On a multisensor forest inventory system
F. G. Bercha and D. H. Currie
The Bercha Group
Suite 250, 120 Kensington Road N.W.
Calgary, Alberta, T2N 3P5
Phone : (403) 270-2221;
Telefcx (403) 270-2014; Telex: 03-827666
Abstract
The development and application of an optimal multi-sensor forest inventory system is described. Following review of state-of-the-art approaches to airborne forest inventory techniques, it was concluded that the combination of a multi spectral imaging sensor to provide planar information on forest characteristics with a laser profilometer to provide vertical sectional information constituted an optimal combination. The two sensors have been utilized in unison, coordinated through a digital computer, to provide a unified three-dimensional description of the forest. The description includes principal forest characteristics such as species composition, forest condition, crown density, and secondary characteristics in the horizontal plane, combined with vertical sectional properties including tree height, vertical stratification, and foliage density. These characteristics may then be used to determine economic parameters such a biomass and timber volume. The system described consisting of a multispectral camera and laser profilometer connected to a digital data acquisition system, ahs been developed up to the operational phase through a series of activities including system integration, hardware acquisition and testing, extensive real time and post-acquisition data processing software development, and pilot survey execution. The latter, which included comparison with ground truth data, showed excellent agreement in the primary geometric properly measurements of the forest and good agreement in the areas of secondary property evaluation. The system has been found to provide an economical and efficient technique for obtaining and forest change monitoring for a range of conditions in both temperate and tropical climates.
Introduction
Te purpose of the work described herein was to develop an airborne data collection and analysis facility which would be useful for forest assessment, either as a stand-alone system or in combination with other airborne and ground based data acquisition systems or in combination with other airborne and ground based data acquisition systems. Current technologies used for assessments of large areas include analysis of airphotos and satellite imagery (Ahern 1987). These approaches all depend heavily upon ground truth data for volume estimation since tree heights can only be estimated. Large-scale photography (Spencer 1987) has been used to obtain tree heights from airborne platforms however the technique is costly and time consuming due to the manual analysis required for each photo pair.
Previous investigations by the authors and others have demonstrated the utility of the laser rangefinder for direct measurements of tree heights in an airborne configuration (Aldred 1985, Bercha 1987, Nelson 1984).by pairing this unique sensor with a second one, capable of obtaining images of the forest canopy, an analyzing the acquired data as an integrated set, a system which could generate a three dimensional representation of the forest canopy in a digital format was predicted.
Prior to this work, the laser systems were often operated in conjunction with a video camera in order to recover the aircraft flight line in post mission processing. These two sensors are complementary in the sense that their operational envelopes overlap (flying height, speed etc). additionally, they offer two very different data sets since the laser measures in the vertical plane, while the video camera produces a horizontal image. Unfortunately, conventional vide imagery is not easily interpreted using the analytical produced by these systems. By using a multispectral video camera, the same image data may be separated into several discrete images representing the scene reflectance at a variety of wavelengths. These images can be captured using a video digitizer and analyzed in a manner analogous to the digital analysis of Landsat imagery.
Airborne System Description
The forest inventory system is constructed fro low-cost components using a central data acquisition computer to ensure system integrity and to provide timing control. The main sensors are a laser rangefinder and a multispectral vide camera. Facilities for film cameras and other alternative sensors are included in the data acquisition package. Figure 1 shows the system installed in a small aircraft.
The laser rangefinder is a gallium-arsenide (GaAs) diode laser capable of pulsing at rates of upto 4000 times per second. The laser pulses are in the near infrared at a wavelength of 902mm. Reflection of the narrow pulses are captured by a sensitive photo detector and the two way travel time is converted to distance using the constant speed of light. Special circuitry is provided to allow discrimination between multiple targets and allow the laser system to report either the shortest or longest range as selected by a data acquisition computer. In an airborne operation, the longest range will represent the ground, whereas the shorter ranges will represent the tree tops.
While range of the treetops is reliably obtained, the range to the ground is not always available due to obstruction of the laser beam by heavy foliage. To maximize the frequency of ground ranges, the laser is pulsed at a rate, which provides a 90% overlap between pulses. The data is filtered in real time so as to give a concise data ser consisting of ground ranges and tree heights. A statistical summary of the number of ranges interpreted by the foliage is recorded to allow estimation of crown closure along with profile.
The multispectral video camera is based on a conventional charge coupled device (CCD) sensor, which is scanned at the North American standard 60 fields per second (NTSC). The sensor resolution is 384 pixels by 491 lines spatially, while the spectral response is from 0.4 to 1.1 mm (Frost, 1985). The sensor is located behind a rotating, shutter, which contains six, user selectable, narrow band spectral filters. The shutter speed is synchronized with the scan frequency of the video circuitry in such a way that each field of video imagery is acquired through a different filter. This provides continuous six band coverage at a rate of ten image sets per second.
In order to relate the video imagery with the laser profiles in post-mission processing, a digital time code is inserted into the video signal prior to recording. These time codes are reliable by a computer controlled video playback unit.
Figure.1 Multisensor System
