo tal. Na takem 
terenu je grobost izražena z naklonom, različnimi 
smermi njegove oblikovitosti in s talnimi ovirami 
ter vpliva na izbiro gozdarskih vozil za spravilo 
lesa. Podatki, ki so na Hrvaškem na razpolago 
(pedološki zemljevidi, uradni gozdnogospodarski 
načrti) in se tičejo talnih ovir (kamnitost/skalna-
tost, druge ovire pa niso omenjene), so še vedno 
podani kot delež na področje in kot taki dejansko 
ne razlikujejo med seboj območij, neprimernih 
za različna gozdarska vozila. Talne ovire so bile 
zabeležene na 319 vzorčnih ploskvah, velikih 10 
× 10 m na razdalji 100 m. Na osnovi prejšnjih 
raziskovalnih smernic sta bili izmerjeni višina in 
pogostost talnih ovir, ki so bile glede na pogostost 
GozdV 75 (2017) 132
Đuka, A., Vusić, D., Poršinsky, T.: Ocena grobosti terena za spravilo lesa z vlačilci hlodov z vitlom
razdeljene v štiri skupine: 1) posamezne, 2) redke, 
3) zmerno pogoste in 4) pogoste; glede na višino so 
bili določeni štirje višinski razredi: 1) H20 (10-30 
cm), 2) H40 (31-50 cm), 3) H60 (51-70 cm) in 
4) H80 (> 71 cm). Analiza talnih ovir (Slika 4) 
je pokazala, da razred 3 (neenakomeren teren) 
prevladuje na večini površine gozdnogospodar-
ske enote (39,59%) ter da razredi 3, 4 in 5 skupaj 
obsegajo 65,87% površine GGE.
Nova klasifikacija grobosti tal za celotno 
gozdnogospodarsko enoto je bila opravljena na 
osnovi dimenzionalnih lastnosti vlačilca hlodov 
Ecotrac 120V: 1) nihanje prednje osi ±11º, 2) 
β1 kot dostopa 38º, 3) β2 odhodni kot 28º, 4) β3 
kot prevračanja 50º, 5) R1 polmeri vzdolžnega 
manevrskega prostora 806 mm, 6) R2 polmeri 
prečnega manevrskega prostora 734 mm, 7) talni 
manevrski prostor 470 mm in 8) krog manevrskega 
prostora 5,1 m.
Na osnovi analize deleža skupin talnih ovir, 
kotov zmožnosti manevriranja in polmera mane-
vrskega prostora vlačilca, ki opisujejo značilnosti 
njegove mobilnosti, so bile določene tri nove 
kategorije grobosti terena: 1) les je mogoče vleči 
preko talnih ovir (20,14% GGE območja), 2) lesa 
je mogoče vleči, če obidemo ovire (24,54% GGE 
območja) in 3) potrebna je gradnja vlak (54,16% 
GGE območja).
Čeprav je vožnja specializiranih gozdarskih 
vozil izven cest običajno prepovedana ali odsve-
tovana v certificiranih gozdovih, bi morali razi-
skovalci in praktični izvajalci imeti informacije 
o terenskih pogojih, preden se podajo v določen 
gozdni sestoj. To bi povečalo učinkovitost in 
spodbujalo racionalizacijo v gozdarstvu (Berg 
1992). Visokoločljivostne LIDAR slike bi zago-
tovo pomagale pri določanju grobosti tal, če pa 
podatkov o grobosti terena ni, lahko izkušen 
operater razvrsti enoto na podlagi okularne ocene, 
neizkušeni pa bi moral skupaj z izkušenim prak-
tičnim izvajalcem med običajnimi aktivnostmi 
pri gozdnem gospodarstvu določiti terenske 
pogoje s pomočjo pojasnjenih vzorčnih ploskev 
in klasifikacijskega sistema.
5 SUMMARY
This paper gives analysis of ground obstacles in 
one Management unit of Forest training and rese-
arch center Zalesina, Croatia, as one of important 
terrain characteristics that limit skidder mobility 
during timber extraction. Research was conduc-
ted in the area of beech and fir selective forests 
of Gorski kotar (hilly and mountainous part of 
Croatia. MU consists of 278.80 hectares of forests, 
of which 274.87 ha is stacked forest area. MU is 
divided into 16 compartments and the average 
growing stock is 446 m3/ha, with annual increment 
of 6.25 m3/ha. This MU is a typical representative 
of high karst with a centrally located peak from 
which terrain descends in all exposition types. 
Terrain is rich in karst phenomenon without a 
developed hydrological network and with highly 
skeletoid mechanical soil composition. In such 
terrain, roughness is defined in terms of slope, 
its various direction forms and ground obstacles 
and it affects the choice of forestry vehicles during 
harvesting operations. Available data (pedologi-
cal maps, official management plans) in Croatia 
regarding ground obstacles (stoniness/rockiness) 
is still given in terms of share per area and as such 
does not really differentiate areas unsuitable for 
various forestry vehicles. Ground obstacles were 
recorded on 319 sampling plots, each of size 10 × 
10 m and with 100 m of distance. Based on previ-
ous research guidelines, height and frequency of 
ground obstacles were measured on each sample 
plot and according to frequency were divided 
into four groups: 1) isolated, 2) infrequent, 3) 
moderately frequent and 4) frequent; as well 
as in four height classes: 1) H20 (10-30 cm), 2) 
H40 (31-50 cm), 3) H60 (51-70 cm) and 4) H80 
(> 71 cm). Analysis of ground obstacles (Figure 
4) showed that class 3 (uneven terrain) prevails 
on most of the management unit area (39.59%) 
and that classes 3, 4 and 5 together comprise to 
65.87% of MU area.
Depending on skidder Ecotrac 120V dimen-
sional features: 1) front axle oscillation ±11º, 2) 
β1 approach angle 38º, 3) β2 departure angle 28º, 
4) β3 break-over angle 50º, 5) R1 longitudinal cle-
arance radii 806 mm, 6) R2 transverse clearance 
radii 734 mm, 7) ground clearance 470 mm and 
8) clearance circle 5.1 m, a new reclassification 
of ground roughness was made for the entire 
management unit.
GozdV 75 (2017) 1 33
Based on share analysis of ground obstacle 
groups, manoeuvrability angles and clearance 
radiuses of skidder, three new terrain trafficabi-
lity categories were defined (Figures 6 and 7): 1) 
skidding timber across ground obstacles (20.14% 
of MU area), 2) skidding timber while by-passing 
ground obstacles (24.54% of MU area) and 3) 
construction of skid roads is necessary (54.16% 
of MU area).
Even though off-road driving of specialized 
forestry vehicles is usually prohibited or not 
recommended in certified forests, researchers 
and practitioners should have information regar-
ding terrain conditions before entering certain 
forest stand which would enhance efficiency and 
promote rationalization in forestry altogether. 
High definition LiDAR images would certainly 
be helpful in determining ground roughness 
conditions, but if no data on terrain roughness 
is available, experienced operator can classify 
unit by visual assessment and the inexperienced 
one should together with a skilled practitioner 
during usual operations in forest management 
determine terrain conditions by using explained 
sample plots and classification system.
6 VIRI
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