COMPUTER DIGITIZATION AND DIGITAL IMAGE ANALYSIS
FOR ASSESSING SOIL DISTURBANCE ON CLEARCUTS
Y. Jim Lee
Pacific Forestry Centre, Forestry Canada
506 West Burnside Road, Victoria, B.C. Canada V8Z 1M5
0.0 ABSTRACT
Soil degradation is caused by certain types and degrees of soil disturbance resulting from forest harvesting
activities. This soil degradation is known to reduce future productivity of trees. The present method of survey
to determine the extent and severity of soil disturbance is a ground-based "grid-point intercept" system which
is time-consuming and costly. Developing digital image analysis techniques for estimating soil disturbance on
the majority of clearcuts would result in greatly increased efficiency and reduced costs. Aerial photography
is useful for determining the area of well defined disturbance caused by landings, roads and skidroads
constructed on bare ground. Non-constructed skidroads and winter-constructed skidroads often have narrow
surfaces obscured by woody debris. Special digital image analysis techniques allow estimation of these poorly
defined types of disturbance. Aerial photographs and ground survey data for two recent clearcuts were
processed by a digital image analysis system and estimates made of the proportion of the ground surface in
landings, roads and skidroads. These estimates were compared with ground survey data. To date, preliminary
work conducted on 1:5000 color aerial photography from the Cariboo Lake area in south-central British
Columbia indicated that, with operator assistance, the digital image processing resulted in estimates of the
extent of disturbance close to those obtained from ground surveying. In addition, the digital image processing
provides areal estimates for (i) "Class A" - "deeply gouged or exposed compacted mineral soil" and (ii) "Class
B" - "exposed mineral soil or possibly woody debris".
1.0 INTRODUCTION
Certain types and degrees of soil disturbance resulting from forest harvesting activities are known to result in
soil degradation and thus in reduced future productivity of trees. Interim soil disturbance guidelines (Lewis
and Carr 1989) have been developed by the British Columbia Forest Service (BCFS) and are becoming part
of silvicultural regulations. Successful application of these guidelines requires that the BCFS and forest
industries conduct soil disturbance surveys on clearcuts. The present survey method is a "grid-point intercept"
ground survey system (Krag and Webb 1987; Curran and Thompson 1990). Since ground surveying is time-
consuming and costly, the alternate use of digital image analysis technology has been proposed frequently but
has not been seriously tested. Ideally, these techniques alone would be used to estimate soil disturbance on
the majority of clearcuts and ground surveys would only be required for contentious cases. The result would
be increased efficiency and reduced survey costs.
A system of computer digitization of aerial photography and digital image analysis is useful for determining the
area of well defined disturbance caused by the construction of landings, roads and skidroads. However, soil
disturbance guidelines now require a distinction between heavily and lightly used non-constructed skidroads and
a tally of winter-constructed skidroads which often have narrow surfaces obscured by woody debris. A system
is needed to allow estimates of the less well defined types of disturbance. In addition, future refinement of the
guidelines may necessitate consideration of the depth of disturbance. A paper describing the preliminary
results of soil disturbance assessment using digital image analysis was published earlier this year (Lee 1990).
This paper describes additional computer digitization and a new procedure of computer digital image analysis
techniques that provides consistent areal estimates of soil disturbance for clearcut units.
123
