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Remote sensing for resources development and environmental management
Damen, M. C. J.

Symposium on Remote Sensing for Resources Development and Environmental Management / Enschede / August 1986
The use of aerial photography for assessing soil disturbance
caused by logging
Forest Research Institute, Rotorua, New Zealand
ABSTRACT: When harvesting takes place in forest plantations, it is common for logging machinery such as
skidders and crawler tractors to traverse 50% or more of the area. The damage caused to the soil through
compaction and soil removal ranges from slight to severe and may lead to slower growth of the subsequent
tree crop and an increased probability of erosion. A technique is described for classifying and mapping
this disturbance using small-format colour aerial photographs. Also mentioned are some aspects of the role
of aerial photography in harvest planning and in determining the effect of soil disturbance on tree growth.
Each year throughout the world millions of
hectares of forest are logged using skidders and
crawler tractors. In New Zealand alone, the area
logged by these machines amounts to about
15 000 ha annually. As the logs and logging
machines pass over the forest floor, it is not
uncommon for 50% or more of the total forest area
to be disturbed by compaction and removal of the
various soil horizons (Murphy 1984). The soil's
airspace is reduced, its bulk density increased,
and its moisture regime altered (Steinbrenner and
Gessel 1955).
With more and more forests in New Zealand
moving into their second and subsequent rotations,
attention is being focused on the growth losses
which may occur in the new crop through the
disturbance done to the soil during thinning and
harvesting the old crop. If the area disturbed
is large, the effect long term, and the reduction
in tree growth significant, the potential exists
for a very serious and expensive problem.
Some years ago, the New Zealand Forest Research
Institute (FRI) decided to investigate harvesting-
related soil disturbance and its effect on tree
growth. The research was divided into four
1. Classifying the disturbance.
2. Mapping the disturbance.
3. Investigating the relationship between soil
disturbance and tree growth.
4. Finding ways to reduce soil damage and
improve the soil.
It was soon found that aerial photography had a
significant part to play in this research and
some aspects of its role are outlined below.
A brief walk through a thinned or logged forest
soon reveals that the disturbance caused by
logging machinery may range from nil (soil is
completely undisturbed) to severe (the litter and
top soil have been completely removed and the
subsoil compacted into an impervious rock-hard
mass). Murphy (1982) has divided this
disturbance into five visual damage classes or
0 - Machines have not travelled over the area.
1 - Machines have travelled over the area but
have not broken through the litter layer.
2 - Machines have broken through the litter layer
and started to compact the topsoil.
3 - Most of the topsoil is puddled, and subsoil
compaction has begun.
4 - Subsoil is puddled and compacted.
Murphy (1984) has successfully used his
classification to assess 18 logging sites around
New Zealand using a line transect method similar
to that tried by Dyrness (1965). However,
walking to and fro over logging slash can be very
arduous. In addition, the transect’ method, while
providing good overall disturbance proportions,
is not suitable for producing maps showing the
pattern of the disturbance. It was thought that
by using aerial photography to bring the logged
area into the office, the assessment of soil
disturbance could be done easily and more
The specifications for the photography were:
Camera - Hasselblad
Format - 55 x 55 mm
Film - Colour negative
- Colour transparency
Negative scales - Between 1:5000 and 1:15000
Three people were asked to interpret the aerial
photographs using Murphy's classification and
then to check their efforts in the field. The
results were disappointing with differences of up
to 59% being obtained.
It was therefore decided to modify Murphy's
classification so that the strata more closely
reflected how soil disturbances appear on the
aerial photographs. Furthermore, objective
elements of interpretation were introduced where
The system shown in Table 1 was devised. Its
main components are:
1. the existence and importance of the skid
trails, and
2. the colour difference between the top soil
and the subsoil.
This new classification was tested by several
people using various sets of aerial photographs of
different logged areas. Consistent results were
obtained between interpreters and there was good
agreement with the ground truth, differences being
under 10%. Subsequent experience with the method
showed that the photographs should be taken as
soon after logging as possible before weed growth
obscurs valuable detail. It was also found that
large negative scales (> 1:10000) are not as
useful as smaller ones (1:10000-1:20000) because
the large scales emphasised irrelevant detail.