THE ALLURE AND PITFALLS OF USING LIDAR TOPOGRAPHY IN HARVEST AND ROAD DESIGN

Airborne laser altimetry (Lidar) can produce topographic maps of amazing detail and accuracy, even where the ground is obscured by forest canopy. Detailed Lidar topography can identify possible landing locations, difficult stream crossings, unstable soils, difficult side-slopes, and useful benches. This detail can reduce field time, guide road designs towards better options, and improve confidence in our cost estimates. Lidar mapping can occasionally fail however, and how these failures are represented will determine Lidar’s reliability and value for road design. We discuss first experiences with an operational Lidar mapping of the Tahoma State Forest, south of Mt. Rainier. This detailed topographic mapping was used in forest operations design such as landing and road locations as part of a watershed-based harvest and transportation plan. Lidar-based in-office designs were subsequently field-verified. Critical to the success of such DEM’s for forest engineering design was the ability (or lack thereof) to distinguish between areas of adequate or marginal ground point coverage leading to excellent or erroneous mapping detail. We discuss various methodologies that would identify areas of marginal Lidar ground point coverage leading to a first set of Lidar data collection requirements mapping contractors should adhere to. SEEING UNDER THE CANOPY A recurring problem in timber harvest and road planning is that the trees that intended for harvest can hide the ground over which logs must be yarded and roads must be built. The topographic maps that are commonly used in planning are based on aerial photographs in which the stands that we now want to harvest have obscured the ground over which we must plan. The resulting topography is thus a map of the top canopy, with an offset for the assumed tree height. Unfortunately, the canopy does not follow the ground exactly, and the minor topographic variations that can be crucial in harvest and road planning are not reflected in the top of the resulting canopy. The topography often includes areas of soil instability, rock outcrops, and uneven topography that can present difficulties in harvest and roading. The canopy can also obscure natural mounds and benches that can serve as convenient landing and road locations. As a result, these topographic maps can only serve as a general guide for design, and critical elements of the operation will need to be based on field verification. Recent developments in airborne laser topographic scanning (Lidar) allow for detailed topographic mapping even under forest canopy. Lidar works by shooting millions of