Symposium on Remote Sensing for Resources Development and Environmental Management / Enschede / August 1986 
441 
The use of aerial photography for assessing soil disturbance 
caused by logging 
J.G.Firth 
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. 
1 INTRODUCTION 
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 
phases. 
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. 
2 CLASSIFYING AND MAPPING SOIL DISTURBANCE 
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 
strata: 
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 
effectively. 
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 
possible. 
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.