Applying GPS Navigation: Seeing the Forest - A Kinematics Study

One can view profiling difficult areas such as forests as a problem or an opportunity. Nortech Geomatics opted for the latter, seeing it as a chance to use the latest technology, including GPS navigation and kinematics tools, advanced computer processing, a data logger, and high-resolution video imagery. In August 1997, a survey and mapping firm in French Guyana hired Nortech to use their helicopter-borne system to map a 600 hectare site for land use studies and to profile a 35 kilometer corridor for road development in various tropical rain forests.

Nortech integrated a 12-channel, L1, C/A-code GPS card into our helicopter system and the base station broadcast unit, located nine kilometers from the site. The base station permitted real-time differential GPS navigation data and supplied satellite range and phase data to the data logger for post-mission kinematics code and phase processing.

A laser provided an infrared pulse stream, which was vertically aligned to a gravity vector and stabilized on a two-axis platform. A vertical reference unit provided real-time kinematics feedback to drive the platform motors and ensure verticality. The platform also housed a video camera to image terrain features. As a video recorder stored the imagery, each frame was time tagged for coordination with the GPS navigation positions and sent to the data logger. A second camera provided a perspective view.

The system worked simply. When passing through foliage, the laser pulses produced multiple surface reflections, which were analyzed from the data logger to record the longest, second longest, and shortest returns. The combination of returns gave us a digital data file, showing a cross-section outline of the forest. By digitizing both the outline's top and bottom, Nortech was able to create profiles of the tree canopy and true ground. Combining multiple ground profiles produced a digital terrain mapping surface model. We then rectified the video imagery for feature extraction.

The GPS navigation and data logger units performed well, yielding real-time accuracies of 1-5 meters and post-processed kinematics positions accuracies of 10-20 centimeters. It took only eight three-hour flights to complete this challenging job.

Adaptation of Work Submitted by: Tony Tubman
Company: Nortech Geomatics International
Location: Calgary, Alberta, Canada

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