LIDAR, acronym for "Light Detection And Ranging ", is an optical remote sensing technology that measures properties of scattered light to find range of a distant target. Typically, multiple (invisible to human eye) infrared lasers or radio waves are pulsed from an array, which is attached to an airplane or helicopter. The range to an object is determined by measuring the time delay between transmission of a pulse and detection of the reflected signal. Using systematic fly-overs, a very detailed terrain map can be generated with differential height data in X,Y & Z coordinates. Using sofisticated algorithms and super computers, very accurate topographic maps are generated (see bottom image). This process can further be translated into a canopy topographic map (see center-left image, courtesty Forestry Tasmania), with the Z coordinate being the height of the tree above ground level. Additionally, the algorithm can be "tweaked" to search only for tree crowns taller than a pre-selected height or even for species type. Save the Redwoods League, Ken Fisher & Redwood National & State Parks co-sponsored a flyover of all the prime redwood forests with LIDAR. The areas covered were Jedediah Smith Redwoods SP, Del Norte Coast Redwoods SP, Prairie Creek Redwoods SP, Redwood Natonal Park, Humboldt Redwoods State Park and also Montgomery Woods State Reserve. The LIDAR data was processed for trees taller than 106m (348') and it locked on to all of the tallest 130 redwoods we already had found since 1990 (except for 1 tree), plus at least another 100 unknowns over 106m. The LIDAR height estimates were generally a little conserative due to the laser pulses not hitting the acutal ground level but rather ferns and bushes just above the ground. Trees that lean over a ravine were usally overestimated in height due to the tree's crown being centered above a ground level which was well below the tree. Several trees on the LIDAR "hit list" were processed as being way over 370', but after locating and measuring these trees on foot, they were found to be leaning over ravines and only slightly over 106m. Most of the trees on this list are indeed over 350', and so far only redwoods make up the list. The tallest LIDAR discovery to date is 369.5' Orion, growing on a spring fed high perched bench. It's big tree too with a dbh of 14.0. I personally am hoping for a super tall douglas fir to be found on the LIDAR "hit list" from Redwood National Park. In theory, a douglas fir 106m or taller might exist in one of our redwood parks, but it is highly unlikely. As of 1/6/2010, there are at least 50 trees on the "hit list" not yet identified. The Tall Trees Club will update the progress of its colleagues tracking down the remaing unknown "hit list" trees. Hyperion & Helios were identified as being #1 and #2 on this list. The Tall Trees Club concludes that the tallest redwoods have been found. Without the aid of LIDAR, finding all these remote tall trees from the ground would likely have taken more than a lifetime for an individual.
The following text is from Wikipedia and offers a more detailed explanation of how LIDAR works.
The use of lasers has become commonplace, from laser printers to laser surgery. In airborne-laser-mapping lidar, lasers are taken into the sky. Instruments are mounted on a single- or twin-engine plane or a helicopter. Airborne lidar technology uses four major pieces of equipment. These are a laser emitter-receiver scanning unit attached to the aircraft; global positioning system (GPS) units on the aircraft and on the ground; an inertial measurement unit (IMU) attached to the scanner, which measures roll, pitch, and yaw of the aircraft; and a computer to control the system and store data. Several types of airborne lidar systems have been developed; commercial systems commonly used in forestry are discrete-return, small-footprint systems. “Small footprint” means that the laser beam diameter at ground level is typically in the range of 6 inches to 3 feet. The laser scanner on the aircraft sends up to 100,000 pulses of light per second to the ground and measures how long it takes each pulse to reflect back to the unit. These times are used to compute the distance each pulse traveled from scanner to ground. The GPS and IMU units determine the precise location and attitude of the laser scanner as the pulses are emitted, and an exact coordinate is calculated for each point. The laser scanner uses an oscillating mirror or rotating prism (depending on the sensor model), so that the light pulses sweep across a swath of landscape below the aircraft. Large areas are surveyed with a series of parallel flight lines. The laser pulses used are safe for people and all living things. Because the system emits its own light, flights can be done day or night, as long as the skies are clear. Thus, with distance and location information accurately determined, the laser pulses yield direct, 3-D measurements of the ground surface, vegetation, roads, and buildings. Millions of data points are recorded, so many that lidar creates a 3-D data cloud. After the flight, software calculates the final data points by using the location information and laser data. Final results are typically produced in weeks, whereas traditional ground-based mapping methods took months or years. The first acre of a lidar flight is expensive, owing to the costs of the aircraft, equipment, and personnel. But when large areas are covered, the costs can drop to about $1 to $2 per acre. The technology is commercially available through a number of sources. 1
Topographic Map Generated With LIDAR. Image From University of California Davis Geography Department
LIDAR Illustrations By Wyoming Department of Interior
LIDAR generated canopy map of a 99m+ tall eucalyptus in Tasmania. Image by Forestry Tasmania
LIDAR generated side profile canopy map of a 111m "hit list" tree from Redwood National Park. This tree appears to have a dead spire top. Image courtesy Bill Kruse